Ocean acidification due to increasing
atmospheric carbon dioxide
Policy document 12/05
June 2005
ISBN 0 85403 617 2
This report can be found
at www.royalsoc.ac.uk

Ocean acidification due to increasing atmospheric carbon dioxideii | June 2005 | The Royal Society
ISBN 0 85403 617 2
© The Royal Society 2005
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Ocean acidification due to increasing atmospheric carbon dioxide
Contents
Page
Summary vi
1 Introduction 1
1.1 Background to the report 1
1.2 The oceans and carbon dioxide: acidification 1
1.3 Acidification and the surface oceans 2
1.4 Ocean life and acidification 2
1.5 Interaction with the Earth systems 2
1.6 Adaptation to and mitigation of ocean acidification 2
1.7 Artificial deep ocean storage of carbon dioxide 3
1.8 Conduct of the study 3
2 Effects of atmospheric CO2 enhancement on ocean chemistry 5
2.1 Introduction 5
2.2 The impact of increasing CO2 on the chemistry of ocean waters 5
2.2.1 The oceans and the carbon cycle 5
2.2.2 The oceans and carbon dioxide 6
2.2.3 The oceans as a carbonate buffer 6
2.3 Natural variation in pH of the oceans 6
2.4 Factors affecting CO2 uptake by the oceans 7
2.5 How oceans have responded to changes in atmospheric CO2 in the past 7
2.6 Change in ocean chemistry due to increases in atmospheric CO2 from human activities 9
2.6.1 Change to the oceans due to CO2 enhancement in recent centuries 9
2.6.2 How oceanic pH will change in the future 9
2.7 The role of carbon chemistry in ocean systems 10
2.7.1 Effects on calcium carbonate and saturation horizons 10
2.7.2 Impacts of acidification on the chemistry of nutrients and toxins 12
2.8 Conclusions 13
3 Biological impacts: effects of changing ocean chemistry on organisms and populations 15
3.1 Introduction 15
3.2 Effects of ocean acidification on photosynthetic and non-photosynthetic micro-organisms 16
3.2.1 Effects on phytoplankton: photosynthetic organisms 16
3.2.2 Effects on non-photosynthetic micro-organisms 18
3.3 Effects of ocean acidification on photosynthesis in benthic organisms 18
3.4 Effects of ocean acidification on multicellular animals 19
3.4.1 Changes to physiology of larger animals 19
3.4.2 Changes to reproduction in larger animals 19
3.5 Effects of ocean acidification on calcifying organisms 20
3.5.1 Introduction 20
3.5.2 Calcified protists and algae 20
3.5.3 Calcified larger animals 21
3.5.4 Functions of calcification and effects of decreased calcification 21
3.5.5 Influence of increased CO2 on calcification 21
3.6 Potential adaptation and evolution resulting from the surface ocean CO2 increase and acidification 22
3.7 Possible impact of ocean acidification on the structure of marine communities 22
3.8 Conclusions 23
4 Ecosystems most at risk from the projected changes in ocean chemistry 25
4.1 Introduction 25
4.2 Impact of ocean acidification on benthic systems 25
4.2.1 Coral reefs 25
4.2.2 Cold-water coral reefs 26
4.2.3 Shallow sediments and benthic organisms 27
4.3 Impact of ocean acidification on pelagic systems 28
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4.3.1 Coastal and open ocean pelagic ecosystems 28
4.3.2 Southern Ocean food webs 29
4.4 Conclusions 30
5 Interaction with the Earth systems 31
5.1 Introduction 31
5.2 Feedback effects of reduced calcification 31
5.3 Other feedbacks within the Earth systems 31
5.4 Conclusions 32
6 Socio-economic effects of ocean acidification 33
6.1 Introduction 33
6.2 Effects on coral reefs 33
6.3 Effects on marine fisheries 34
6.4 More general ecosystem effects 34
6.5 Ecosystem services and vulnerability 34
6.6 Corrosion 35
6.7 Conclusions 35
7 Engineering approaches to mitigation of ocean pH change 37
8 Conclusions and recommendations 39
8.1 Conclusions 39
8.2 Recommendations 42
Annexes
1 A brief account of measures of acidity such as pH, and the acid–base chemistry of the 43
CO2– carbonate system in the sea
A1 The meaning of pH 43
A2 Dissolved inorganic carbon in seawater 43
A3 The carbonate buffer and seawater pH 43
A4 The calcium carbonate saturation horizon 44
2 List of respondents 45
3 Abbreviations and glossary 47
4 References 51
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The members of the working group involved in producing this report were as follows:
Chair
Prof John Raven FRS School of Life Sciences, University of Dundee
Members
Dr Ken Caldeira Energy and Environment Directorate, Lawrence Livermore National Laboratory, USA
Prof Harry Elderfield FRS Department of Earth Sciences, University of Cambridge
Prof Ove Hoegh-Guldberg Centre for Marine Studies, University of Queensland, Australia
Prof Peter Liss School of Environmental Sciences, University of East Anglia
Prof Ulf Riebesell Leibniz Institute of Marine Sciences, Kiel, Germany
Prof John Shepherd FRS National Oceanography Centre, University of Southampton
Dr Carol Turley Plymouth Marine Laboratory
Prof Andrew Watson FRS School of Environmental Sciences, University of East Anglia
Secretariat
Mr Richard Heap Manager, The Royal Society
Mr Robert Banes Science Policy Officer, The Royal Society
Dr Rachel Quinn Senior Manager, The Royal Society
Membership of Working Group
Ocean acidification due to increasing atmospheric carbon dioxide The Royal Society | June 2005 | v

Summary
The oceans cover over two-thirds of the Earth’s surface.
They play a vital role in global biogeochemical cycles,
contribute enormously to the planet’s biodiversity and
provide a livelihood for millions of people.
The oceans are absorbing carbon dioxide (CO2) from the
atmosphere and this is causing chemical changes by
making them more acidic (that is, decreasing the pH of
the oceans). In the past 200 years the oceans have
absorbed approximately half of the CO2 produced by
fossil fuel burning and cement production. Calculations
based on measurements of the surface oceans and our
knowledge of ocean chemistry indicate that this uptake
of CO2 has led to a reduction of the pH of surface
seawater of 0.1 units, equivalent to a 30% increase in the
concentration of hydrogen ions.
If global emissions of CO2 from human activities continue
to rise on current trends then the average pH of the
oceans could fall by 0.5 units (equivalent to a three fold
increase in the concentration of hydrogen ions) by the
year 2100. This pH is probably lower than has been
experienced for hundreds of millennia and, critically, this
rate of change is probably one hundred times greater
than at any time over this period. The scale of the changes
may vary regionally, which will affect the magnitude of
the biological effects.
Ocean acidification is essentially irreversible during our
lifetimes. It will take tens of thousands of years for ocean
chemistry to return to a condition similar to that occurring
at pre-industrial times (about 200 years ago). Our ability
to reduce ocean acidification through artificial methods
such as the addition of chemicals is unproven. These
techniques will at best be effective only at a very local
scale, and could also cause damage to the marine
environment. Reducing CO2 emissions to the
atmosphere appears to be the only practical way to
minimise the risk of large-scale and long-term
changes to the oceans.
All the evidence collected and modelled to date indicates
that acidification of the oceans, and the changes in ocean
chemistry that accompany it, are being caused by
emissions of CO2 into the atmosphere from human
activities. The magnitude of ocean acidification can be
predicted with a high level of confidence. The impacts of
ocean acidification on marine organisms and their
ecosystems are much less certain but it is likely that,
because of their particular physiological attributes, some
organisms will be more affected than others.
Predicting the direction and magnitude of changes in a
complex and poorly studied system such as the oceans is
very difficult. However, there is convincing evidence to
suggest that acidification will affect the process of
calcification, by which animals such as corals and
molluscs make shells and plates from calcium carbonate.
The tropical and subtropical corals are expected to be
among the worst affected, with implications for the
stability and longevity of the reefs that they build and the
organisms that depend on them. Cold-water coral reefs
are also likely to be adversely affected, before they have
been fully explored.
Other calcifying organisms that may be affected are
components of the phytoplankton and the zooplankton,
and are a major food source for fish and other animals.
Regional variations in pH will mean that by 2100 the
process of calcification may have become extremely
difficult for these groups of organisms particularly in the
Southern Ocean. Some shallow water animals, which
play a vital role in releasing nutrients from sediments, also
calcify, and may be affected by changes in the chemistry
of the oceans. Some studies suggest that growth and
reproduction in some calcifying and non-calcifying
marine species could be reduced due to the projected
changes in ocean chemistry.
From the evidence available it is not certain whether
marine species, communities and ecosystems will be able
to acclimate or evolve in response to changes in ocean
chemistry, or whether ultimately the services that the
ocean’s ecosystems provide will be affected. Research
into the impacts of high concentrations of CO2 in
the oceans is in its infancy and needs to be
developed rapidly. We recommend that a major,
internationally coordinated effort be launched to
include global monitoring, experimental,
mesocosm and field studies. Models that include
the effects of pH at the scale of the organism and
the ecosystem are also necessary. The impacts of
ocean acidification are additional to, and may
exacerbate, the effects of climate change. For this
reason, the necessary funding should be additional
and must not be diverted from research into
climate change.
Oceans play a very important role in the global carbon
cycle and Earth’s climate system. There are potentially
important interactions and feedbacks between changes
in the state of the oceans (including their pH) and
changes in the global climate and atmospheric chemistry.
Changes in the chemistry of the oceans will reduce their
ability to absorb additional CO2 from the atmosphere,
which will in turn affect the rate and scale of global
warming. The knowledge of these impacts and effects is
currently poor and requires urgent consideration. The
understanding of ocean acidification and its impacts
needs to be taken into account by the
Intergovernmental Panel on Climate Change and
kept under review by international scientific bodies
such as the Intergovernmental Oceanographic
Commission, the Scientific Committee on Oceanic
Research and the International Geosphere-
Biosphere Programme.
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The socio-economic effects of ocean acidification could
be substantial. Damage to coral reef ecosystems and the
fisheries and recreation industries that depend on them
could amount to economic losses of many billions of
dollars per year. In the longer term, changes to the
stability of coastal reefs may reduce the protection they
offer to coasts. There may also be direct and indirect
effects on commercially important fish and shellfish
species.
Marine ecosystems are likely to become less robust as a
result of the changes to the ocean chemistry and these
will be more vulnerable to other environmental impacts
(such as climate change, water quality, coastal
deforestation, fisheries and pollution). The increased
fragility and sensitivity of marine ecosystems needs
to be taken into consideration during the
development of any policies that relate to their
conservation, sustainable use and exploitation, or
the communities that depend on them.
If the risk of irreversible damage arising from ocean
acidification is to be avoided, particularly to the
Southern Ocean, the cumulative future human
derived emissions of CO2 to the atmosphere must be
considerably less than 900 Gt C (gigatonnes of
carbon) by 2100.
In setting targets for reductions in CO2 emissions,
world leaders should take account of the impact of
CO2 on ocean chemistry, as well as on climate change.
These targets must be informed by sound science.
Ocean acidification is a powerful reason, in addition
to that of climate change, for reducing global CO2
emissions. Action needs to be taken now to reduce
global emissions of CO2 to the atmosphere to avoid
the risk of irreversible damage to the oceans. We
recommend that all possible approaches be
considered to prevent CO2 reaching the atmosphere.
No option that can make a significant contribution
should be dismissed.
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1.1 Background to the report
Covering around 70% of the planet, the oceans play a
central role in the Earth’s major processes. They are host
to thousands of species of organisms, which live in a
variety of habitats and ecosystems. Carbon dioxide (CO2)
emitted to the atmosphere by human activities is
absorbed by the oceans, making them more acidic
(lowering the pH—the measure of acidity). Initial
evidence shows that the surface waters of the oceans,
which are slightly alkaline, are already becoming more
acidic: we refer to this process as ocean acidification.
There is growing concern that as atmospheric
concentrations of CO2 continue to rise, the increasing
acidity will have significant effects on the marine system.
In recent years global warming and the resulting climate
changes, has received considerable global attention.
There is now a clear scientific consensus that increasing
atmospheric levels of CO2 (one of the major greenhouse
gases), resulting mainly from human activities, are
causing global mean surface temperatures to rise (IPCC
2001).
Ocean acidification is an additional concern to that of
climate change, but the threat it poses to the marine
environment has only recently been recognised. Parts of the
international scientific community are beginning to take this
issue seriously, for example the 2004 UNESCO symposium
on the Oceans in a High-CO2 World. An understanding of
the chemical processes involved when CO2 is absorbed from
the atmosphere and dissolves in seawater is fairly well
established. However, much less is known about the oceans
and the biological and chemical processes of the life within
them. Therefore predicting the impacts of ocean
acidification is a complex and significant challenge.
For this reason the Royal Society has undertaken this study
to provide a concise overview of the present state of
scientific knowledge of ocean acidification and its likely
impacts on marine organisms. This report will be of interest
to those taking decisions and making policies on climate
change, energy policy and environmental protection; for
scientists studying the oceans, atmosphere and climate;
and for anyone who is interested in the impact of human
activities on the natural processes of our planet.
1.2 The oceans and carbon dioxide:
acidification
Carbon dioxide is being produced in substantial
quantities mainly through the combustion of fossil fuels,
cement production, agriculture and deforestation. The
concentration of CO2 in the atmosphere has been
increasing from its recent pre-industrial level of about 280
parts per million (ppm) to about 380 ppm today. What is
significant for biological systems is that the rate of this
increase is unprecedented since the peak of the last Ice
Age—for at least 20 000 years (IPCC 2001). Atmospheric
CO2 levels are predicted to continue to increase for at
least the next century and probably longer, and unless
emissions are substantially reduced, may well reach levels
exceeding 1000 ppm by 2100, higher than anything
experienced on Earth for several million years.
Oceans play a fundamental role in the exchange of CO2
with the atmosphere. Over the past 200 years, since pre-
industrial times, the oceans have absorbed about a half of
the CO2 emissions produced from burning fossil fuels and
cement manufacture. This demonstrates the integral role
that oceans play within the natural processes of cycling
carbon on a global scale—the so-called carbon cycle.
The oceans and the organisms they support contain
about 38000 Gt C (gigatonnes of carbon; 1 Gt C =
1015 grams) (Figure 1). This accounts for about 95% of all
the carbon that is in the oceans, atmosphere and
terrestrial system, constituting a substantial reservoir of
carbon. As we explain in Section 2, the chemical properties
of the dissolved carbon in this system enable the oceans to
buffer, or neutralise, changes in acidity due to the uptake
of CO2 emissions. However, as absorption of the CO2
emissions from human activities increases (currently about
2 Gt C per year), this reduces the efficiency of the oceans
to take up carbon. Carbon dioxide exchange is a two-way
process, with the oceans and atmosphere absorbing and
releasing CO2. A decrease in the amount of CO2 absorbed
by the oceans will mean that relatively more CO2 will stay
in the atmosphere. This will make global efforts to reduce
atmospheric concentrations of CO2 and the associated
climate change more difficult.
The surface waters of the oceans are slightly alkaline, with
an average pH of about 8.2, although this varies across
the oceans by ±0.3 units because of local, regional and
seasonal variations. Carbon dioxide plays an important
natural role in defining the pH of seawater (a brief
account of measures of acidity such as pH, and the
acid–base chemistry of the CO2–carbonate system in the
oceans, is given in Annex 1). When CO2 dissolves in
seawater it forms a weak acid, called carbonic acid. Part
of this acidity is neutralised by the buffering effect of
seawater, but the overall impact is to increase the acidity.
This dissolution of CO2 has lowered the average pH of the
oceans by about 0.1 units from pre-industrial levels
(Caldeira & Wickett 2003). Such a value may seem small
but because of the way pH is measured, as we explain in
Section 2, this change represents about a 30% increase in
the concentration of hydrogen ions, which is a
considerable acidification of the oceans. Increasing
atmospheric concentration of CO2 will lead to further
acidification of the oceans.
In Section 2 we outline the main chemical reactions
associated with ocean acidification. We look at the effects
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1 Introduction

on ocean chemistry that CO2 emissions from human
activities have already caused and consider how the
chemistry, nutrients and trace metals of the oceans may
change with future emissions. These changes will affect
the many important natural processes that are affected by
its acidity/alkalinity (pH).
1.3 Acidification and the surface oceans
In this report we use the term ‘surface oceans’ to describe
the near-surface waters where exchange of CO2 occurs.
Only the near-surface waters, or surface layers, of the
oceans (down to about 100 m on average) are well mixed
and so in close contact with the atmosphere. Carbon
dioxide in the atmosphere dissolves in the surface waters
of the oceans and establishes a concentration in
equilibrium with that of the atmosphere. Molecules of
CO2 exchange readily with the atmosphere and on
average only remain in the surface waters for about 6
years. However mixing and advection (vertical motions,
sinking and upwelling) with the intermediate and deep
waters of the oceans (down to about 1000 m and
4000 m respectively) is much slower, and takes place on
timescales of several hundred years or more. Over time
this mixing will spread the increased atmospheric uptake
of CO2 to the deeper oceans. Owing to this slow mixing
process most of the carbon stored in the upper waters of
the oceans will be retained there for a long time. This
makes the impacts in the surface waters greater than if
the CO2 absorbed from the atmosphere was spread
uniformly to all depths of the oceans.
1.4 Ocean life and acidification
Most of the biological activity in the oceans (and all of the
photosynthesis) takes place in the near-surface waters
through which sunlight penetrates; the so-called photic
zone.
Marine organisms are, by definition, adapted to their
environment. However, changes in ocean chemistry,
especially rapid modifications such as ocean acidification,
could have substantial direct and indirect effects on these
organisms and upon the habitats in which they live. Direct
effects include the impact of increasing CO2
concentration and acidity, which may affect all stages of
the life cycle. Indirect effects include the impact on
organisms arising from changes in availability or
composition of nutrients as a result of increased acidity.
One of the most important implications of the changing
acidity of the oceans relates to the fact that many marine
photosynthetic organisms and animals, such as corals,
make shells and plates out of calcium carbonate (CaCO3).
This process of ‘calcification’, which for some marine
organisms is important to their biology and survival, is
impeded progressively as the water becomes acidified
(less alkaline). This adverse effect on calcification is one of
the most obvious and possibly most serious of the likely
environmental impacts of ocean acidification.
Any changes in the biological processes in the surface
ocean waters will also affect the deeper water of the
oceans. This is because organisms and habitats living at
the lower levels of the oceans — far from the sunlight —
rely mainly on the products created by life in the surface
waters. On a longer timescale, these organisms may also
be vulnerable to acidification and changes in ocean
chemistry as higher levels of CO2 mix throughout the
oceans.
In Section 3 of the report we explore the biological
systems of the oceans and highlight processes and groups
of species that may be vulnerable to changes in ocean
chemistry. We examine how effects on organisms may
affect populations of species; how these will affect
interactions between species; and finally we consider
whether species will acclimatise or evolve in response to
ocean acidification. Section 4 looks at how these changes
will affect ecosystems most likely to be at risk, such as
coral reefs. Coral structures provide a valuable habitat for
many other species, but being composed of CaCO3 could
be most at risk from increasing surface ocean CO2
concentrations.
1.5 Interaction with the Earth systems
Ocean acidification will not occur in isolation from the
rest of the Earth systems. Oceans play a significant role in
the regulation of global temperature and so affect a
range of climatic conditions and other natural processes.
The Earth’s climate is currently undergoing changes as a
result of global warming, which is having an impact
across many chemical and biological processes.
Considerable interactions may exist between all these
processes, which may have beneficial or adverse impacts,
alongside those of ocean acidification. In Section 5 we
identify the important interactions and consider the
possible impacts of changes in ocean chemistry on other
global processes.
1.6 Adaptation to and mitigation of ocean
acidification
Any changes in natural resources as a result of ocean
acidification could impact upon the livelihoods of people
who rely on them. In Section 6 we look at the areas where
there could be large socio-economic effects and evaluate
the potential costs of these impacts.
Apart from reducing emissions to the atmosphere,
engineering approaches (such as adding limestone, a
carbonate material) have been suggested for tackling
ocean acidification. These approaches aim to reduce
some of the chemical effects of increased CO2 through
the addition of an alkali to the oceans. In Section 6 we
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2.1 Introduction
The oceans are a significant store of carbon within the
Earth systems. They readily exchange carbon in the form
of CO2 with the atmosphere and provide an important
sink for CO2. Human activities are releasing CO2 that
would otherwise be locked away from the atmosphere in
geological reservoirs. Because of these changes,
atmospheric concentrations of CO2 are higher today than
for at least 420000 years (IPCC 2001).
Approximately one-half of the CO2 produced by fossil fuel
burning and cement production as a result of human
activities in the past 200 years is being taken up by the
oceans. This absorption process is chemically changing the
oceans, in particular increasing its acidity. In this section we
consider the evidence of increased uptake of CO2 by the
oceans over the past century and how this reflects changes
in atmospheric CO2 levels and ocean acidity. We provide
an overview of the chemical processes involved as CO2
dissolves in the oceans; how ocean chemistry responds to
changes in CO2 levels; and an introduction to how these
changes may affect the biological systems, which are
considered further in Sections 3 and 4.
2.2 The impact of increasing CO2 on the
chemistry of ocean waters
2.2.1 The oceans and the carbon cycle
Carbon exists throughout the planet in several ‘reservoirs’
and in a variety of forms (Figure 1). The exchange of
carbon between the important reservoirs of the
biosphere, atmosphere and oceans is known as the
carbon cycle. One of the more commonly known
exchanges of carbon in this cycle is its absorption, in the
form of CO2, by trees and herbaceous plants on land
during photosynthesis, also known as primary production
(the production of organic from inorganic carbon), and
subsequent release back into the atmosphere by
respiration. Carbon dioxide also dissolves in the oceans
and can be released back into the atmosphere, making
the oceans a considerable point of exchange in the
carbon cycle. Organisms within the surface ocean
exchange CO2 in much the same way as the biological
processes on land. Although the biological uptake of CO2
per unit area of the surface oceans is lower than that in
most terrestrial systems, the overall biological absorption
is almost as large as that in terrestrial environment. This is
because the surface area of the oceans is so much larger
(Field et al 1998).
The oceans are a substantial carbon reservoir. When
measured on short timescales of hundreds of years,
their greatest exchanges of carbon are with the
atmosphere. The pre-industrial oceanic carbon reservoir
has been estimated at about 38 000 Gt, compared with
about 700 Gt in the atmosphere and somewhat less
than 2 000 Gt in the terrestrial biosphere
(approximately 700 Gt as biomass and 1 100 Gt as soil)
(Brovkin et al 2002). These reservoirs exchange
quantities of carbon each year that are large relative to
the amount of carbon stored within them. Figure 1
illustrates that the oceans are acting as an important
carbon sink, absorbing 2 Gt C per year more CO2 than
they are releasing into the atmosphere. This is small in
comparison to the amount of carbon that is cycled
between the different reservoirs but is a significant
proportion of the 6 Gt C per year released into the
atmosphere from human activity (Figure 1).
The carbon buried in some reservoirs, such as rocks and
organic-rich shale, exchanges with the other reservoirs
on geologically long timescales. As a result, carbon in
these reservoirs will not affect the atmosphere or oceans
on short timescales (up to about 103 years) unless
exchange rates are artificially increased by human
activity such as limestone mining, oil, gas and coal
production. It is the carbon released by human activities
that has produced increased atmospheric
concentrations of CO2 to levels unprecedented for at
least 420000 years and possibly for the past tens of
millions of years (IPCC 2001).
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2 Effects of atmospheric CO2 enhancement on ocean chemistry
Atmosphere: 700 Gt (3 years)
Alive
70 Gt
(5 years)
Dead
1100 Gt
(20 years)
Terrestrial biosphere
Surface ocean
600 Gt (6 years)
Intermediate ocean
7000 Gt (100 years)
Deep ocean
30 000 Gt (100000 years)
Fossil fuels and shales
12000 Gt (1000 years)
Marine sediments
30 million Gt (100 m years)
6 60 122 102 100 0.3
<0.1 0.3
60
Figure 1. Diagram of the global carbon cycle showing
sizes of carbon reservoirs (units are Gt (gigatonnes): 1 Gt
= 1015 grams) and exchange rates (‘fluxes’) between
reservoirs (units are gigatonnes per year) in the terrestrial
(green) and the oceanic (dark blue) parts of the Earth
system. Also shown are ‘residence times’ (in years) of
carbon in each reservoir: however, some mixing between
the deep oceans and marine sediments does occur on
shorter timescales. Carbon exchanges readily between
the atmosphere, the surface oceans and terrestrial
biosphere. However, the residence time of carbon in the
atmosphere, oceans and biosphere combined, relative to
exchange with the solid Earth, is about 100000 years.
(Reprinted and redrawn from Holmen (2000) with
permission from Elsevier.)

function for the burning of the remaining fossil-fuel
resources, emissions of CO2 from human activities have
peaked around 2 150. However, the CO2 remains in the
atmosphere at a high concentration for several thousand
years, before it is absorbed by the oceans or the terrestrial
biosphere. The initial changes in the surface ocean pH are
rapid, but as CO2 continues to be absorbed from the
atmosphere it is slowly transferred to the deep oceans
(including ocean sediments) by mixing and through the
biological pump, with subsequent changes in pH.
The study predicts a possible decrease in pH of 0.5 units
by the year 2100 in the surface oceans, which, as stated
earlier, corresponds to a three fold increase in the
concentration of hydrogen ions from pre-industrial times.
The simulated atmospheric CO2 exceeds 1900 ppm at
around 2300, and the maximum reduction in pH in the
oceans’ surface would be 0.77 units (Figure 6). The
lowering of pH would be moderated slightly by factors
such as changes in temperature, but by less than 10%.
Taking this example, even at high CO2 concentrations,
with a significant lowering of ocean pH, the carbonate
buffer means that the oceans are still slightly alkaline (ie a
pH of less than 7). These changes in ocean pH are outside
the range of natural variability and, as we evaluate in
Sections 3 and 4, could have a substantial affect on and
biological processes in the surface oceans.
Even the current level of ocean acidification is essentially
irreversible during our lifetimes. It will take tens of
thousands of years for ocean chemistry to return to a
condition similar to that occurring at pre-industrial times,
about 200 years ago.
The delay arises because it takes tens of thousands of
years for mixing to take place throughout the oceans.
This mixing is required to bring up compounds from the
oceans’ sediments to buffer the changes as a result of
increased CO2 on ocean surface chemistry. As we have
highlighted above, the oceans have in the past been
able to adapt to increased atmospheric CO2 levels
because the rate of change took place over time periods
that enabled sufficient mixing with deeper waters.
Warming of the oceans, as a result of global warming,
may also reduce the rate of mixing with deeper waters
and this would further delay recovery.
2.7 The role of carbon chemistry in ocean
systems
Below we provide an introduction to the important
chemical changes that are predicted to affect marine
biological processes.
2.7.1 Effects on calcium carbonate and
saturation horizons
Marine organisms that construct CaCO3 structures, such
as shells, are dependent on the presence of bicarbonate
and carbonate forms of dissolved inorganic carbon in
seawater. Once formed, CaCO3 will dissolve back into the
water unless the surrounding seawater contains
sufficiently high concentrations of carbonate ions (CO3
2–)
(ie it is saturated) (Annex 1). Calcium carbonate also
becomes more soluble with decreasing temperature and
increasing pressure, and hence with ocean depth. It is also
affected by the flow of CO2-rich deep waters between
ocean basins (Section 2.3). A natural boundary develops
in seawater as a result of these different variables. This is
known as the ‘saturation horizon’ and it identifies a clear
depth of seawater above which CaCO3 can form, but
below it dissolves.
Ocean acidification due to increasing atmospheric carbon dioxide10 | June 2005 | The Royal Society
Figure 5. Projected change in pH of the oceans due to release
of CO2 from human activities. The top layer of the diagram
shows past and projected emissions of CO2 to the
atmosphere in Gt C per year. Carbon dioxide emission trends
are based on mid-range CO2 emission scenario (IPCC IS92a
scenario (Houghton et al 2001)). The decrease in emissions
after 2150 assumes that human activities will continue at
levels until the eventual burning of the remaining fossil fuel
resources . The second layer shows historical atmospheric CO2
levels to 1975, observed atmospheric CO2 concentrations
from 1975 to 2000 and predicted concentrations to year
3000. The decrease in atmospheric CO2 levels after 2400
reflects the excess of uptake of CO2 by the oceans and
sequestered down to the oceans’ sediments. The coloured
sections at the bottom of the figure show the projected
average change in ocean pH with depth. The Figure assumes
the chemistry and pH is uniform on the horizontal plane
throughout the oceans. The maximum reduction in the
surface oceans shown is 0.77 units. The vertical variations
relate to mixing over time of the surface waters with deeper
parts of the oceans and chemical reactions in the deep
oceans. These CO2 effects were estimated using the
Lawrence Livermore National Laboratory (LLNL) ocean
general-circulation model. Physical oceanographic modelling
is based on Bryan (1969) and Cox (1984). (Reprinted with
permission from Nature in Caldeira & Wickett
Anthropogenic carbon and ocean pH. Copyright 2003,
Macmillan Publishers Ltd. http://www.nature.com).
Emissions
pCO2
–0.7
–0.4
–0.3
–0.2
–0.1
1750 2000 2250 2500 2750 3000
20
10
0
2000
1000
0
1
2
3
4 ∆pH
pp
m
Year
G
t C
/
yr
D
ep
th
(k
m
)

The marine organisms that produce CaCO3 shells live
above the saturation horizon, where CaCO3 does not
readily dissolve. Increasing CO2 levels and the resultant
lower pH of seawater decreases the saturation state of
CaCO3 and raises the saturation horizon closer to the
surface. Therefore increases in surface ocean CO2 levels
could have severe consequences for organisms that make
external CaCO3 shells and plates.
Calcium carbonate exists in two commonly occurring
forms: aragonite and calcite (Box 1). Because the
aragonite form of CaCO3 is more soluble than calcite, the
aragonite saturation horizon is always nearer the surface
of the oceans than the calcite saturation horizon.
Therefore calcifying organisms that produce the calcite
form of CaCO3 (coccolithophores and foraminifera) are
possibly less vulnerable to changes in ocean acidity than
those that construct aragonite structures (corals and
pteropods) (see Figure 7 and Table 2 for a description of
the key groups of relevant marine life within the oceans).
Projected changes in pH and the calcite and aragonite
saturation state in the surface oceans in 2100 under a
range of possibilities for future CO2 emissions are shown
in Figure 6 (based on IPCC 2000). When modelled under
one of the more modest emission scenarios (B1), which
projects cumulative CO2 emissions to be about 900 Gt C
by 2100, the Southern Ocean is already undersaturated
with respect to aragonite. This would mean that any
aragonite CaCO3 structure produced by organisms in this
environment would dissolve. The cold Southern Ocean is
particularly vulnerable to changes in saturation levels
because it currently has very low saturation levels. The
impacts of changing carbonate saturation on ecosystems,
particularly for coral reefs, and the Southern Ocean are
examined in greater detail in Sections 4.2.1 and 4.2.2.
The saturation level of the calcite and aragonite horizons
can be mapped, and the changes can be shown between
the present and pre-industrial locations of the horizons
resulting from penetration of CO2 into seawater from
human activities (Feely et al 2004). Lower seawater
temperature towards the poles means that the saturation
levels are lower at higher latitudes (Figure 5). Across most
of the oceans the saturation horizon, for the calcite form,
occurs at depths between 1.5 to 5 km and, for the
aragonite form, at depths between 0.5 to 2.5 km below
the surface of the oceans.
The production of CaCO3 by free-floating organisms
plays a key role in the biological pump (Figure 3). Once
the organism dies, the CaCO3 causes it to sink, thereby
taking its carbon content to the deeper waters. If the
saturation horizon moves closer to the surface, this
would lead to both a reduced production of CaCO3 by
organisms and an increased dissolution of calcified
structures. These effects may have serious consequences
for the amount of carbon that is produced in the near-
surface waters and sequestered into the deeper oceans.
However effects of this kind may be offset by an increase
in the production of other organic carbon compounds by
surface marine organisms. The biological role of CaCO3
and organic carbon is discussed in more detail in Section
3.5.
The formation of CaCO3 leads to an increased CO2
concentration in the water. This apparently
counterintuitive behaviour arises because two ions of
bicarbonate (HCO3
–) react with one ion of doubly
charged calcium (Ca2+) to form one molecule of CaCO3,
which leads to the release of one molecule of CO2. Some
of this released CO2 is converted to bicarbonate by the
buffering process, outlined above and in Annex 1. Under
current conditions, for each molecule of CO2 produced
during calcification about 0.6 molecules are released,
potentially to the atmosphere, while the rest is taken up
by the bicarbonate-carbonate buffer (Ware et al 1992).
As the capacity of the oceans to buffer CO2 decreases
with increasing acidity, this ratio is expected to increase.
Specifically, if CO2 emissions continue at their present
rate, atmospheric concentration is estimated to reach
about 700 ppm by 2100 (IPCC 2001). It is then estimated
that, by 2150, for each CO2 molecule produced from
calcification 0.79 molecules would be potentially
released to the atmosphere (Zondervan et al 2001). This
would increase the total emission of CO2 from the
oceans as a result of the calcification process by about
0.22 Gt C per year, and this would lead to a change in
CO2 emissions from about 0.63 Gt C per year in 1850 to
about 0.85 Gt C per year in 2150 (Zondervan et al 2001).
However, it is important to stress that this calculation
assumes that the rate of calcification would remain
constant, at about 1 Gt C per year, and not be affected
by changes in the acidity. A decrease in calcification
resulting from increased acidity would counteract this
increase and according to some estimates, would
decrease the total emission from the oceans, providing a
Ocean acidification due to increasing atmospheric carbon dioxide The Royal Society | June 2005 | 11
Box 1 Forms of calcium carbonate
Calcium carbonate (CaCO3) exists in two main
structures. These are (i) aragonite, which has
orthorhombic symmetry in its structure, and (ii) calcite,
which is trigonal. Both aragonite and calcite are
abundant in organisms. However, because of its
structure, calcite is less soluble.
As we describe in Sections 3 and 4, these CaCO3
structures are widespread and integral to the biology of
key species in the marine ecosystem, including primary
producers and herbivores.
Throughout the report the phrase undersaturated with
respect to CaCO3 (aragonite, calcite or both), refers to
the situation where there is insufficient CaCO3
2–
available to prevent CaCO3 dissolving in sea water. In
practice this also means there is insufficient CO3
2–
available for organisms to make CaCO3 structures. This
results from factors, such as ocean acidification,
changing ocean chemistry.

availability over at least 10% of the oceans’ surface (Boyd
et al 2000). Therefore any effect of pH on iron speciation
may be important in these areas and reduce the iron
limitation, as one effect of reduced pH would be to increase
the proportion of soluble to insoluble iron (Morel et al
2003).
A decrease in pH will also probably change the nature and
form of particle surfaces in seawater and hence have an
additional indirect effect on the speciation of trace metals
(Mouvet & Bourg 1983). Indeed, the main impact of
change in pH on speciation of trace metals may be to
release previously bound metals from the sediment to the
water column. This could be of particular importance in
shallow seas.
There appear to have been no studies of the impact
of change in pH on nutrient speciation. It therefore
remains unclear whether such changes would
significantly affect biological systems. However,
theoretical speciation diagrams allow a qualitative
prediction of the effect of pH on nutrient speciation
(Zeebe & Wolf-Gladrow 2001). These show significant
changes in speciation of phosphate, silicate, iron
and ammonia within the range of decrease in pH
considered in this report. Reduced pH would, for
example, lower the concentrations of ammonia (NH3)
in seawater in preference to ammonium (NH4
+).
In mesotrophic coastal waters with frequently high
nutrient concentrations, a change of pH from 8.1 to
7.8 halves the fraction of NH3 in the ratio of NH4
+:NH3
(Raven 1986). Further quantitative research is
required on the impacts of pH on nutrient speciation,
concentration and availability and their non-
equilibrium behaviour that is typical of natural
systems.
The biological implications of these changes in chemical
speciation are considered in Section 3.
2.8 Conclusions
The chemical changes in the oceans caused by increases
in the concentration of CO2 in the atmosphere will
include a lowering of the pH, an increase in dissolved CO2,
a reduction in the concentration of carbonate ions and an
increase in bicarbonate ions.
The oceans are currently taking up about one tonne
of CO2 associated with human activities per year for
each person on the planet (IPCC 2001). Almost half of
the CO2 produced in the past 200 years by burning
fossil fuels and cement manufacture has been absorbed
by the oceans. This has already resulted in a change
to ocean chemistry, reducing surface seawater pH
by about 0.1 units, which corresponds to an increase
of about 30% in the concentration of hydrogen
ions.
The affects of possible impacts of future cumulative CO2
emissions by 2100 have been modelled for the oceans.
These models, including analyses from the GLODAP,
International Carbon-cycle Model Intercomparison
Project Phase 3 (OCMIP3) and the LLNL presented in this
Section, rely on the same basic modelling approaches
(Bryan 1969; Cox 1984). These studies show that even at
a modest future projection of CO2 emissions, of about
900 Gt C, direct impact of ocean acidification is very likely
to cause the Southern Ocean to become undersaturated
with respect to aragonite. This would lead to severe
consequences for organisms that make the aragonite
form of CaCO3 shells and plates.
Ocean acidification due to increasing atmospheric carbon dioxide The Royal Society | June 2005 | 13
Table 1. Changes to ocean chemistry and pH estimated using the OCMIP3 models calculated from surface ocean
measurements and our understanding of ocean chemistry. Note that the concentration of bicarbonate ion (HCO3
–) and
carbonic acid (H2CO3) increase with rising atmospheric concentration of CO2 while carbonate ion (CO3
2–) decreases. The
average pH of the surface ocean waters decreases with increasing atmospheric CO2 concentration. (Assumptions used
in model: Total alkalinity = 2324 mol/kg, temperature = 18° C. All other assumptions as per OCMIP3 (Institut Pierre
Simon Laplace 2005). Aragonite and calcite saturation calculated as per Mucci & Morse (1990). Physical oceanographic
modelling is based on Bryan (1969) and Cox (1984).
Pre-industrial Today 2 ✕ pre- 3 ✕ pre- 4 ✕ pre- 5 ✕ pre-` 6 ✕ pre-
industrial industrial industrial industrial industrial
Atmospheric 280 ppm 380 ppm 560 ppm 840 ppm 1120 ppm 1400 ppm 1680 ppm
concentration of CO2
H2CO3 (mol/kg) 9 13 19 28 38 47 56
HCO3– (mol/kg) 1768 1867 1976 2070 2123 2160 2183
CO32– (mol/kg) 225 185 141 103 81 67 57
Total dissolved inorganic 2003 2065 2136 2201 2242 2272 2296
carbon (mol/kg)
Average pH of surface 8.18 8.07 7.92 7.77 7.65 7.56 7.49
oceans
Calcite saturation 5.3 4.4 3.3 2.4 1.9 1.6 1.3
Aragonite saturation 3.4 2.8 2.1 1.6 1.2 1.0 0.9

As more CO2 enters the atmosphere from human
activity, more will be taken up by the oceans (Table 1). If
CO2 emissions continue on current trends, this could
result in the average pH of the surface oceans
decreasing by 0.5 units below the level in pre-industrial
times, by 2100. This is beyond the range of natural
variability and represents a level probably not
experienced for at least hundreds of thousands of years
and possibly much longer (Caldeira & Wickett 2003).
Critically, the rate of change is also at least 100 times
higher than the maximum rate observed during this time
period. These changes are so rapid that they will
significantly reduce the buffering capacity of the natural
processes that have moderated changes in ocean
chemistry over most of geological time.
In Sections 3 and 4 we consider the biology of the marine
system and examine the implications of the projected
changes in ocean chemistry. We then examine the
species, populations and ecosystems that may be
vulnerable to changes in ocean chemistry.
Ocean acidification due to increasing atmospheric carbon dioxide14 | June 2005 | The Royal Society

3.1 Introduction
The increase in surface ocean CO2 and the corresponding
decrease in pH predicted for 2100 represent an extent of
change in sea surface chemistry that has not been
experienced for at least the past 420000 years and
possibly much longer (Section 2.5). As significant is the
rate of this change, which is at least 100 times the
maximum rate observed during this time period (Section
2.5). Such extents, and rates, of change will inevitably
have effects on organisms in the oceans. Here we discuss
the effects these changes in ocean chemistry have on
present-day marine organisms (Figure 7; Table 2).
We also consider the possibilities of acclimation or
adaptation of organisms to the changing environment
within the constraints of their current genetic
composition, as well as the potential for evolution
(genetic changes selected in relation to the changed
ocean chemistry).
Photosynthetic marine organisms play a major ecological
role, providing more than 99% of the organic matter
used by marine food webs. Converting dissolved CO2 to
organic carbon, they account for just under half of the
total primary productivity on Earth (Field et al 1998). The
vast majority occur as free-floating microscopic
phytoplankton, over the whole of the surface oceans.
They fix about 50 Gt of carbon per year into their bodies
(Field et al 1998), with significant additional extracellular
dissolved organic carbon (Engel et al 2004). Most of the
organic matter produced by these photosynthetic
organisms is immediately or ultimately consumed by
other micro-organisms (Figure 7).
The other major primary producers in the oceans are the
benthic (bottom-dwelling) photosynthetic organisms,
such as seaweed, sea grasses and corals. Limited to
shallower waters, they have a net primary productivity of
about 1 Gt of carbon per year (Field et al 1998). Despite
their limited role in global primary productivity, these
benthic primary producers are crucial components of
coastal ecosystems, providing, for example, habitats and
breeding grounds for other species, as well as recycling
nutrients and reducing coastal erosion by reducing the
power of waves.
Higher up the marine food webs are the multicellular
animals. Although these are dependent on the availability
of the micro-organisms for food, and are therefore
affected indirectly by changes in ocean carbon chemistry,
the physiology of these complex organisms may also be
affected directly by changes in ocean chemistry. For
animals, such as fish, these direct effects of ocean
acidification on their physiology, with other pressures
such as fishing, are additional to any changes in the
availability of their food species caused by changes in
ocean chemistry.
Of all the organisms in the surface oceans the impacts are
very likely to be most severe for the calcifying species
(Table 2), such as molluscs, crustaceans, echinoderms,
corals, large calcareous algae, foraminifera and some
phytoplankton. This will occur principally because of a
reduction in the availability of the chemical constituents
needed for calcified shells and plates, although their
physiology may also be affected.
In this section we outline what is known about these
organisms and, starting at the base of the food chain,
consider how they are likely to be affected by
acidification. We consider the direct effect of CO2 and pH
changes on the organisms and the indirect effects
Ocean acidification due to increasing atmospheric carbon dioxide The Royal Society | June 2005 | 15
3 Biological impacts: effects of changing ocean chemistry on
organisms and populations
CO2 in
atmosphere
CO2
dissolve
Respiration
Respiration Respiration
by animals
Photosynthesis by
phytoplankton
Phytoplankton
cells (organic
carbon)
Zooplankon cells
(organic carbon)
Higher trophic
levels:
fish, cephalopods,
cetaceans*, birds*
Dissolved
(organic
carbon)
Microbial cells
(organic carbon)
Death
Death
Consumption
Consumption
Consumption
Figure 7. Importance of micro-organisms (phytoplankton
and non-photosynthetic zooplankton and microbial cells)
and of larger animals in the marine carbon cycle. The
thickness of the lines indicates relative carbon flow
through the pathway. *Cetaceans (whales and dolphins)
and birds release CO2 to the atmosphere, not to water,
but this is a minor pathway. The cycle assumes no net
input of carbon or loss from the oceans. A similar diagram
showing CO2 flow patterns could also be produced for
benthic primary producers, bringing in, for example
echinoderms (such as starfish and sea urchins) among the
larger animals, but with a similar major role for micro-
organisms.
Consumption

through changes in other resources on which they
depend. In many cases there are few data, so for example
we are generally unable to say whether organisms will be
able to adapt to the changing environment in the short
term, or evolve in the long term.
3.2 Effects of ocean acidification on
photosynthetic and non-photosynthetic
micro-organisms
This section looks at the some of the possible major
effects of increased CO2 and the associated change in
ocean pH on micro-organisms. As we discussed in Section
2.7, calcifying organisms could be particularly affected by
changes in ocean pH. We therefore look at calcification in
detail in Section 3.5. The discussion below considers the
other possible effects, aside from calcification, from
changes to ocean chemistry on marine organisms.
On land, plants take up CO2 primarily by diffusion,
although some have mechanisms that actively take up
CO2. As a result, increasing atmospheric CO2 generally
has a positive effect on photosynthesis, productivity and
growth (Ainsworth & Long 2005). In contrast, most
marine phytoplankton have concentrating mechanisms
that actively take up inorganic carbon; either as CO2 or
bicarbonate ions (HCO3
–) or both. Because they actively
concentrate CO2, changes in the pH and CO2 content of
the water have less effect on their photosynthesis
(Giordano et al 2005). Carbon-concentrating
mechanisms are thought to be less widespread in benthic
photosynthetic organisms (Giordano et al 2005).
Variation in the availability of nutrients caused by
increasing CO2 concentration and pH change may also
have an indirect effect on the growth of photosynthetic
organisms. In particular, changes in pH, as discussed in
Section 2.7.2, will affect the availability of nutrients. This
may alter rates of growth and photosynthesis. Changes
may also occur in the cell composition, which could affect
the nutritional value of the micro-organisms to the
animals that feed on them.
3.2.1 Effects on phytoplankton:
photosynthetic organisms
Photosynthesis
As we underline throughout the report, a reduction in
ocean pH (acidification), is one of a number of changes to
the oceans that results from increased atmospheric CO2
levels. It is impossible to differentiate unequivocally
between the effects of increased CO2 and those of
decreased pH in experiments on marine organisms, since
there is significant co-variance of these environmental
factors (see Annex 1). In almost all of the phytoplankton
species examined to date, it has been found that doubling
the present atmospheric CO2 concentration has only a
small direct effect, 10% or less, on the rate of
photosynthesis (Beardall & Raven 2004; Schippers et al
2004; Giordano et al 2005; Figure 8). This conclusion
comes from experiments performed on several species to
examine their photosynthetic response to altered
atmospheric CO2 concentrations. In some experiments,
algae were grown at present-day concentrations, and
then exposed to varying CO2 levels for measurement of
their photosynthetic rate. In other experiments, species
have also been grown for several generations at altered
CO2 concentrations before having their photosynthetic
rates measured (Burkhardt et al 2001; Rost et al 2003;
Beardall & Raven 2004). Generally, both types of
experiment gave similar results, namely relatively small
increases in photosynthesis. This lack of response is
primarily due to the presence of carbon-concentrating
mechanisms, which mean that even at today’s CO2
concentration photosynthesis is saturated with inorganic
carbon.
Ocean acidification due to increasing atmospheric carbon dioxide16 | June 2005 | The Royal Society
Table 2.Groups of calcified marine organisms with indications of their ability to photosynthesise, the form of CaCO3
deposited and their habitat. Aragonite dissolves more rapidly at low carbonate concentrations than calcite.
Photosynthesis in foraminifera and warm-water corals depends on symbiosis with algae (Lowenstam (1981), Berner &
Berner (1996) and references in Section 3.5).
Organisms Photosynthetic or Form of calcium Habitat
non-photosynthetic carbonate
Foraminifera Some photosynthetic Calcite Benthic
Non-photosynthetic Calcite Planktonic
Coccolithophores Photosynthetic Calcite Planktonic
Macroalgae* Photosynthetic Aragonite or calcite Benthic
Corals: warm water Photosynthetic Aragonite Benthic
cold water Non-photosynthetic Aragonite Benthic
Pteropod molluscs Non-photosynthetic Aragonite Benthic
Non-pteropod molluscs* Non-photosynthetic Aragonite + calcite Benthic or Planktonic
Crustaceans* Non-photosynthetic Calcite Benthic or Planktonic
Echinoderms Non-photosynthetic Calcite Benthic
*Not all members of the group are calcified.

An important exception to this has been found in the
case of the globally very abundant calcifying
phytoplankton Emiliania huxleyi (Figure 8). This organism
exhibits a greater increase in photosynthetic rate in
response to elevated CO2. It might be expected to
benefit from elevated CO2 as a result of this
photosynthetic response. However, it is also observed
to decrease calcification at elevated CO2, which might
offset any beneficial effect due to increased
photosynthesis in terms of the growth and
competitiveness of the organism (Section 3.5). A
complete inhibition of photosynthesis (and growth) was
found in two species of marine planktonic dinoflagellate
24 hours after exposure to high CO2 (Dason & Colman
2004), although the CO2 increase and corresponding pH
decrease in those experiments was much greater than
those considered likely by this report.
The evidence for the general lack of effect on
photosynthesis due to changes in inorganic carbon
concentrations resulting from increased atmospheric CO2
has been derived mainly from laboratory studies. Many
laboratory experiments have involved artificially altering
the total inorganic carbon concentrations, the pH value,
or both. As a result, the ratios of CO2 and HCO3
– do not
mimic the change that would be found if atmospheric
CO2 was increased (Giordano et al 2005). Some
experiments that change the gas phase of CO2, to mimic
the situation in the real world, have been carried out in
the laboratory and in mesocosms. The results are similar
to those from the other types of manipulation of the
inorganic carbon system (Burkhardt et al 2001; Rost et al
2003).
Most of the experiments conducted on marine
phytoplankton have been short term and did not
provide sufficient time for any genetic modifications
that might allow them to adapt to the change in
conditions. Further long-term experiments are required to
determine if the organisms will evolve to take advantage
of the increased CO2. This is discussed further in Section
3.6.
Growth and composition
Experiments carried out so far show that increased (even
double) atmospheric CO2 concentrations have relatively
little direct effect on the growth rate and the elemental
composition of marine phytoplankton (Burkhardt et al
1999; Gervais & Riebesell 2001). There is also the
possibility of indirect effects through changes in nutrient
availability and in the toxic substances, resulting from the
decreased pH (Section 2.7.2).
In recent mesocosm studies (Riebesell personal
communication) there was an increase in inorganic
carbon assimilation, and also in the ratios of carbon to
nitrogen and carbon to phosphorus in phytoplankton, at
750 ppm CO2 compared with the present day (380 ppm)
CO2 levels. Other studies showed little effect of such
changes in CO2 on the productivity of a natural
assemblage of phytoplankton species (Hein & Sand-
Jensen 1997; Tortell et al 2002).
The effect that increasing CO2 has on growth rate and
cell composition needs to be superimposed on the
availability of other resources. Some of these resources can
be limiting: for example, nitrogen for much of the world’s
oceans; and for parts of the oceans or at particular times,
iron and phosphorus. Climate change may alter this
availability, with rising temperatures tending to increase
stratification and reduce mixing with nutrient-enriched
deeper waters. In areas when there is deep mixing (tens of
metres or more) of the surface oceans, light can become a
limiting factor for phytoplankton growth. It is known that,
measured over short periods, the dependence of
photosynthesis on the CO2 concentration during growth is
affected by the availability of these resources (Giordano et
al 2005).
Many of the data on growth and composition have come
from laboratory experiments. However, these
experiments have not always been performed under the
nutrient-limiting conditions that may occur in the oceans.
Further experiments are needed to show the effects of
total nutrient concentrations and ratios of nutrient
species, such as nitrogen, phosphate, iron and (for
diatoms) silicic acid.
Ocean acidification due to increasing atmospheric carbon dioxide The Royal Society | June 2005 | 17
Figure 8. Photosynthesis of phytoplankton species differ
in sensitivity to CO2 concentrations. Although most
species (here Skeletonema costatum and Phaeocystis
globosa) reach their maximum photosynthetic rate under
present-day ambient CO2 levels (14.7 µmol per litre),
some species, such as the coccolithophore Emiliania
huxleyi show increased rates of photosynthesis when CO2
is increased above present levels. This raises the possibility
that coccolithophores may benefit directly from the
current increase in atmospheric CO2. The range in
photosynthetic rates for each organism reflects the extent
of which the physiology of the organism changes as a
function of the concentration of CO2 at which they are
grown. (Reprinted from Rost & Riebesell (2004) with
permission of Springer Science and Business Media).
S. costatum
P. globosa
E. huxleyi
100
75
50
25
0
Re
la
tiv
e
ph
ot
os
yn
th
es
is
(%
)
0 10 20 30 40 50 60 70 80
Present atmospheric
concentration
CO2 (µmol L
–1)

There are insufficient data to draw any conclusions about
the impacts that increasing CO2 might have on the
growth and composition of marine phytoplankton. In
addition, experiments so far have not been long enough
to indicate if existing species will be able to evolve to
changing conditions. However, any reduction in the total
biomass production by phytoplankton, either through
reduced photosynthesis or from greater energy demand
to obtain nutrients, could have significant implications
for marine food webs. In addition, these effects of
increasing CO2 concentrations need to be considered in
relation to other environmental changes. Further
experiments are needed on the effect of changes in pH
on phytoplankton nutrition for nutrients supplied as
solutes whose extent of ionisation varies with pH in the
relevant range of pH values; such experiments should use
both growth-limiting and growth-saturating
concentrations of the nutrients.
Extracellular polysaccharides
For phytoplankton, a significant component of the
organic carbon produced by photosynthesis is dissolved
extracellular organic carbon, of which much is
polysaccharide. Production of extracellular dissolved
organic carbon is a rather variable fraction of
photosynthesis, with 3–40% found for different natural
marine planktonic systems. This provides an important
food source for non-photosynthetic micro-organisms in
the surface oceans. The proportion of the total organic
carbon produced by cells that appears as extracellular
polysaccharides is increased by higher CO2
concentrations, but additional photosynthesis (for
example in E. huxleyi: Figure 8) with increased CO2 could
increase the rate of extracellular organic carbon
production (Engel 2002; Engel et al 2004, 2005).
These polysaccharides are also important in the
aggregation and then sedimentation of phytoplankton
cells and other particles (Engel et al 2004). This means
that increased CO2 could potentially increase particle
sedimentation in the ‘biological pump’ (Section 2.2.2).
However, this needs to be balanced against reductions in
calcification by coccolithophores caused by increased CO2
levels (Riebesell et al 2000; Section 2.7.1), reducing the
input of mineral ballast and hence decreasing the rate of
sinking of the aggregates (Section 5.2).
Further work is needed to understand how increases in
CO2 affect organic carbon sedimentation, and the
consequences for feedback effects on surface CO2
concentrations and on the absorption of CO2 from the
atmosphere, and hence to the global carbon cycle.
3.2.2 Effects on non-photosynthetic
micro-organisms
Non-photosynthetic micro-organisms in the oceans
include archaea, bacteria, fungi and a range of protists
including flagellates and ciliates. They feed on organic
particles, dissolved organic matter and inorganic
nutrients, producing CO2 and other gases such as halo-
carbons (Section 5). These organisms play major roles in
the marine carbon cycle, because they are the main
means by which primary productivity in the oceans is
recycled to CO2 (del Giorgio & Williams 2005; Figure 7).
They are abundant in the open oceans as well as in muddy
and sandy benthic habitats (Section 3.3). Although their
physiology suggests that they will respond to increased
CO2 in the surface oceans and to decreased pH, there are
few data available to demonstrate any responses and
therefore impact that this might have on the functioning
of the organisms in their natural environment.
Like photosynthetic micro-organisms, non-
photosynthetic micro-organisms are affected by changes
in the concentration and speciation of nutrients as a result
of changes in pH (Section 2.7.2). Seawater pH also affects
the mechanisms that transport these solutes into the
cells. Apart from depending on photosynthetic micro-
organisms for food, the non-photosynthetic micro-
organisms that grow in the illuminated surface oceans are
in direct competition with them for inorganic nutrients,
but not for CO2. Any changes in this nutrient supply may
alter the productivity of either group.
Compared with marine photosynthetic phytoplankton,
non-photosynthetic micro-organisms have a greater
metabolic diversity. One example of a metabolic process is
nitrification (conversion of ammonia (NH3) and
ammonium (NH4
+) to nitrate), which occurs mainly in the
non-illuminated part of the oceans. The organisms
responsible for this part of nitrification can only use
ammonia as their substrate. In experiments simulating the
pH decrease likely to occur in very high CO2
concentrations, Heusemann et al (2002) found a drastic
reduction in the rate of nitrification. If this concentration
was reached in surface waters it could lead to
accumulation of ammonium instead of nitrate. However,
these experiments were done at significantly greater
concentrations of CO2 than is likely to occur as a result of
human-produced emissions of CO2 to the atmosphere.
Micro-organisms and dead organic particles provide a
valuable food supply for benthic invertebrate detrital and
suspension feeders, again producing CO2 and inorganic
nutrients. The micro-organisms living deeper in muddy
and sandy sediments are less exposed to the direct
influence of increases in surface ocean CO2 than are those
living nearer the sediment surface or in the plankton
(Section 4.2.3).
3.3 Effects of ocean acidification on
photosynthesis in benthic organisms
The dominant primary producers growing on the sea bed,
in areas where the oceans are shallow enough to allow
photosynthesis to occur, are large algae (seaweeds),
marine flowering plants (seagrasses) and, in some
Ocean acidification due to increasing atmospheric carbon dioxide18 | June 2005 | The Royal Society

but significant loss (40%) of sperm motility of Pacific
oysters (Crassostrea gigas) has been shown in much
larger changes in pH from 8 to 6 units (Dong et al 2002);
however, this is beyond any expected changes in the
oceans due to increases in atmospheric CO2
concentration up to the year 2300. Experiments have also
demonstrated significant decreases in egg production by
copepods, but no effect on survival of adults, over the 8
days tested using CO2 values well in excess of what is
expected in future oceans (Kurihara et al 2004a, 2004b).
In another experiment, lower pH decreased hatching of
the egg sacs of a gastropod mollusc (Babylonia areolata)
farmed in the Far East (Luo et al 2004). Other studies
using very high concentrations of CO2, beyond what is
expected in the future oceans, have also shown effects on
the embryos and larvae of silver sea bream (Pagrus major)
(Ishimatsu et al 2004) and on the reproductive success of
sea urchins (Kurihara et al 2004a). These experiments
suggest that the early stages of the life cycle may show
the greatest sensitivities to increased CO2 concentrations,
with potentially widespread implications for population
size, community structure and biodiversity. However, as
with effects on other life-cycle stages, the CO2
concentrations used in these were greater than those
considered likely in the future oceans. There is an urgent
requirement for experiments addressing the effects of
smaller increases in CO2 on the reproduction of marine
animals.
3.5 Effects of ocean acidification on calcifying
organisms
3.5.1 Introduction
The formation of shells or plates of CaCO3, by
calcification, is a widespread phenomenon among marine
organisms, such as most molluscs, corals, echinoderms,
foraminifera and calcareous algae. Although it is not
always clear what function this calcification has, it seems
integral to their biology; so any decrease in calcification,
as a result of increased CO2, is therefore likely to have
significant consequences such as the weakening of coral
skeletons and reef structures generally.
Calcification is a tightly controlled process and generally
requires biological initiation even in the present day
surface seawater which is super-saturated with respect to
CaCO3. However, the mechanism of calcification is
currently poorly understood. It can be intracellular, in
specific calcifying compartments, or on external surfaces,
which suggests that large differences exist in the
mechanisms and transport routes leading up to the
precipitation reaction. The various groups of calcifying
organisms also differ in the crystal structure and chemical
composition of their carbonate skeletons (Section 2.7.1;
Table 2). Whereas corals and a group of molluscs called
pteropods precipitate aragonite, coccolithophores
(calcifying phytoplankton) and foraminifera (protist
plankton) produce the less soluble calcite, generally in
internal compartments. Mollusc shells consist of layers of
either all aragonite or inter-layered aragonite and calcite.
Echinoderms, which include sea urchins, sea stars and
brittle stars, form calcite structures that are high in
magnesium and have a very high porosity often exceeding
50%. Calcareous benthic algae precipitate either high-
magnesium calcite or aragonite and perform the
important function of ‘gluing’ the skeletons of corals
together to create reefs. Despite this variation the principal
steps of calcification are likely to be similar in each case.
To make these calcareous structures, seawater has to be
supersaturated with calcium (Ca2+) and carbonate (CO3
2–)
ions to ensure that once formed the CaCO3 does not
dissolve. Lower pH reduces the carbonate saturation of
the seawater, making calcification harder and also
weakening any structures that have been formed.
Although calcite is less soluble than aragonite, making it
less susceptible to pH changes, the incorporation of
magnesium into either form increases their solubility.
3.5.2 Calcified protists and algae
Foraminifera are important in global ocean calcification in
both the plankton and in bottom-dwelling environments.
They are thought to form high-magnesium calcite during
the initial stages of calcification, but without going though
a step involving an amorphous precursor of high solubility
(Erez 2003). This is in contrast with the echinoderms
considered below (Section 3.5.3). The encapsulation of
seawater has been proposed as the starting point for
calcification for some calcifying organisms such as
foraminifera (Erez 2003). During the calcification process
the alkalinity of the captured water is lowered (the pH
decreases). To compensate for this the organism is
thought to actively raise the pH, which immediately
increases the carbonate ion (CO3
2–) concentration. If the
pH of the ambient water is already decreased, this process
will therefore require more energy, which may explain the
decrease in calcification with decreasing pH. Allocating
more energy to calcification to counteract the effect of
decreasing pH could mean that less is available for other
vital metabolic processes, which may reduce the growth
and fitness of the organism.
In laboratory experiments on the symbiont-bearing
foraminiferans, the shell weights of Orbulina universa and
Globigerinoides sacculifer varied with the carbonate ion
concentration of the seawater (Bijma et al 1999, 2002).
Similar results were obtained for the foraminifera
Amphistegina lobifera and A. hemprichii (Erez 2003). A
strong reduction in the calcification rate occurred as pH
decreased from 9 to 7. The trend reverses above pH 9,
with calcification rate decreasing as pH increases. A
positive correlation between foraminiferal shell weight
and the carbonate ion concentration (and the carbonate
saturation state) is also observed in the
palaeoceanographic record as a response to known
glacial–interglacial changes in atmospheric CO2 of the
past 50000 years (Barker & Elderfield 2002). Plotting
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calcification rate versus the carbonate ion concentration
reveals a similar slope, with a two- to threefold increase in
calcification rate with CO3
2– increasing from 100 to
400 µM (Erez 2003).
Coccolithophores are planktonic algae whose
photosynthesis, in the species investigated, is not
saturated at present levels of CO2 (Section 3.2.1). The
coccolithophores are considered to be the most
productive calcifying organisms on Earth. They are
covered with one to several layers of minute calcite
platelets, the coccoliths. The coccoliths are produced
inside the coccolithophore cells, and do not involve the
internalisation of seawater (Berry et al 2002). Benthic
macroalgae deposit either calcite or aragonite
externally.
3.5.3 Calcified larger animals
Corals, crustaceans, echinoderms and molluscs are
important calcified benthic animals, and calcified
crustaceans and molluscs are significant members of
many pelagic marine communities. As we show in
Sections 3.5.4 and 4.2.1, the mechanical function of
calcification is obvious in these calcified animals.
Echinoderms are among several groups that may be
especially sensitive to ocean acidification. The larvae of
sea urchins and also of molluscs have been shown to form
skeletal parts consisting of magnesium-bearing calcite
that form through an amorphous precursor phase, which
is 30 times more soluble than calcite without magnesium
(Politi et al 2004). The same type of skeletal material is
used by most adult echinoderms. Lower pH in the oceans
may inhibit the formation of these highly soluble
amorphous calcite precursors, which echinoderms need
in the construction of their shells. Further work is needed
on the mechanism of calcification in echinoderms, in view
of their importance in many coastal ecosystems: in some
cases they are keystone predators (sea stars) or very
important grazers (urchins). This is also true of coastal
molluscs and arthropods, which in addition are also of
great economic importance. Crustacea may be a
particularly vulnerable group because of their
dependence on the availability of calcium and
bicarbonate ions for the mineralization of their
exoskeleton after moulting. In freshwater species this
mineralization the animals’ ability to control blood pH is
affected by changes in water composition in intensive
culture, particularly in recirculation systems (Brown et al
1991). Similar effects may occur in marine species,
although further work is needed to establish the
significance of effects within the range of seawater CO2
concentrations considered in this report.
3.5.4 Functions of calcification and effects of
decreased calcification
Decreased calcification may equally influence the
organism’s competitive fitness, but this ultimately
depends on the functional role of the calcareous
structures or the metabolic function of the calcification
process. The function of calcareous structures is obvious
in some groups of organisms, such as for the structural
frameworks of corals or the exoskeletons serving as
mechanical protection in pteropods and foraminifera.
Reduced calcification in corals can translate into declining
coral cover and reduced, possibly disappearing, reef
frameworks (Gattuso et al 1999; Kleypas et al 2001). A
decrease in carbonate supersaturation may also weaken
carbonate skeletal structures, making them more
susceptible to erosion (both physical and biological) and
dissolution. This effect, which will be most pronounced in
aragonitic groups such as the pteropods, would
presumably compromise the fitness of these organisms
and could shift the ecological and evolutionary
competitive advantage to non-calcifying organisms.
The function of calcification is less clear in other groups
such as the coralline algae and coccolithophores. A
variety of protective, metabolic and adaptive functions
have been proposed for calcification in coccolithophores
(Brownlee & Taylor 2004), but the evidence remains for
the most part inconclusive.
In addition to structural roles for calcification, calcium
carbonate deposits can also influence the acid-base
balance in anoxia (absence of oxygen) (Booth et al 1984;
Lindinger et al 1984). Anoxia occurs, counter-intuitively,
when intertidal organisms are exposed to air at low tide
and seal themselves off from exposure to the air in limiting
water loss at the expense of preventing entry of oxygen.
As calcium carbonate structures make organisms denser,
increased sinking rate is therefore an unavoidable
consequence of calcification in planktonic organisms that
are unable to swim. This includes the most abundant, non-
flagellated (flagellates are used in propulsion) stages of
coccolithophores, and planktonic foraminifera, because
calcite is denser than other components of the organisms.
This argument also applies to the aragonite-depositing
pteropods after they have died and can no longer swim. It
is not clear what competitive advantage this increased
sinking caused by calcification might offer and therefore it
is difficult to conclude how it might have evolved (Raven
and Waite 2004). However, regardless of the evolutionary
basis for calcification, a reduction in calcification decreases
the amount of ballast available for the sedimentation of
organic matter and CaCO3, which has implications for the
operation of the biological pump (Sections 2.2.2 and 5.2).
Since the functions of calcification are not fully
understood, the long-term implications of reduced
calcification on the physiological and ecological fitness of
calcifying organisms are unknown.
3.5.5 Influence of increased CO2 on calcification
Because of the dissimilarities in the calcification process,
it is surprising to find a rather narrow range of
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in other phytoplankton organisms were observed at
increased CO2 and temperature in ship-borne microcosm
incubations carried out in the Bering Sea (Hutchins
personal communication). These contradictory results
indicate that the impact of rising CO2 on phytoplankton
species composition may be community- or regime
specific and is hence likely to differ seasonally and
geographically.
For many ecosystems the impact that such changes in
species composition would have on their productivity and
the services that they provide is unclear. It is possible that
different species may dominate and fill the gap in the
food web left by those that are adversely affected by
ocean acidification.
3.8 Conclusions
Effects of ocean acidification on micro-
organisms
The evidence considered in Section 3.2 suggests that the
increase of CO2 in the surface oceansexpected by 2100 is
unlikely to have any significant direct effect on
photosynthesis or growth of most micro-organisms in the
oceans. Changes in elemental composition of the cells
also appear to be small and variable. More work is needed
before these negligible, or small, effects of increased CO2
can be generalised and applied with confidence to the
oceans as an indication of future trends. Moreover, much
of the work that has been done relates to the
acclimatisation of cells to increased CO2 and has not been
conducted for long enough to determine whether the
organisms will be able to evolve, through natural
selection, to the changed environment.
Changes in CO2 concentrations and pH will lead to
changes in nutrient concentrations and speciation.
Insufficient data is currently available to identify the
indirect impacts these changes might have.
A substantial increase in information is required if we are
to arrive at widely applicable conclusions on the effect of
increased surface ocean CO2 on the functioning of non-
photosynthetic micro-organisms in the oceans.
Effects of ocean acidification on multicellular
animals
In the short term (20–40 years), projected increases in
atmospheric CO2 will produce minor impacts on
multicellular marine animals. Climate change scenarios
that involve three- to fourfold increases in atmospheric
CO2 are more likely to stress their physiology and may
reduce their growth. Increased CO2 in the seawater may
also increase mortality, particularly for organisms with
high metabolic rates such as squid. Reproduction of
multicellular animals is strongly influenced by CO2 at the
very high concentrations tested. However, much more
work is needed to establish the effects of the changes in
surface ocean CO2 concentrations expected over the
next century. In particular, we need to examine the
effects of increased CO2 on the complete life cycle of
multicellular animals to determine the validity of the
suggestion that the gametes and juvenile (larval) stages
are more sensitive to high CO2 and low pH than is the
adult stage.
The influence of ocean acidification on marine organisms
other than those exerted through calcification, could
include decreased reproductive potential, slower growth
or increased susceptibility to disease. These responses
could have cascading effects through food webs, with
possible consequences for ecosystem structure and
elemental cycling.
Effects of ocean acidification on calcification
From our understanding of ocean chemistry and
available evidence, it is clear that increasing the acidity
of the oceans will reduce the concentration and
therefore the availability of carbonate ions. It is expected
that calcifying organisms will find it more difficult to
produce and maintain their shells and hard structures.
However, the lack of a clear understanding of the
mechanisms of calcification and its metabolic or
structural function means that it is difficult, at present,
to reliably predict the full consequences of CO2-induced
ocean acidification on the physiological and ecological
fitness of calcifying organisms. What is clear is that any
decrease in calcification, as a result of increased CO2, is
likely to have significant consequences such as the
weakening of coral skeletons and reef structures
generally (Section 4.2). The widespread use of
calcification in the oceans suggests that it is an essential
aspect of the biology and ecology of many calcifying
organisms. As outlined in Section 2, any changes in
calcification will also have important implications for the
global carbon cycle.
Organisms will continue to live in the oceans wherever
nutrients and light are available, even under conditions
arising from ocean acidification. However, from the data
available, it is not known if organisms at the various levels
in the food web will be able to adapt or if one species will
replace another. It is also not possible to predict what
impacts this will have on the community structure and
ultimately if it will affect the services that the ecosystems
provide. Without significant action to reduce CO2
emissions into the atmosphere, this may mean that there
will be no place in the future oceans for many of the
species and ecosystems that we know today. This is
especially likely for some calcifying organisms.
It is unclear how direct effects of increasing CO2 interact
with those of other environmental changes occurring
simultaneously, such as those of climate and nutrient
supply, the extent of oxygenation of sediments, surface
ocean mixing and stratification. Independent of these
Ocean acidification due to increasing atmospheric carbon dioxide The Royal Society | June 2005 | 23

uncertainties it is possible to say with some confidence
that that various groups of marine organisms will
experience significant alterations in their geographical
ranges (Boyd & Doney 2003).
Having evaluated the possible effects of ocean
acidification on species and communities of organisms, in
the following section we look at the wider picture and
consider which marine ecosystems may be most
vulnerable to the projected changes in ocean chemistry.
Ocean acidification due to increasing atmospheric carbon dioxide24 | June 2005 | The Royal Society

4.1 Introduction
Section 2 shows that ocean chemistry is being, and will
increasingly be, altered by rising atmospheric CO2 levels.
As the CO2 concentration in the surface oceans increases,
this raises their acidity (reduces their pH), and importantly
for organisms that calcify, lowers the carbonate
saturation status of the surface oceans. The extent and
rate of change to ocean chemistry that is predicted to
occur by 2100 has no precedent for at least hundreds of
thousands of years and possibly much longer. Section 3
demonstrates that, while the effects of these changes in
ocean chemistry on marine organisms are not well
understood, significant effects are expected, especially
for some calcifying organisms. Based on the conclusions
in Section 3, here we examine the likely impacts of these
changes in ocean chemistry on the ecosystems that are
most likely to be at risk.
4.2 Impact of ocean acidification on benthic
systems
4.2.1 Coral reefs
Photosynthetic coral reefs require calcified structures to
survive. Considerations of changes in ocean chemistry in
the tropics and subtropics suggest that these systems are
liable to significantly decline as a result of surface ocean
acidification.
Coral reefs cover an estimated 1.28 million square
kilometres of the world’s tropical and subtropical oceans
(Spalding et al 2001). They occur primarily in well-lit and
shallow waters, characterised by low water-column
turbidity (Spalding et al 2001). In contrast to cold-water
corals, warm-water reef-building corals are symbiotic with
single-celled dinoflagellate algae (Table 2). High rates of
calcification allow the building of a scaffolding or
framework, which becomes home to hundreds of
thousands of other species that dwell in coral reefs (Reaka-
Kudla 1996). Without the presence and activities of reef-
building corals, coral reefs would be vastly different from
the diverse and productive ecosystems that typify shallow
tropical and subtropical oceans today. Despite representing
less than 1.2% of the world’s continental shelf area, coral
reefs are critical resources to people and industries in many
countries (Spalding et al 2001). The socio-economic value
of coral reefs is considered in Section 6.
Warm-water coral reefs generally exist within a narrow
environmental envelope defined largely by sea
temperature, light and the aragonite saturation state
(Kleypas et al 1999a). The growth of coral reefs is minimal
outside this envelope. Climate change, through its effect
on ocean temperature and aragonite saturation state
(Section 2.7.1), has already had an impact on the world’s
coral reefs (Hoegh-Guldberg 1999). Almost 30% of
warm-water corals have disappeared since the beginning
of the 1980s, a change largely due to increasingly
frequent and intense periods of warm sea temperatures.
High ocean temperatures cause corals to bleach (ie lose
their pigmented dinoflagellate symbionts and, if
conditions are warm enough for long enough, die). The
increase in atmospheric CO2 is compounding this effect,
by lowering the aragonite saturation state of seawater,
making carbonate ions less available for calcification
(Section 2.7). Gattuso et al (1999) and Kleypas et al
(1999b) have proposed that the calcification rates of
corals would decrease by 10–30% under a doubling of
atmospheric CO2 concentrations. Ample evidence now
indicates that coral calcification is directly proportional to
the aragonite saturation (Gattuso et al 1999; Marubini &
Atkinson 1999; Langdon et al 2000, 2003; Leclercq et al
2000, 2002; Marubini et al 2001, 2002). These studies,
done in laboratory or mesocosm settings, reveal that
doubling atmospheric CO2 causes a 3–54% decrease in
carbonate production.
Some studies have suggested that the decrease due to
declining concentrations of carbonate ions will be more
than offset by increases in calcification due to the increase
in ocean temperature (Lough & Barnes 2000; McNeil et al
2004). These assessments are based on the assumption
that calcification increases in direct proportion to
temperature, despite the fact that all studies so far have
shown that coral calcification reaches a maximum at or
just below current-day temperatures. For this and other
reasons, these theoretical counter-perspectives have been
regarded as ‘seriously flawed’ (Kleypas et al 2005). Other
suggestions that coral reefs may expand significantly to
higher latitudes as seas warm have been countered by the
observation that carbonate ion concentrations decrease
towards the poles and will reach even lower values at high
latitudes than is the case now as atmospheric CO2
increases (Kleypas et al 2001). This, and the fact that light
for photosynthesis also decreases at higher latitudes,
suggests that the latitudinal limit for coral reefs will
remain within a few hundred kilometres of where it is
today.
The synergistic interaction of elevated sea temperature
and atmospheric CO2 is likely to produce major changes
to coral reefs over the next few decades and centuries.
Under most IPCC emission scenarios (IPCC 2000), corals
are unlikely to remain abundant on reefs and could be
rare on tropical and subtropical reefs by the middle of this
century if CO2 doubles or triples above present levels.
Over longer timescales, reef frameworks that are critical
for the protection of coastlines across tropical and
subtropical regions may start to disappear as the rate of
erosion starts to exceed calcification rates (Burke 2004).
Ocean acidification due to increasing atmospheric carbon dioxide The Royal Society | June 2005 | 25
4 Ecosystems most at risk from the projected changes in
ocean chemistry

groups. By far the most common cold-water coral is the
cosmopolitan stony coral Lophelia pertusa, a habitat-
forming, reef-building species which occurs in dense
girdles stretching from the southwestern Barents Sea
along the eastern Atlantic continental margin down to
West Africa. Evidence is emerging of a similar belt along
the western margin of the Atlantic off Nova Scotia to the
Florida Straits and into the Gulf of Mexico. Other groups
with global distribution include the genera Madrepora
and Solenosmilia (Figure 9). Cold-water reefs and
structures harbour a distinct and rich ecosystem,
providing niches and nursery grounds for a variety of
species, including commercial fish species.
So far, there are no studies on the sensitivity of cold-water
corals to CO2-related changes in seawater chemistry.
However, because of the comparatively uniform response
of all calcifying organisms tested so far, it is reasonable to
expect that calcification of cold-water corals will also be
reduced as carbonate supersaturation declines. Because
the carbonate saturation state generally decreases with
latitude and water depth, the conditions in waters
typically inhabited by cold-water corals are less favourable
for calcification to start with. This may cause cold-water
corals to be affected earlier and more strongly by CO2-
related ocean acidification than their warm-water
counterparts. Model calculations, based on a situation
where CO2 emissions continue on current trends, indicate
that the aragonite saturation horizon (the depth below
which aragonite dissolves) will rise closer to the oceans’
surface) by several hundred metres for most of the ocean,
and may reach the surface in the Southern Ocean by
2100 (Caldeira & Wickett in press). Thus, not only is the
ocean-wide decrease in the aragonite saturation level
likely to decrease cold-water coral calcification, large
areas of the oceans may also become completely
uninhabitable for cold-water corals as the impacts of
increased CO2 are greater at higher latitudes.
If CO2 emissions continue on current trends, ocean
acidification will threaten the existence of cold-water
corals before we have even started to understand and
appreciate their biological richness and importance for
the marine ecosystem.
4.2.3 Shallow sediments and benthic organisms
Sediment-dominated habitats, which occupy a large
fraction of the area of the oceans, play a crucial role in
several key ecosystem functions and processes in shelf sea
environments. For example, in shallow (less than 50 m)
coastal areas productivity in the overlying water column
relies on the sediment system, with up to 80% of the
nitrogen required by phytoplankton coming from the
bacterial regeneration of organic matter within the
seabed (Dale & Prego 2002). Phytoplankton are the main
marine primary producers upon which the whole marine
food chain depends, so the supply of nutrients for their
growth from the sediment is essential for sea shelf
productivity. This benthic–pelagic (bottom dwelling –
surface living) coupling is two way, with the sediment
receiving the ‘fall-out’ of organic matter from the
overlying primary production, by the biological pump
(Section 2.2.3). This material acts as the primary food
resource for benthic organisms, which in turn support a
benthic food web with demersal fish (such as cod and
plaice) being at the top. Hence both bottom dwelling and
pelagic fisheries in shelf seas are dependent on a healthy
sediment ecosystem and on benthic–pelagic coupling.
Changes to the biological pump, and therefore the supply
of food to the sediment-dwelling organisms, will affect
the benthic food web and the benthic–pelagic coupling.
Ocean acidification has many potential implications for
benthic organisms and their ecosystems. This section
focuses on benthic systems, particularly those concerning
animals living in soft mud or on the sandy floor. This
excludes temperate and tropical reefs (considered
benthic) which are considered above (Section 4.2); and
muddy or sandy bottoms occupied by seagrass beds,
which have been mentioned in Section 3.3.
Sediments in unmixed settings are characterised by
strong geochemical gradients. For example, the pH at
depths of 30 cm or so can be as much as one unit lower
than the pH of the sediment surface and overlying water
(Fenchel & Riedl 1970). In the face of such geochemical
variability it is difficult to conceive how benthic sediment
systems could be affected by the relatively small pH
changes expected from surface ocean acidification. A
number of processes in the surface layers of sediments
(typified by pH values that are closer to overlying
seawater) may be affected by changes expected under
ocean acidification. By considering some of the potential
changes to the distribution and activities of sediment-
dwelling organisms, it is possible to identify areas where
these small changes could have potentially large
consequences for sediment communities and
biogeochemical processes.
The communities of benthic sediments are strongly
stratified, with different species characteristically
occupying different depths (Barnes & Hughes 1988). The
surface layer is the most densely inhabited and is home to
most of the animals, whereas only those species capable
of oxygenating their immediate environment are able to
dwell below the depth where the rate at which oxygen
diffusion from overlying water is inadequate to supply the
respiratory requirements of organisms (Furukawa 2001).
Microbial communities and the functions they perform
(for example nitrification and denitrification) also differ
considerably in their ability to tolerate oxygen and hence
in where they are situated between the surface sediments
and the deeper layers devoid of oxygen. The presence of
these depth-constrained niches means that although
benthic systems as a whole are already subject to a
relatively large range in pH, many of the organisms and
processes that exist within them are not. Although most
larger animal species are restricted to the upper oxic
(oxygen-containing) layers, others are able to inhabit the
Ocean acidification due to increasing atmospheric carbon dioxide The Royal Society | June 2005 | 27

deeper sediment layers. They do so by the creation of
permanent burrows through which they are able to draw
down oxygenated water from above. The burrows
created by these larger animals experience significant
oscillation in pH (as much as two pH units) and dissolved
oxygen concentration (between saturation and near
anoxia (absence of oxygen)), generated by the periodic
ventilation of burrows (Furukawa 2001). Such oscillation
is absent at the water–sediment interface. Consequently,
animals that inhabit permanent burrows may have a
greater tolerance to changes in pH than those that do not
build burrows. This potential difference in pH tolerance
between benthic species could lead to the selection of
more tolerant species and thereby substantial changes in
the structure and function of sediment communities in
the face of changing levels of pH. Recent work has
already identified significant variability in pH sensitivity for
several different benthic groups (Shirayama et al 2004).
Even among organisms that depend on CaCO3 structures,
variability in tolerance has been observed with
echinoderms showing less tolerance to pH change than
molluscs (Shirayama et al 2004). Such variability in
sensitivity could have considerable implications for the
diversity and functioning of sediment communities in
shallow seas with declining pH.
It has long been recognised that organisms living within
the sediment have the ability to alter the physical and
chemical characteristics of their immediate environment,
contributing significantly to the rate of nutrient fluxes at
the sediment–water interface (Banta et al 1995; Nedwell
& Walker 1995; Widdicombe & Austen 1998) and the
maintenance of biodiversity (Widdicombe & Austen
1998; Widdicombe et al 2000). In soft sediment habitats
the single most important mechanism by which nutrients
are exchanged with and released from the sediments is
bioturbation: the disturbance and mixing of the
sediments by animals as they go through them (Rhoads &
Young 1970). Any factor that alters the rate of
bioturbation will have considerable implications for the
functioning of marine ecosystems and, as some
bioturbating species rely on calcareous structures, an
increase in the acidity of coastal seas would be such a
factor. Two of the most important groups of bioturbating
organisms are the burrow-forming thalassinidean
crustaceans (such as some shrimps) and the highly mobile
subsurface echinoderms. Although both groups actively
bioturbate the sediment, the mechanisms by which they
do this are very different, which has important
implications for their impact on the sediment
environment. Burrowing shrimps create a permanent
system of tunnels which enables oxygen and other water-
borne substances to penetrate deep into the sediment.
This has been shown to significantly increase the uptake
of nitrate and the release of ammonium, as well as
stimulating the process of denitrification by
microorganisms (Howe et al 2004). Conversely,
burrowing urchins ‘bulldoze’ through the surface
sediment, creating a deep, uniform oxic layer which
decreases the flux of nitrate into the sediment
(Widdicombe & Austen 1998). Organisms that create
permanent burrows (for example thalassinidean shrimps)
are already exposed to low and variable levels of pH
whereas those species that inhabit the upper few
centimetres of the sediment and do not create permanent
burrows are not (for example burrowing urchins). In
addition, the body structure of echinoderms would seem
to make them particularly susceptible to changes in pH as
they rely on a calcareous skeleton and have more ready
exchange of CO2 between ambient seawater and the
internal body cavities than is the case for other marine
animals of a similar size. This potential vulnerability of
echinoderms to changes in pH is supported by the small
amount of evidence currently available, which indicates
that echinoderms are significantly impacted by a pH
decrease of as little as 0.3 units (Shirayama et al 2004).
Most benthic animals have a pelagic larval stage to
enhance their distribution. Bivalve larvae that are settling
from the pelagic to the benthic phase have an extremely
high mortality (greater than 98%) in the first days and
weeks. This has been linked to undersaturation of
carbonates which makes calcification difficult and
weakens the shells (Green et al 2004).
Such major differences in the potential response of
organisms from different taxa and functional groups to
reduced pH could have a considerable impact on the
functioning of marine sediments and the ‘goods and
services’ they provide. Consequently, whilst changes in
seawater pH may have a direct effect on sediment and
seawater chemistry through changes in nutrient
speciation, this effect may be exacerbated by changes in
the type and intensity of megafaunal bioturbation.
4.3 Impact of ocean acidification on pelagic
systems
4.3.1 Coastal and open ocean pelagic
ecosystems
Calcified organisms are very significant components of
pelagic ecosystems and any changes in the functioning of
these calcified components in response to surface ocean
acidification would have large implications on the
functioning of the ecosystem. Pelagic ecosystems supply
the organic carbon used by organisms deeper in the oceans
and are important in the interactions of the surface oceans
with the atmosphere, such as in the exchange of CO2.
A diverse and often abundant community of
coccolithophores, foraminifera and pteropods inhabits
the sunlit (euphotic) zone of all open and coastal ocean
areas, with the exception of the Southern Ocean (Section
4.3.2) (south of the polar front) and the Arctic Ocean.
Massive blooms of coccolithophores, which can cover
hundreds of thousands of square kilometres of surface
oceans and can each produce up to one million tonnes of
calcite (Holligan et al 1993), are seen from satellites in
Ocean acidification due to increasing atmospheric carbon dioxide28 | June 2005 | The Royal Society

temperate and subpolar latitudes (Iglesias-Rodriguez et al
2002). However, even outside these blooms, which are
predominantly formed by only one species, Emiliania
huxleyi, coccolithophores are a common and often
dominant component of the phytoplankton community
(for example in subtropical circulation currents). Although
the spatial and temporal distributions of coccolithophores
are consistent with a requirement for high-calcite
saturation states (Tyrrell and Merico 2004), which is also
reflected by their absence in low calcite saturation areas
as such as the polar oceans, it is not clear what role this
factor plays for the occurrence of this group. Laboratory
and field experiments show that calcification of
coccolithophores decreases with decreasing saturation
state (Riebesell et al 2000; Zondervan et al 2001).
Ocean acidification is likely to also affect calcification of
foraminifera, a group of calcite-producing protists. As
with other calcifying organisms, changes to the
distribution and abundance of these protists could have
significant effects on the global carbon cycle.
As saturation states decline globally, it might be expected
that calcifying organisms in near-polar areas, where
saturation state is the lowest, will be affected first. Owing
to the higher solubility of aragonite, pteropods, that
produce CaCO3 in its aragonite form, will be more strongly
affected than the coccolithophores and foraminifera
which produce the calcite form. If CO2 emissions continue
on current trends the aragonite saturation horizon will rise
to the surface of the oceans before the end of this century,
making aragonite skeletons unstable throughout the
water column over the entire Southern Ocean (Section
4.2.1). The highest-latitude coccolithophore blooms are
currently in the Barents Sea at more than 70° N. Satellite
images suggests that these blooms are probably more,
rather than less, numerous today than during the 1980s
and early 1990s (Smyth et al 2004). Changes other than
ocean acidification, such as global warming and related
enhanced stratification at these high latitudes, may have
favoured the northward extension of coccolithophore
blooms in recent years.
The shift from coccolithophore to a diatom-dominated
phytoplankton community, or vice versa, can have strong
impacts on the entire ecosystem of the region where it
occurs. In the eastern Bering Sea such a regime shift
occurred in the late 1990s. Although previously
dominated by diatoms, extensive blooms of Emiliania
huxleyi have occurred throughout the eastern Bering Sea
between 1996 and 2000 (Merico et al 2003). This change
in the dominant primary producer was correlated with a
restructuring of the region’s ecosystem at all trophic
levels. One of the results of this was a strong decline in
salmon catches during these years.
The finding that coccolithophore blooms are now
occurring at high latitudes shows that there are
significant factors determining the abundance of
coccolithophores other than the calcite saturation state.
However, it seems very likely that calcified planktonic
organisms, including coccolithophores, will decline as
CO2 in the surface oceans increases, with important
implications for trophic interactions and ecosystem
function (Section 5). It is important to note that we have
much less understanding of the factors determining the
occurrence of coccolithophore blooms than of the
determinants of diatom blooms (Flynn 2001).
4.3.2 Southern Ocean food webs
Southern Ocean food webs are of particular concern
(Section 2.7.1). Calcifying organisms in the Southern
Ocean will be among the first to be affected from ocean
acidification. Aragonite-producing pteropods are the
dominant calcifiers in the Southern Ocean. These pelagic
molluscs account for a significant proportion of the
biological pump of the Antarctic polar front, and in the
Ross Sea (Section 2.2.2, Gardener et al 2000; Seibel &
Dierssen 2003). Pteropods have a global distribution, but
reach their highest abundances (up to 1000 individuals
per cubic metre) in polar and subpolar waters. These
pelagic molluscs are an important food source for marine
predators in the Antarctic food web and sometimes
replace krill as the dominant zooplankton group in parts
of the Southern Ocean. The response of pteropods to
increased CO2 has not yet been tested. However, it is likely
to be consistent with the other biogenic carbonate
producers (Seibel & Fabry 2003). If pteropods are
restricted to aragonite-saturated waters, then their
habitat will become increasingly limited latitudinally and
vertically as the aragonite saturation horizon moves closer
to the oceans’ surface.
In the event that pteropods were eliminated from the
Southern Ocean, there would be a reduction in the
biological pump to the deeper oceans of particulate
inorganic and organic carbon in the region. Although
pteropods contribute to the diet of carnivorous
zooplankton, North Pacific salmon, mackerel, herring,
cod and baleen whales (LeBrasseur 1966; Takeuchi
1972), most of these would probably be able to switch
to other prey types. However, this could result in greater
predation pressure on juvenile fishes. Gymnosomes, the
shell-less planktonic molluscs that are believed to prey
exclusively on pteropods, would probably shift their
geographical distribution with that of their pteropod
prey (Lalli and Gilmer 1989). Decreasing carbonate
saturation state will impact the depth ranges of
pteropods in several other regions of the world’s oceans,
such as the subarctic Pacific, where saturation levels are
already low, as well as upwelling areas associated with
the Benguela Current, the western Arabian Sea and the
Peru Current, where pteropod abundances can be high
(Fabry 1990; Hitchcock et al 2002).
Over the next few centuries, as the aragonite saturation
horizon moves progressively closer to the surface of the
oceans in these regions, and the calcite saturation state of
the euphotic zone decreases, the trophic dynamics and
Ocean acidification due to increasing atmospheric carbon dioxide The Royal Society | June 2005 | 29

other ecosystem processes, including the cycling of CaCO3
and organic matter will be affected (Feely et al 2004).
4.4 Conclusions
According to the evidence available, changes in ocean
chemistry will present severe challenges to some of the
components of these vast and important ecosystems
considered in this section. Changes to important coastal
ecosystems such as coral reefs, to the carbon cycle of the
Southern Ocean and to deep-water reef ecosystems will
have large-scale ramifications for this and other
interconnected ecosystems. That said, we recognise that
there are large gaps in our knowledge of how these
processes might be affected and the outcome of the
changes being exerted on them.
Ocean acidification due to increasing atmospheric carbon dioxide30 | June 2005 | The Royal Society

5.1 Introduction
Modelling of aspects of the effect of future climate
change on the oceans has incorporated physical and
biological processes. Such studies have considered the
ability of the oceans to absorb CO2 (see, for example,
Sarmiento et al 1998; Sarmiento & LeQuere 1996) and on
the emission of gases, such as dimethyl sulphide (DMS),
from the ocean (Bopp et al 2003). However, these studies
have generally not addressed the role of pH change.
In this section, we address the possible direct effects of
enhanced ocean acidification by CO2 on the Earth
systems. In particular we look at the exchange of gases
between the oceans and atmosphere, which is important
for the chemistry of the atmosphere and which might
affect global climate change. The processes behind each
of these feedback effects are very complex,
understanding the overall impact on the Earth systems is a
significant challenge.
5.2 Feedback effects of reduced calcification
One can expect that the effect of lowered pH on the
amount of CO2 absorbed by the oceans will be very
dependent on any changes that may occur in the
composition of species within surface ocean
communities. For example, if the amount of calcification
is reduced by the lower pH (as discussed in Sections 3 and
4), there will be a net increase in CO2 uptake as a result of
a reduced return flux of the gas released from the
calcification reaction (as discussed in Section 2). Model
calculations indicate that if CO2 concentrations continue
to increase at current levels, the projected decrease in
calcification would lead to a reduction in atmospheric
CO2 of 4–10 ppm by 2100 (Zondervan et al 2000; Heinze
2004). Making global ocean calcification proportional to
the declining calcite saturation state causes a reduction of
19 ppm in atmospheric CO2 by 2100 (Klepper & de Haan
1995).
A reduction and possibly regional cessation of
calcification by organisms in the oceans would strongly
affect ecosystem regulation and the flow of organic
material to the seafloor. As discussed in Section 2, the
biological pump removes carbon from the surface waters.
It has been suggested that CaCO3 acts as a mineral ballast
for the export of organic carbon, such as plankton cells
and other particulate matter, in the biological pump
(Section 3.2.1) (Klaas & Archer 2002). Any reduction in
CaCO3 production will reduce the amount of ballast
available to the biological pump and may therefore
diminish the flow of carbon to the deep oceans. However,
because any link between the flow of this mineral ‘ballast’
and the flow of organic matter is unknown, the
significance of this effect remains uncertain (Passow
2004).
Another potential effect of ocean acidification may be the
disappearance of coccolithophore blooms (Section 4.3.1).
These massive blooms add to the albedo affect of the
Earth. This means that they quantifiably increase the
amount of sunlight that is reflected back into space,
which cannot then contribute to global warming. It has
been projected that the loss of these blooms could reduce
the global albedo by up to 0.13%, and could therefore
enhance global warming (Tyrrell et al 1999).
5.3 Other feedbacks within the Earth systems
As discussed in Section 3, it is unclear what impact rising
atmospheric CO2 will have on the physiology of
phytoplankton (such as diatoms and flagellates). As a
result it is uncertain whether it will lead to greater
productivity and therefore draw down more CO2 or
reduce productivity thus absorbing less.
Apart from the CO2, the climate is affected by a number of
other gases that are produced by marine organisms,
including the greenhouse gases nitrous oxide (N2O) and
methane (CH4). In addition, some groups of plankton
produce DMS, a gas that when oxidised in the atmosphere
produces cloud-forming particles which can lead to climatic
cooling. Changes in DMS production under elevated CO2
conditions, as with the preceding discussion on
calcification, will be very dependent on which plankton
species are most affected by the changed conditions.
For example, it is well established that there is wide
variation in the ability of different groups of phytoplankton
to produce DMS from its biochemical precursor,
dimethylsulphoniopropionate (DMSP). For example in the
phytoplankton, diatoms form little DMSP whereas the alga
Phaeocystis and coccolithophores are prolific producers
(Liss et al 1994). Changes in the abundance of these groups
would affect the size of the feedback.
Notwithstanding this uncertainty, a recent modelling
study (Gunson, personal communication) suggests a
considerable climatic sensitivity to changes in DMS
emissions from the oceans. These models indicate that a
relatively small (two-fold) increase in DMS emission, if
occurring globally, would produce an atmospheric
temperature decrease in the order of 1-2oC. Such a
cooling would clearly be significant and changes in DMS
emissions of this size are certainly possible. However, it
should be stressed that there are considerable
uncertainties in the modelling, including the mechanisms
of particle and cloud formation on oxidation of DMS in
the atmosphere, as well as in the representation in models
of the variations in DMS production by different plankton
species.
In this context it is important to note that organisms
forming CaCO3 plates, such as the coccolithophores, are
major producers of DMS. This means that a decline in
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5 Interaction with the Earth systems

coccolithophore productivity may also reduce DMS-
related cloud formation (Malin & Steinke 2004) and so
have a feedback effect on climate. In some recent
laboratory and mesocosm studies it was found that under
elevated CO2 conditions the amounts of DMS and DMSP
produced by the phytoplankton were halved (Avgoustidi
personal communication). However, it needs to be
stressed that such studies are very short-term (days to
weeks in duration) and that the organisms are subject to
rapid change in CO2 level compared with what will
happen in the real-world situation of relatively slow pH
change.
Other gases formed by marine organisms can affect air
quality; these include organo-halogens, non-methane
hydrocarbons and oxygenated organics. Once released
from the oceans into the troposphere (lower
atmosphere), these gases can affect the formation of
particles and destroy oxidants in the atmosphere. These
oxidants, such as ozone, are important for improving air
quality by breaking down organic pollutants.
In addition, other gases are emitted that affect the acidity
of atmospheric particles and precipitation; these include
ammonia, sulphur dioxide, and other products of DMS
oxidation.
5.4 Conclusions
In the case of the gases N2O, CH4 and DMS, which can affect
atmospheric chemistry, there is not enough information
about how their emission to the atmosphere might change
as the ocean pH decreases. This uncertainty is increased by
the current lack of understanding of the processes
controlling their production in marine surface waters.
There are potentially important interactions between
changes in the state of the oceans (including their pH) and
changes in the global climate and atmospheric chemistry.
Our knowledge of these impacts and effects is currently
poor and requires urgent consideration as part of the
broader drive to understand Earth Systems processes.
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6.1 Introduction
It is clear that the full range and magnitude of the
biological and biogeochemical effects of acidification of
the surface oceans are still so uncertain that a reliable and
quantitative estimate of the likely socio-economic effects
is not yet possible. However, some general indications of
the possible size of such effects can be obtained for at
least some of the aspects discussed above, and a
preliminary attempt to set such effects in context, by
estimating at least the economic value of the resources at
risk, even if the extent of damage is uncertain, is given
below.
6.2 Effects on coral reefs
The likely effects of ocean acidification on coral reefs
(Section 4.2.1) are probably the best understood and
documented of those considered in this report. These
alone are sufficient to give rise to serious concern. Coral
reefs support many millions of people globally through
subsistence food gathering. Many more people are
supported through industries, such as tourism and
fisheries that bring in billions of dollars annually (Hoegh-
Guldberg & Hoegh-Guldberg 2004). The projected
impacts and the implications for the ecosystems
associated with the coral reefs (sea grass, mangrove) are
likely to have adverse effects on tourism and fisheries,
both of which are extremely important for the economic
welfare of local populations, particularly in areas where
alternative economic opportunities are scarce.
Research into the socio-economic effects of coral reef
degradation has focused on damage to corals as a result
of global warming. Studies in this area have not yet
included the effects of ocean acidification on corals, such
as declining calcification rates. However, they provide
indicative values on socio-economic impacts of the loss of
coral reefs from adverse environmental changes. As
ocean acidification has not been included, these studies
have to be considered conservative in terms of potential
global impacts.
Potential socio-economic impacts of reef health have
been explored for the world’s largest coral reef
ecosystem, the Great Barrier Reef. Hoegh-Guldberg &
Hoegh-Guldberg (2004) assessed how climate-driven
changes on the mortality of coral reefs would affect the
industries of local coastal Queensland. The study
developed a new measure of the importance of ‘reef-
interested’ tourism for the five regions of coastal
Queensland – a measure that considerably increases
previous estimates. In terms of contribution to the
regional economies (gross regional product, GRP), the
estimated reef-interested component accounts for 68%
of the total – AU$1.4 billion (about £58000 million,
using the exchange rate in May 2005). The reef-
proportion of the interested component is highest in
Far North Queensland (AU$800 million of a total tourism
GRP of AU$900 million in 1999). The authors then
calculate the diminishing effect of the tourist appeal of
the Great Barrier Reef as the bleaching and mortality of
corals (and the associated organisms) changes with
increases in temperature. This effect was calculated using
the possible future greenhouse gas emission scenarios
developed by the IPCC (2000). Even under a modest
emission scenario, leading to an atmospheric
concentration of CO2 of about 600 ppm by 2100, climate
change is predicted to cost local economies
substantially—a minimum of AU$2.5 billion over 19 years
to 2020. Under scenarios in which global greenhouse
emissions continue to grow rapidly (IPCC 2000), leading
to an atmospheric concentration CO2 of over 800 ppm by
2100, losses for the coastal economies of Queensland rise
to over AU$14 billion.
The World Resources Institute (Burke et al 2004) has
estimated that in 2000, Caribbean coral reefs alone
‘provided annual net benefits in terms of fisheries, dive
tourism, and shoreline protection services with an
estimated value between US$3.1 billion and US$4.6
billion’ (about £2000 million and £3000 million, using
the exchange rate in May 2005); and that the loss of
income by 2015 from these degraded reefs may be
several hundred million dollars per annum. As pointed out
by several authors (see, for example, Hoegh-Guldberg &
Hoegh-Guldberg 2004), these baseline estimates are
minimal estimates of the economic value of coral reefs
because they do not value indirect contributions that
coral reefs make such as the stabilisation of coastlines and
their role in creating the circumstances for other
ecosystems such as mangroves and sea grasses to
flourish. These, when considered with the future
accumulated benefits, make the value of coral reefs
significantly higher than those associated with the direct
benefits to industry.
Coastal reefs in Hawaii have been estimated to generate
almost US$364 million each year in added value (Cesar et
al 2002). Here ‘added value’ is the net business revenues
(ie income minus costs), directly and indirectly from
residents and tourists going snorkelling and diving on
Hawaii’s reefs. About 84% of this added value (US$304
million) is generated from snorkelling and diving on reefs
and emerges from US$700–$800 million in gross sales
per year. Added value from property generates another
US$40 million each year. This number reflects the higher
prices for homes and higher occupancy and room rates
for commercial properties adjacent to healthy coral reefs.
In addition, there is a ‘non-use value’ (US$7 million),
which estimates the amount of money residents, visitors
and non-visitors are willing to pay to ensure active
management of a healthy reef. Finally, the annual added
value for near-shore fisheries within the main Hawaiian
islands’ coastal reefs is about US$2.5 million per annum.
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6 Socio-economic effects of ocean acidification

This amount reflects the revenues of fishermen minus
their costs.
It is clear that globally the economic value of coral reefs is
many billions of dollars per annum, and that the
likelihood of significant degradation of reefs would be a
sufficient cause for action to protect them.
6.3 Effects on marine fisheries
Revenue from global capture fisheries in 2002 was
around US$78 billion, with marine capture fisheries
accounting for about 90% of this (FAO 2004). Although
the major adverse effect on marine fisheries at present is
that of the mortality generated by large-scale fishing
(Pauly et al 1998), any further adverse effect on their
productivity would nevertheless lead to very substantial
social and economic losses.
Direct effects on marine fish include physiological effects
such as hypercapnia, which has been studied on some
species of fish in elevated CO2 levels (Section 3.4.1). There
is insufficient information to predict whether future high
levels of CO2 will lead to significant adult fish mortality.
Similarly, it is not possible to predict if there will be any
changes in the reproductive success of marine organisms
(Section 3.4.2), which would have wide-ranging impacts
on marine fisheries.
One of the major indirect impacts on tropical coastal
fisheries may be associated with the loss of coral. In
addition to the effect on tourism of rising acidity on coral
growth, the potential loss of coral-dominated reef
systems in the tropics may lead to large-scale changes to
the critical habitats of many commercial fish species.
These organisms, many of which are completely
dependent on coral for food and shelter, would be
expected to undergo a major contraction as coral
populations dwindle. Organisms that are associated with
coral but not as dependent may be expected to be
relatively unaffected. Several studies already support this
proposal. The orange-spotted filefish (Oxymonacanthus
longirostris), which is unable to survive without coral,
rapidly disappeared from Okinawan reefs after the 1998
bleaching event (Kokita & Nakazono 2001). This
particular species, like many others such as butterflyfish
and angelfish, requires coral for food, larval settlement
and shelter. Although this species is not of great
commercial concern, it illustrates the likely impacts of
large-scale changes to the habitat of tropical coastal
fishes. Other fish associated with coral reefs may stand to
benefit from reefs that change from coral domination to
reefs that are mostly dominated by algal species.
Herbivorous fish populations appear to have increased
after the 1998 mass bleaching event on Tanzanian reef
systems (Lindahl et al 2001). Similar conclusions have
been seen at other sites by Chabanet (2002). Our
understanding of the relationships between organisms on
coral reefs is relatively scant; hence our ability to project
how changes in coral cover will affect overall species
diversity and community structure on coral reefs is
limited. However, the impact of the loss of carbonate
structures along tropical coastal areas, and therefore this
important habitat, could be substantial.
These considerations show that there could be very
significant effects of increased ocean surface CO2 on
fisheries.
6.4 More general ecosystem effects
As noted in previous sections, pH is such a fundamental
variable for marine biogeochemical processes that
pervasive effects on ecosystem structure and function are
possible. The widespread effects on the ecosystem of the
eastern Bering Sea caused by the coccolithophorid
blooms (Section 4.3.1) in 1997 may serve as an example
of the significant impacts on an ecosystem that can result
from small changes in the Earth systems. In this case it
was a change in temperature (Pitcher & Calder 2000),
which caused an opposite effect on coccolithophores
than would be expected with ocean acidification.
Observations of the coccolithophorid blooms in 1997
showed a detrimental affect on the mortality of diving
seabirds and the migration of sockeye salmon. The
number of sockeye salmon migrating in Bristol Bay was so
low – an estimated 5 million to 12 million animals failed to
appear – that the fishery was declared a ‘commercial
failure’ (Fisheries 1998).
6.5 Ecosystem services and vulnerability
The oceans provide numerous ecosystem services that
benefit human-kind (Millennium Ecosystem Assessment
2005). These services, such as fisheries contribute
significantly to global employment and economic activity.
Other services provided by the oceans, for example
providing a site for the release of wastes and pollutants,
and protection of coastal area by coral from wave energy,
are not sufficiently accounted for by standard economic
measures. Attempts to quantify some of these services
have produced estimates of many billons of dollars
(Millennium Ecosystem Assessment 2005). Therefore any
significant degradation (even by a few per cent) of these
services could have a considerable impact on the global
economy.
The Millennium Ecosystem Assessment report highlights
established but incomplete, evidence that changes made
to an ecosystem increase the likelihood of further
nonlinear changes to that ecosystem (Millennium
Ecosystem Assessment 2005). These may include
accelerated, abrupt and potentially irreversible changes to
ecosystem structure which would have significant
consequences for human well-being. Owing to projected
changes in ocean chemistry, marine ecosystems are likely
to become less resilient to further change and more
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Ocean acidification due to increasing atmospheric carbon dioxide36 | June 2005 | The Royal Society

This section considers the practicality of some of the
engineering approaches that have been suggested as
ways to mitigate the chemical effects on the oceans of
increased atmospheric CO2 concentrations.
There are many approaches to preventing the emissions
of CO2 to the atmosphere (for example improved
efficiency, renewable energy, carbon sequestration). A
discussion of these approaches is beyond the scope of this
report; however, the Royal Society has commented in
detail on the national and international implementation
of these methods in previous reports (for example, Royal
Society & Royal Academy of Engineering 1999; Royal
Society 2002, 2003, 2005).
Some of the approaches to reduce the accumulation of
CO2 emissions to the atmosphere, for example direct
injection of CO2 into the deep oceans or fertilization of
the upper oceans with iron (Coale 1996; Boyd et al 2000),
have the potential to exacerbate chemical changes to the
oceans. These chemical changes may have a range of
important biological consequences, including some
potentially negative impacts, and require further
consideration.
It has been suggested that some of the chemical effects of
CO2 addition could be mitigated with the addition of
alkalinity to the oceans (Kheshgi 1995; Rau & Caldeira
1999). However, this mitigation would be at best partial. If
enough alkalinity were added to restore the mineral
saturation state of ocean carbonate, only half of the pH
change would be mitigated (Annex 1). If enough alkalinity
were added to restore ocean pH, the oceans would
become more saturated with carbonate minerals (Caldeira
& Wickett in press). This in itself could potentially lead to
ecosystem level changes. A major concern relating to this
approach is the ecological damage from the amount of
limestone mining that would be required (see below).
The challenges in achieving mitigation at a global scale,
or even a local scale are formidable. Alkalinity could be
provided by highly soluble minerals such as magnesium
hydroxide, Mg(OH)2 (Kheshgi 1995). Unfortunately, such
minerals are relatively rare and probably could not be
obtained at the required scale. Thus, most attention has
focused on carbonate minerals (for example limestone).
The scale of the mining and transportation operation
that would be required for a global effort appears to be
prohibitively large and would also require a great deal of
energy. This energy is likely to come from fossil fuel
sources and would increase rates of greenhouse gas
emissions. To counteract the changes in acidity caused by
today’s ocean uptake of roughly 2 Gt C per year (IPCC
2001) would require roughly 20 Gt CaCO3 per year
(Caldeira & Rau 2000), which, for a limestone layer
100 m thick, would require the removal of roughly
60 km2 each year. This limestone would need to be
coastally located, or transportation costs would likely be
prohibitive (Rau & Caldeira 1999). Thus, features such as
the white cliffs of Dover could be rapidly consumed.
Therefore the introduction of limestone to offset ocean
acidification would raise a host of additional
environmental problems. Furthermore, limestone does
not dissolve in surface waters, so additional processing,
and energy, would be needed (Kheshgi 1995; Rau &
Caldeira 1999).
Although the vast amounts of carbonate minerals needed
may make this approach infeasible at the scale required to
mitigate global changes in ocean chemistry, this approach
is widely used by salt-water aquarists to promote coral
growth in fish tanks. Thus, it might be possible to use
alkalinity addition to save specific coral reefs (Rau &
Caldeira 2002), but such ideas have never been tested in
situ and therefore must be regarded as speculative.
Reducing the scale of future changes to the chemistry and
acidity of the oceans is only possible by preventing the
accumulation of CO2 in the atmosphere. Alternative
solutions, such as adding chemicals to counter the effects
of acidification, are likely to be only partly effective and
only at a very local scale.
Ocean acidification due to increasing atmospheric carbon dioxide The Royal Society | June 2005 | 37
7 Engineering approaches to mitigation of ocean pH change

8.1 Conclusions
1 The oceans are absorbing CO2 produced from
human activities from the atmosphere and this
is causing chemical changes which make them
more acidic (ie there is a decrease in the pH of
the oceans).
a Ocean acidification, like global warming, is a
predictable response to those human
activities that increase the atmospheric
concentration of CO2. The magnitude and
rate of the acidification of the oceans can be
predicted with more confidence than the
rise in temperature due to global warming,
as they are less dependent on climate-
system feedbacks.
b Predictions of the consequences of CO2 emissions
must take into consideration the close chemical
and physical coupling between the oceans and
the atmosphere. For example, changes in the
chemistry of the oceans will affect its ability to
absorb additional CO2, which will in turn affect
the rate and scale of global warming. Rising sea
temperatures caused by global warming also
affect ocean chemistry, as well as other physical
and biological processes.
c The oceans are currently taking up about one
tonne of human-derived CO2 per year for each
person on the planet. Almost a half of the CO2
produced in the past 200 years from burning
fossil fuels and cement manufacture has been
absorbed by the oceans. Calculations based on
measurements of the surface oceans and our
understanding of ocean chemistry indicate that
this uptake of CO2 has already reduced surface
seawater pH by about 0.1 units, which
corresponds to an increase of about 30% in the
concentration of hydrogen ions.
d As CO2 continues to enter the atmosphere from
human activity, a proportion will be taken up
by the oceans. If CO2 emissions arenot
regulated, this could result in the pH decreasing
by 0.5 units by the year 2100. This is beyond
the range of current natural variability and
probably to a level not experienced for
at least hundreds of thousands of years and
possibly much longer. Critically, the rate of
change is also probably at least 100 times
higher than the maximum rate during this time
period. These changes are so rapid that they
will significantly reduce the buffering capacity of
the natural processes that have moderated
changes in ocean chemistry over most of
geological time.
e The chemical changes in the oceans caused by
increases in the concentration of CO2 in the
atmosphere will include a lowering of the pH,
an increase in dissolved CO2, a reduction in the
concentration of carbonate ions, and an
increase in bicarbonate ions. All of these will
affect the organisms and processes in the
oceans.
f There appears to be no practical way to
remove this additional CO2 from the oceans
after it has been absorbed, nor any realistic
way to reverse its widespread chemical and
probable biological effects. It will take many
thousands of years for natural processes to
remove this excess CO2 and return the oceans to
a level close to their pre-industrial state. Thus, it
appears that the only practical way to minimise
long-term consequences for the oceans is to
reduce CO2 emissions to the atmosphere.
2 These changes in ocean chemistry will impact
on marine organisms and ecosystems.
a Seawater pH is a critical variable in marine
systems; even small changes will have a large
impact on ocean chemistry. These changes are
likely to alter the biodiversity of marine
ecosystems, and may affect the total productivity
of the oceans. The impacts will be greater for
some regions and ecosystems. However, apart
from a few ecosystems such as coral reefs and
the Southern Ocean, the direction and
magnitude of these impacts are very uncertain.
Most of the available evidence suggests that
these changes are likely to reduce the resilience
of marine ecosystems.
b The best scientific information currently
available suggests that these changes in ocean
chemistry will almost certainly have major
adverse effects on corals and the reefs they
build in tropical and subtropical waters. This will
affect the stability and longevity of the
structures supporting these ecosystems with
implications for the biodiversity and their
ecological sustainability. Cold-water coral reefs
are also likely to be adversely affected, before
they have been fully explored.
c Future changes in ocean chemistry, resulting
from enhanced atmospheric CO2, specifically
the likely reduction in the concentration of
carbonate ions, will make it more difficult for
animals to produce hard structures such as
carbonate skeletons and shells. The evidence
available suggests that this will probably
adversely affect most of these organisms. For
Ocean acidification due to increasing atmospheric carbon dioxide The Royal Society | June 2005 | 39
8 Conclusions and recommendations

example many coastal animals and specific
groups of phytoplankton and zooplankton will
be affected.
d Increased CO2 and the associated reduction in
seawater pH may be particularly stressful for
organisms with a high metabolism such as
squid. Some studies indicate that the physiology
of other large marine animals may also be
affected, possibly restricting their growth,
reproduction or both.
e Ecosystems near the ocean floor, especially on the
seabed of shallow waters, provide an essential
service for other organisms in the surface oceans:
receiving food from the sunlit waters above in the
form of sinking particles, they also recycle
nutrients from the sediments, which are required
for the growth of other organisms. Differences in
the response of these diverse organisms to
reduced pH could have a considerable impact on
the functioning of marine sediments and the
‘ecosystem goods and services’ they provide.
f Organisms will continue to live in the oceans
wherever nutrients and light are available, even
with ocean acidification. However, from the data
available, it is not known if organisms at the
various levels in the food web will be able to adapt
or if one species will replace another. It is also not
possible to predict what impacts this will have on
the community structure and ultimately if it will
affect the services that the ecosystems provide.
Without significant action to reduce CO2 emissions
into the atmosphere, this may mean that there will
be no place in the future oceans for many of the
species and ecosystems that we know today.
3 Oceans play a very important role in the global
carbon cycle and Earth’s climate system.
a There are potentially important interactions
between changes in the state of the oceans
(including their pH) and changes in the global
climate and atmospheric chemistry. Our
knowledge of these impacts and effects is
currently poor and requires urgent
consideration.
b Both ocean acidification and climate change
result from increasing atmospheric CO2
emissions. Effects of climate change, particularly
increases in ocean temperature, will impact on
marine ecosystems. Occurring at the same time,
the impacts of climate change will be additional
to (and possibly synergistic with) those due to
increased CO2 and ocean acidification.
4 The socio-economic effects of ocean
acidification could be substantial.
a Serious damage to coral reef ecosystems, and
the human activities that are based on them,
such as fisheries and recreation, could amount
to economic losses of many billions of dollars
per year. This will have very serious impacts on
the vulnerable societies that depend upon these
ecosystems.
b Ocean acidification is likely to have significant
impacts on some marine fish and shellfish
species. It is very hard to determine the
economic value of many of these impacts, but
would present a risk to some unknown fraction
of the global economic value of these resources
(about $100 billion per year).
c Ocean acidification will probably cause
significant changes in the whole marine
biogeochemical system and the ecosystem
services that it provides, to an extent and in
ways that cannot at present be foreseen.
d Changes to the stability of coastal reefs in
tropical and subtropical parts of the globe may
lead to serious decreases in coastal protection
over longer time periods. These changes could
fundamentally change the nature of entire
coastlines and the resources available to human
societies that depend on these coastal reefs.
5 Reducing the scale of future changes to the
chemistry and acidity of the oceans is only
possible by preventing the accumulation of
CO2 in the atmosphere.
a Solutions other than preventing CO2 emissions
reaching the atmosphere, such as adding
chemicals to counter the effects of acidification,
are likely to be only partly effective and only at a
very local scale. It would be impossible at a
regional or global level, and could also cause
damage to the marine environment.
b There are many possible approaches to
preventing the accumulation in the atmosphere
of CO2 produced by human activities: for
example improved energy efficiency, use of
renewable energy and carbon sequestration in
geological reservoirs. Some approaches, such
as direct injection of CO2 into the deep oceans,
have the potential to further exacerbate
chemical changes to the oceans.
c Unless significant reductions are made in
global emissions of CO2 (to ensure
atmospheric CO2 concentration is under
twice its current levels by 2100) the entire
Ocean acidification due to increasing atmospheric carbon dioxide40 | June 2005 | The Royal Society

depth of the Southern Ocean will become
undersaturated for aragonite, which is
required by some organisms to make
carbonate skeletons and shells. Further
increases in atmospheric concentration of CO2,
above this doubling, would make the impacts
progressively worse for the oceans as a whole.
6 We conclude on the basis of current evidence
that ocean acidification is an inevitable
consequence of continued emissions of CO2
into the atmosphere, and the magnitude of this
acidification can be predicted with a high level
of confidence. However, its impacts,
particularly on marine organisms, are much less
certain and require a substantial research
effort.
a Research into the impacts on ocean with a high
CO2 concentration is in its infancy and needs to
be developed rapidly. Among the priority areas
for further research are: the identification of
those species, functional groups and
ecosystems that are most sensitive to increased
surface ocean CO2, and the rate at which
organisms can adapt to these changes; the
interaction of increased CO2 in the surface
oceans with other factors such as temperature,
the carbon cycle, sediment processes, and the
balance of reef accretion and erosion;
feedbacks of increased ocean surface CO2 on
air–sea exchange of CO2, dimethylsulphide and
other gases important for climate and air
quality; and larger-scale manipulation
experiments on the effect of increased CO2 on
biota in the surface oceans.
7 Ocean acidification is a powerful reason, in
addition to climate change, for reducing global
CO2 emissions.
Ocean acidification due to increasing atmospheric carbon dioxide The Royal Society | June 2005 | 41

A1 The meaning of pH
The term pH describes the acidity of a liquid. It is defined
as:
pH = –log10 [H
+] (1)
This negative logarithmic dependence on hydrogen ion
concentration, [H+], in moles per litre means that if [H+]
increases 10-fold then pH decreases by 1unit. The square
brackets refer to the concentration of H+ and
subsequently for the concentration of other substances.
Water has the formula H2O. Only a small proportion of
the water molecule splits up into H+ and OH–. In pure
water, the amounts of [H+] and [OH–] are equal and the
concentration of each is 10–7 in the units mole per litre.
This means that a neutral solution has pH = 7. Acid
solutions have an excess of [H+] and a pH less than 7.
Alkaline (basic) solutions have an excess of [OH–] and pH
greater than 7, up to 14.
A2 Dissolved inorganic carbon in seawater
As CO2 dissolves in seawater, it reacts with seawater to
produce H+ and various negatively charged forms of
dissolved carbon (see Figure A). Carbon dioxide
dissolved in seawater first reacts with the water
molecule (H2O) to form carbonic acid (H2CO3). Not all
the CO2 dissolved in seawater reacts to make carbonic
acid and therefore seawater contains dissolved gaseous
CO2; a point that is easily appreciated from a familiarity
with carbonated drinks (which are solutions of carbonic
acid). According to Henry’s Law, in a simple aqueous
solution we would expect the concentration of CO2 in
the air to be proportional to that in the solution of
carbonic acid.
[CO2] + [H2O] ⇒ [H2CO3] (2)
Carbonic acid is an acid because it can split up into its
constituents, releasing an excess of H+ to solution and so
driving pH towards lower values. Carbonic acid splits up
by adding one H+ to solution along with HCO3
– (a
bicarbonate ion):
[H2CO3] ⇒ [H
+] + [HCO3
–] (3)
This increase in H+ causes some CO3
2– (called carbonate
ion) to react with H+ to become HCO3
–:
[H+] + [CO3
2–] ⇒ [HCO3
–] (4)
Thus, the net effect of the dissolution of CO2 in seawater
is to increase concentrations of H+, H2CO3 and HCO3
–,
while decreasing concentrations of CO3
2–.
The decrease in carbonate ion concentration [CO3
2–] has
important consequences for the chemistry of carbonate
minerals commonly used by marine biota to form shells or
skeletons. The formation and dissolution of carbonate
minerals can be represented as:
←mineral formation
CaCO3 ⇔ [Ca
2+] + [CO3
2–] (5)
dissolution→
Because the dissolution of CO2 in seawater decreases
[CO3
2–], this reaction moves to the right, impeding the
formation of carbonate minerals and promoting their
dissolution. Dissolution of carbonate minerals (Equation
5) provides carbonate ions that can react to consume H+
according to Equation 4. Thus, the dissolution of
carbonate minerals tends to decrease [H+] (increase pH),
counteracting some of the pH effects of added CO2 (see
A3).
A3 The carbonate buffer and seawater pH
The term ‘carbonate buffer’ is used to describe how the
dissolved inorganic carbon system in seawater acts to
diminish changes in ocean H+ concentration, and thus pH.
If a process, such as CO2 dissolution (Equations 2 and 3),
adds H+ to seawater, some of the added H+ reacts with
Ocean acidification due to increasing atmospheric carbon dioxide The Royal Society | June 2005 | 43
Annex 1 A brief account of measures of acidity such as pH, and the
acid–base chemistry of the CO2–carbonate system in the sea
Range of seawater
HCO3
–
CO3
2–
CO2
4 5 6 7 8 9 10 11
100
10–1
10–2
10–3
Fr
ac
tio
na
l l
og
[c
on
ce
nt
ra
tio
n]
Figure A. Relative proportions of the three inorganic
forms of CO2 dissolved in seawater. Note the ordinate
scale (vertical axis) is plotted logarithmically.
pH

carbonate (CO3
2–) ion to convert it to bicarbonate (HCO3
–)
(Equation 4). Because initially most of the added H+ would
be consumed in this way, the change in pH is much less
than it would otherwise be. But this process also
consumes some carbonate ion; therefore this pH-
buffering capacity would diminish as CO2 concentrations
increase. Because CO2 is absorbed at the sea surface, it is
the surface oceans that are most affected.
On the longer time scales of ocean mixing, interaction
with CaCO3-rich sediments tends further to buffer the
chemistry of the seawater so that changes in pH are
lessened. For example, if the deep oceans start to become
more acidic such as through the addition of CO2, which
decreases concentrations of CO3
2–, some carbonate ion
will be dissolved from sediments (Equation 5).
The carbonate buffer acts to stabilise the average pH of
seawater at approximately pH = 8 because of the
following two processes: (i) uptake of CO2 from the
atmosphere (for example from volcanoes and now,
importantly, from fossil fuel burning) and (ii) interaction of
seawater with oceanic sediments composed of CaCO3.
Because CaCO3 is abundant in sediments, the pH of the
deep oceans cannot change by large amounts over
timescales of 10000 years. However, over historical
timescales, significant changes in surface and near
surface ocean pH can occur.
A4 The calcium carbonate saturation horizon
As discussed in this report, small organisms living in open
ocean waters construct shells or plates of CaCO3 that, on
death of the organism, fall to the sea floor. Their preservation
in sea floor sediments depends on the solubility of CaCO3 in
seawater and on the concentration of carbonate ions.
There is a critical concentration of carbonate ions in
seawater (the saturation concentration) below which
CaCO3 will start to dissolve. Because CaCO3 solubility
increases with decreasing temperature and increasing
pressure, the critical concentration occurs at a depth, the
‘saturation horizon’, below which seawater is under-
saturated and CaCO3 will tend to dissolve and above
which seawater is super-saturated and CaCO3 will tend to
be preserved. Because the CaCO3 mineral calcite is less
soluble than the form aragonite, the aragonite saturation
horizon is shallower. Because added CO2 decreases the
carbonate ion concentration, the saturation horizons will
become shallower with increasing releases of human-
derived CO2 to the atmosphere.
Ocean acidification due to increasing atmospheric carbon dioxide44 | June 2005 | The Royal Society

To inform the study we issued an open call for evidence in August 2004. We are very grateful to those who responded
to this call and to those who provided additional information at the request of the Working Group.
Name of submitter Organisation
Dr Martin Angel Southampton Oceanography Centre
Ms Valia Avgoustidi University of East Anglia
Dr Richard Bellerby University of Bergen, Bjerknes Centre for Climate Research
Dr Jelle Bijma Alfred Wegener Institute
Dr Philip Boyd University of Otago
Dr J S Burnell-Gray Institute of Corrosion
Dr Victoria Fabry California State University San Marcos
Dr Richard Feely NOAA, USA
Prof André Freiwald Friedrich Alexander Universität Erlangen Nürnberg
Dr Jim Gunson The Hadley Centre
Prof G Fogg FRS CBE Personal submission
Dr Jason Hall-Spencer University of Plymouth
Dr Peter Haugan University of Bergen
Dr Donald Hawthorn Personal submission
Prof Christoph Heinze University of Bergen
Prof Dave Hutchins University of Delaware
Dr Ian Joint Plymouth Marine Laboratory
Prof Les Kaufman Boston University Marine Program
Dr Dan Laffoley English Nature
Dr Martina Langenbuch Alfred Wegener Institute
Prof G Langer Alfred Wegener Institute
Dr Ben McNeil University of New South Wales
Dr Douglas Mills Institute of Corrosion
Dr Corinne Le Quéré Max-Planck-Institut für Biogeochemie
Dr Tim McClanahan World Conservation Society
Prof Paul Pearson Cardiff University
Dr Martin Pêcheux Consultant/IPCC Large Foraminifer Institute
Ocean acidification due to increasing atmospheric carbon dioxide The Royal Society | June 2005 | 45
Annex 2 List of respondents

Albedo Proportion of sunlight reflected by the Earth back into space. A reduction or increase of the
Earth’s albedo has an impact on global warming
Algae Photosynthetic organisms lacking characteristic features of land plants, including
phytoplankton and seaweeds
Anoxic Without oxygen
Aragonite Form of CaCO3 with orthorhombic (crystal system based on three unequal axes at right
angles to each other) symmetry in its crystal stucture
Archaea Single-celled creatures that, with bacteria, make up a category of life called the prokaryotes
(organisms composed of a cell having no nuclear membrane)
Bacteria Single-celled creatures that, with archaea, make up a category of life called the prokaryotes
(organisms composed of cells having no nuclear membrane)
Benthic Bottom dwelling
Bioturbation The disturbance and mixing of the sediments by animals as they move through them
CaCO3 Calcium carbonate. Compound used in the formation of shells and plates of marine
organisms
Calcite Form of CaCO3 with a trigonal (rhombohedral) symmetry in its crystal structure
CCS Carbon capture and storage
Cephalopoda A class of molluscs including squids, cuttlefish and octopuses
Cetacea An order of large aquatic mammals including whales, dolphins and porpoises
CH4 Methane
CO2 Carbon dioxide
CO3
2– Carbonate ion
Coccolithophore Calcifying planktonic algae
Demersal Living near the bottom of the sea
DIC Dissolved inorganic carbon
Dinoflagellate Members of the algal class Dinophyceae, occuring as members of the marine phytoplankton
and as photosynthetic symbionts in some benthic marine foraminifera and invertebrates
Dissociation The capacity for certain compounds to form ions including H+; this causes, and is caused by,
pH changes
DMS Dimethylsulphide, a breakdown product of DMSP which is volatile and can, in the
atmosphere, be chemically transformed to compounds that can increase cloud formation
DMSP Dimethylsulphoniopropionate a compound produced by some marine phytoplankton
organisms, and whose breakdown produces DMS in globally significant quantities.
Euphotic zone Zone of the sea where light penetration is sufficient for photosynthesis
Ocean acidification due to increasing atmospheric carbon dioxide The Royal Society | June 2005 | 47
Annex 3 Abbreviations and glossary

Foraminifera An order of protozoans, the members of which have a shell that is usually calcareous
GLODAP Global Data Analysis Project
GRP Gross regional product
Gt Gigatonne (109 tonnes)
Gymnosomata Carnivorous marine planktonic molluscs which feed exclusively on pteropods
Halocarbon A compound in which the hydrogen of a hydrocarbon is replaced by halogens
Halogen Any of the group of elements fluorine, chlorine, bromine, iodine and astatine
HCO3
– Bicarbonate ion
H2CO3 Carbonic acid
H2O Water
IPCC Intergovernmental Panel on Climate Change
Krill A small shrimp-like planktonic crustacean of the order Euphausiacea, important as food for
fish, and for some whales and seals
Henry’s Law The concentration of a solute gas in a solution at a given temperature and salinity is directly
proportional to the partial pressure of that gas above the solution
Hypercapnia CO2-induced acidification of body fluids
LLNL Lawrence Livermore National Laboratory
Mesocosm Experimental apparatus or enclosure designed to approximate natural conditions for tonnes
of seawater, and in which environmental factors can be manipulated
Microcosm Experimental apparatus or enclosure designed to approximate natural conditions for
kilograms of seawater, and in which environmental factors can be manipulated
Mg(OH)2 Magnesium hydroxide
Orthorhombic Crystal structure based on three unequal axes at right angles to each other
N2O Nitrous oxide
Oxic Containing oxygen
Pelagic Living in the middle depths and surface waters of the sea
pH A measure of the acidity of a solution; a pH below 7 indicates acidity, and above 7 denotes
alkalinity
Planktonic Of plankton, floating or drifting organisms
Protist A member of the Protista, a group of unicellular organisms regarded as intermediate
between, or more properly distinct from, plants and animals.
Protozoan A member of the Protozoa, a group of protists with animal-like features
Ocean acidification due to increasing atmospheric carbon dioxide48 | June 2005 | The Royal Society

Pteropods Planktonic marine molluscs with wing-like extensions to the foot, commonly called sea
butterflies. The herbivorous Thecosomata typically have an aragonitic shell and are the
organisms referred to in the text as Pteropods. The carnivorous Gymnosomata lack a shell and
are sometimes considered to be pteropods
Saturation horizon A natural boundary in seawater, above which CaCO3 can form, and below, it dissolves. The
boundary forms through the interplay of several variables, including pH
Speciation In the context of this report, it is the chemical form of an element in seawater
Surface ocean Near surface waters where exchange of CO2 occurs
Symbionts Partners in a symbiotic association
Symbiosis Close spatial and functional association of different organisms over much or all of their lives
Thecosomata Herbivorous planktonic marine molluscs with an aragonitic shell, considered in the text to be
equivalent to pteropods
Trigonal Three-sided; having three fold symmetry
UNFCCC United Nations Framework Convention on Climate Change
Upwelling Bulk movement of water from the deep ocean to the surface as part of global ocean
circulation or, on a smaller scale, as part of other upward water movements
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