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 dioxide ii | June 2005 | The Royal Society ISBN 0 85403 617 2 �� The Royal Society 2005 Requests to reproduce all or part of this document should be submitted to: Science Policy Section The Royal Society 6-9 Carlton House Terrace London SW1Y 5AG email science.advice@royalsoc.ac.uk Copy edited and typeset by The Clyvedon Press Ltd, Cardiff, UK

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 CO 2 enhancement on ocean chemistry 5 2.1 Introduction 5 2.2 The impact of increasing CO 2 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 CO 2 uptake by the oceans 7 2.5 How oceans have responded to changes in atmospheric CO 2 in the past 7 2.6 Change in ocean chemistry due to increases in atmospheric CO 2 from human activities 9 2.6.1 Change to the oceans due to CO 2 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 CO 2 on calcification 21 3.6 Potential adaptation and evolution resulting from the surface ocean CO 2 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 Ocean acidification due to increasing atmospheric carbon dioxide The Royal Society | June 2005 | iii

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 CO 2 ��� 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 Ocean acidification due to increasing atmospheric carbon dioxide iv | June 2005 | The Royal Society

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 (CO 2 ) 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 CO 2 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 CO 2 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 CO 2 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 CO 2 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 CO 2 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 CO 2 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 CO 2 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. Ocean acidification due to increasing atmospheric carbon dioxide vi | June 2005 | The Royal Society

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 CO 2 to the atmosphere must be considerably less than 900 Gt C (gigatonnes of carbon) by 2100. In setting targets for reductions in CO 2 emissions, world leaders should take account of the impact of CO 2 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 CO 2 emissions. Action needs to be taken now to reduce global emissions of CO 2 to the atmosphere to avoid the risk of irreversible damage to the oceans. We recommend that all possible approaches be considered to prevent CO 2 reaching the atmosphere. No option that can make a significant contribution should be dismissed. Ocean acidification due to increasing atmospheric carbon dioxide The Royal Society | June 2005 | vii

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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 (CO 2 ) 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 CO 2 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 CO 2 (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-CO 2 World. An understanding of the chemical processes involved when CO 2 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 CO 2 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 CO 2 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 CO 2 with the atmosphere. Over the past 200 years, since pre- industrial times, the oceans have absorbed about a half of the CO 2 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 CO 2 emissions. However, as absorption of the CO 2 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 CO 2 . A decrease in the amount of CO 2 absorbed by the oceans will mean that relatively more CO 2 will stay in the atmosphere. This will make global efforts to reduce atmospheric concentrations of CO 2 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 CO 2 ���carbonate system in the oceans, is given in Annex 1). When CO 2 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 CO 2 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 CO 2 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 Ocean acidification due to increasing atmospheric carbon dioxide The Royal Society | June 2005 | 1 1 Introduction

on ocean chemistry that CO 2 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 CO 2 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 CO 2 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 CO 2 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 CO 2 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 CO 2 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 (CaCO 3 ). 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 CO 2 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 CaCO 3 could be most at risk from increasing surface ocean CO 2 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 CO 2 through the addition of an alkali to the oceans. In Section 6 we Ocean acidification due to increasing atmospheric carbon dioxide 2 | June 2005 | The Royal Society

briefly evaluate the potential of some of these methods to mitigate ocean acidification. 1.7 Artificial deep ocean storage of carbon dioxide Our report focuses on ocean acidification as a result of increasing CO 2 being absorbed from the atmosphere. We do not directly address the issue of the release and storage of CO 2 on the ocean floor and in the deep oceans as part of a carbon capture and storage (CCS) programme. As the report does address the possible effects of increased CO 2 on organisms and ocean chemistry, some of our findings will be relevant to those interested in CCS. The concept of CCS is to capture emissions of CO 2 from power generation for example, and to store them, for thousands of years, in places that are isolated from the atmosphere, such as in liquid form on the seabed in the deep oceans and in underground geological structures. This subject is part of a forthcoming special report on carbon capture and storage by the Intergovernmental Panel on Climate Change (IPCC), due in late 2005. 1.8 Conduct of the study The Royal Society convened a working group of international experts across several scientific disciplines to write this report. The Council of the Royal Society has endorsed its findings. We are very grateful to those individuals and organisations (listed in Annex 2) who responded to our call for evidence to inform this study. These have been valuable contributions, and in many cases have been reflected in our report. Ocean acidification due to increasing atmospheric carbon dioxide The Royal Society | June 2005 | 3

Ocean acidification due to increasing atmospheric carbon dioxide 4 | June 2005 | The Royal Society

2.1 Introduction The oceans are a significant store of carbon within the Earth systems. They readily exchange carbon in the form of CO 2 with the atmosphere and provide an important sink for CO 2 . Human activities are releasing CO 2 that would otherwise be locked away from the atmosphere in geological reservoirs. Because of these changes, atmospheric concentrations of CO 2 are higher today than for at least 420000 years (IPCC 2001). Approximately one-half of the CO 2 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 CO 2 by the oceans over the past century and how this reflects changes in atmospheric CO 2 levels and ocean acidity. We provide an overview of the chemical processes involved as CO 2 dissolves in the oceans how ocean chemistry responds to changes in CO 2 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 CO 2 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 CO 2 , 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 CO 2 in much the same way as the biological processes on land. Although the biological uptake of CO 2 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 CO 2 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 CO 2 to levels unprecedented for at least 420000 years and possibly for the past tens of millions of years (IPCC 2001). Ocean acidification due to increasing atmospheric carbon dioxide The Royal Society | June 2005 | 5 2 Effects of atmospheric CO 2 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.)

2.2.2 The oceans and carbon dioxide Carbon dioxide, like other gases, obeys Henry���s law, which means that an increase in the atmospheric level of CO 2 increases the concentration of CO 2 in the surface oceans. Carbon dioxide in the atmosphere is a chemically unreactive gas but, when dissolved in seawater, becomes more reactive and takes part in several chemical, physical, biological and geological reactions, many of which are complex (Annex 1). One of the overall effects of CO 2 dissolving in seawater is to increase the concentration of hydrogen ions, ([H+]), within it. This is the result of an initial reaction between water (H 2 O) and CO 2 to form carbonic acid (H 2 CO 3 ). This weak acid readily releases the hydrogen ions to form the other types of dissolved inorganic carbon (Annex 1). As we explain in Annex 1, acidity is determined by the concentration of hydrogen ions. This is measured on the pH scale, with an acid having a pH of less than 7 and alkali having a pH of greater than 7 units. The more acidic a solution, the more hydrogen ions are present and the lower the pH. Therefore the amount of CO 2 that dissolves in seawater has a strong influence on the resultant acidity/alkalinity and pH of the oceans. In the oceans, CO 2 dissolved in seawater exists in three main inorganic forms collectively known as dissolved inorganic carbon (DIC). These are: (i) aqueous CO 2 (about 1% of the total) in this report this term also includes carbonic acid (H 2 CO 3 ), (as aqueous CO 2 can be in either form), and two electrically charged forms, (ii) bicarbonate (HCO 3 ���, about 91%) and (iii) carbonate ions (CO 3 2��� about 8%). Thus under current ocean conditions, bicarbonate is the most abundant form of CO 2 dissolved in seawater followed by carbonate and then aqueous CO 2 (Figure 2). There is approximately an order of magnitude difference in abundance between each of the three forms however, amounts vary somewhat with seawater temperature, salinity and pressure. All three forms of dissolved CO 2 are important for the biological processes of marine organisms. These processes include photosynthesis by marine algae (mostly phytoplankton), the production of complex organic carbon molecules from sunlight, and calcification, providing structures such as CaCO 3 shells. When these organisms die or are consumed, most of the carbon either stays in the surface waters or is released back into the atmosphere. However, some of this CaCO 3 and organic material falls as particle sediments to the deep oceans (Figure 3). The process whereby carbon is transferred from the atmosphere to the deep ocean waters and sediments is referred to as the ���biological pump���. By removing carbon from the surface waters and taking it to greater depths, the pump increases the capacity for the oceans to act as a sink for atmospheric CO 2 . Any changes in the strength of this pump would have significant consequences on the amount of carbon being sequestered to the deep ocean environments and therefore removed from the atmosphere. 2.2.3 The oceans as a carbonate buffer The relative proportion of the three forms of DIC (CO 2 , HCO 3 ��� and CO 3 2���) reflects the pH of seawater and maintains it within relatively narrow limits. This DIC operates as a natural buffer to the addition of hydrogen ions���this is called the ���carbonate buffer���. If an acid (such as CO 2 ) is added to seawater, the additional hydrogen ions react with carbonate (CO 3 2���) ions and convert them to bicarbonate (HCO 3 ���). This reduces the concentration of hydrogen ions (the acidity) such that the change in pH is much less than would otherwise be expected (Annex 1). When atmospheric CO 2 dissolves in seawater, the oceans increase in acidity but, because of the carbonate buffer, the resultant solution is still slightly alkaline. The capacity of the buffer to restrict pH changes diminishes as increased amounts of CO 2 are absorbed by the oceans. This is because when CO 2 dissolves, the chemical processes that take place reduces some carbonate ions, which are required for the ocean pH buffer (Annex 1). 2.3 Natural variation in pH of the oceans Surface oceans have an average pH globally of about 8.2 units. However, pH can vary by ��0.3 units due to local, regional and seasonal factors. The two primary factors Ocean acidification due to increasing atmospheric carbon dioxide 6 | June 2005 | The Royal Society Range of seawater HCO3��� CO32��� CO2 4 5 6 7 8 9 10 11 100 10���1 10���2 10���3 Fractional log [concentration] Figure 2. Relative proportions of the three inorganic forms of CO 2 dissolved in seawater. The green arrows at the top indicate the narrow range of pH (7.5���8.5) that is likely to be found in the oceans now and in the future. Note the ordinate scale (vertical axis) is plotted logarithmically (see Table 1 for numeric details and Annex 1 for further explanation). pH

governing the spatial distribution (Figure 4) of ocean pH are (i) temperature of the surface oceans and (ii) upwelling of CO 2 -rich deep water into the surface waters. Lower surface water temperatures tend to increase CO 2 uptake, whilst surface warming drives its release. When CO 2 is released from the oceans, at constant temperatures, pH increases. In the deep oceans, the CO 2 concentration increases as sinking organic matter from biological production (which varies seasonally) is decomposed. These additions of CO 2 to the deep oceans cause its pH to decrease as the deep waters transit from the North Atlantic to the Pacific Ocean. When this CO 2 -rich deep water upwells to the surface, it creates regions with lower- pH in the surface waters. Seasonal changes such as those in temperature and in bio-productivity, including variations in photosynthesis and respiration, contribute to fluctuations in ocean pH (Gonzalez-Davila et al 2003). Coastal waters are more likely to be affected by the terrestrial system, such as run off from rivers, leading to wider variations in ocean pH in these areas (Hinga 2002). Geographic pH variation for the global surface oceans (50 m) for the year 1994 is shown in Figure 4. The pH values are calculated using data from the Global Data Analysis Project (GLODAP). Surface values range from 7.9 to 8.25 with a mean value of 8.08 (Sabine personal communication). The lowest values are observed in upwelling regions (eg Equatorial Pacific, Arabian Sea) where subsurface waters with lower pH values are brought to the surface. The highest values are observed in regions of high biological production and export. In these regions DIC is converted into organic carbon by phytoplankton and exported by the biological pump into the deeper oceans resulting in higher pH values in the surface waters. In Sections 2.5 and 2.6 we evaluate the affect of increased atmospheric CO 2 from human activities on surface ocean chemistry and pH, and compare these changes to natural and historical variations. 2.4 Factors affecting CO 2 uptake by the oceans Several chemical, physical and biological factors have the potential to affect the uptake of CO 2 by the oceans (Houghton et al 2001). Chemical processes that may affect CO 2 uptake include changes to the CO 2 buffering capacity (Sarmiento et al 1995) and the effects of temperature on CO 2 solubility. Physical factors that affect uptake include increased ocean stratification due to increasing global temperatures. Warming of the oceans leads to increased vertical stratification (decreased mixing between the different levels in the oceans), which would reduce CO 2 uptake, in effect, reducing the oceanic volume available to CO 2 absorption from the atmosphere. Stratification will reduce the return flow of both carbon and nutrients from the deep oceans to the surface. Biologically linked processes, discussed in greater detail below and in Section 3, are perhaps the most difficult to evaluate however, the removal of nutrients from the upper oceans with a slower return flow from the deep oceans could have negative impact on life in the surface oceans. In addition to its effects upon CO 2 uptake and ocean chemistry, any acidification of marine surface waters may influence parts of the Earth systems through the emission of gases to the atmosphere. These broader factors are considered in more detail in Section 5. 2.5 How oceans have responded to changes in atmospheric CO 2 in the past We are only certain of the atmospheric CO 2 concentrations over the past 420000 years, the time for which we have an archive of ancient air trapped in bubbles of Antarctic ice (IPCC 2001). During this period the atmospheric concentrations have always been lower than today. With less certainty, we can reconstruct CO 2 concentrations much further back in time. To do this we can use inferred reconstructions from ���proxy��� data as diverse as boron isotopes in carbonates (Pearson and Palmer 2000), rocks derived from ancient soils (Cerling 1991), the stomata on fossil leaves, and models of the processes we believe Ocean acidification due to increasing atmospheric carbon dioxide The Royal Society | June 2005 | 7 Figure 3. Diagram of the carbonate buffer and biological pump in the surface oceans. After absorption of CO 2 into the oceans it is converted by the carbonate buffer. Calcification in the oceans also releases CO 2 , some of which is returned to the atmosphere. The biological pump (represented as descending wiggly arrows) converts CO 2 from the atmosphere into organic carbon (Corg) and CaCO 3 and transfers it to the deep ocean waters and sediments. The vertical scale is compressed: the process depicted by the equations and the ���box��� occur within the surface oceans (top few hundred metres). This is far above the carbonate saturation horizon (Section 2.7.1), which for the calcite form, occurs at depths in the range of about 1.5 to 5 km and for the aragonite form, at depths in the range of about 0.5 to 2.5 km. (Reprinted with permission from Elderfield (2000) copyright ASSA). + O2 + H2O + CO2 CO2 Ca2+ + 2HCO3��� CO32��� Corg CaCO3 Carbonate saturation horizon Clay Carbonate ooze ��� ���