Atal lthough Earth has undergone many periods of significant environmen- change, the planet���s environment has been unusually stable for the past 10,000 years1���3. This period of stability ��� known to geologists as the Holocene ��� has seen human civilizations arise, develop and thrive. Such stability may now be under threat. Since the Industrial Revolution, a new era has arisen, the Anthropocene4, in which human actions have become the main driver of global envi- ronmental change5. This could see human activities push the Earth system outside the stable environmental state of the Holocene, with consequences that are detrimental or even catastrophic for large parts of the world. During the Holocene, environmental change occurred naturally and Earth���s regu- latory capacity maintained the conditions that enabled human development. Regular temperatures, freshwater availability and biogeochemical flows all stayed within a rela- tively narrow range. Now, largely because of a rapidly growing reliance on fossil fuels and industrialized forms of agriculture, human activities have reached a level that could dam- age the systems that keep Earth in the desirable Holocene state. The result could be irrevers- ible and, in some cases, abrupt environmental change, leading to a state less conducive to human development6. Without pressure from humans, the Holocene is expected to continue for at least several thousands of years7. Planetary boundaries To meet the challenge of maintaining the Holocene state, we propose a framework based on ���planetary boundaries���. These A safe operating space for humanity Identifying and quantifying planetary boundaries that must not be transgressed could help prevent human activities from causing unacceptable environmental change, argue Johan Rockstr��m and colleagues. Figure 1 | Beyond the boundary.���The inner green shading represents the proposed safe operating space for nine planetary systems. The red wedges represent an estimate of the current position for each variable. The boundaries in three systems (rate of biodiversity loss, climate change and human interference with the nitrogen cycle), have already been exceeded. A t m o s p h e r i c B doi i v e r s i t y l o s s i l d u s e S t r a t o s p h e r i c Ocean acidifi ca tio n Climate change C h e m i c a l p o l l u t in o ( n o t y e t q u a n t i fid e) a e r o s o l l o a d i n g ( n o t y e t q u a n t i fi e d ) o z o n e d e p l e t i o SUMMARY ��� New approach proposed for defining preconditions for human development ��� Crossing certain biophysical thresholds could have disastrous consequences for humanity ��� Three of nine interlinked planetary boundaries have already been overstepped boundaries define the safe operating space for humanity with respect to the Earth system and are associated with the planet���s bio- physical subsystems or processes. Although Earth���s complex systems sometimes respond smoothly to changing pressures, it seems that this will prove to be the exception rather than the rule. Many subsystems of Earth react in a nonlinear, often abrupt, way, and are par- ticularly sensitive around threshold levels of certain key variables. If these thresholds are crossed, then important subsystems, such as a monsoon system, could shift into a new state, often with deleterious or potentially even disastrous consequences for humans8,9. Most of these thresholds can be defined by a critical value for one or more control vari- ables, such as carbon dioxide concentration. Not all processes or subsystems on Earth have well-defined thresholds, although human actions that undermine the resilience of such processes or subsystems ��� for example, land and water degradation ��� can increase the risk that thresholds will also be crossed in other processes, such as the climate system. We have tried to identify the Earth-system processes and associated thresholds which, if crossed, could generate unacceptable envi- ronmental change. We have found nine such processes for which we believe it is neces- sary to define planetary boundaries: climate change rate of biodiversity loss (terrestrial and marine) interference with the nitrogen and phosphorus cycles stratospheric ozone depletion ocean acidification global fresh- water use change in land use chemical pol- lution and atmospheric aerosol loading (see Fig. 1 and Table). In general, planetary boundaries are values for control variables that are either at a ���safe��� distance from thresholds ��� for processes with evidence of threshold behaviour ��� or at dangerous levels ��� for processes without 472 Vol 461|24 September 2009 FEATURE �� 2009 Macmillan Publishers Limited. All rights reserved

evidence of thresholds. Determining a safe distance involves normative judgements of how societies choose to deal with risk and uncertainty. We have taken a conservative, risk-averse approach to quantifying our plan- etary boundaries, taking into account the large uncertainties that surround the true position of many thresholds. (A detailed description of the boundaries ��� and the analyses behind them ��� is given in ref. 10.) Humanity may soon be approaching the boundaries for global freshwater use, change in land use, ocean acidification and interfer- ence with the global phosphorous cycle (see Fig. 1). Our analysis suggests that three of the Earth-system processes ��� climate change, rate of biodiversity loss and interference with the nitrogen cycle ��� have already transgressed their boundaries. For the latter two of these, the control variables are the rate of species loss and the rate at which N2 is removed from the atmosphere and converted to reactive nitrogen for human use, respectively. These are rates of change that cannot continue without signifi- cantly eroding the resilience of major compo- nents of Earth-system functioning. Here we describe these three processes. Climate change Anthropogenic climate change is now beyond dispute, and in the run-up to the climate negotiations in Copenhagen this December, the international discussions on targets for climate mitigation have intensified. There is a growing convergence towards a ���2 ��C guard- rail��� approach, that is, containing the rise in global mean temperature to no more than 2 ��C above the pre-industrial level. Our proposed climate boundary is based on two critical thresholds that separate quali- tatively different climate-system states. It has two parameters: atmospheric concentration of carbon dioxide and radiative forcing (the rate of energy change per unit area of the globe as measured at the top of the atmos- phere). We propose that human changes to atmospheric CO2 concentrations should not exceed 350 parts per million by volume, and that radiative forcing should not exceed 1 watt per square metre above pre-industrial levels. Transgressing these boundaries will increase the risk of irreversible climate change, such as the loss of major ice sheets, accelerated sea- level rise and abrupt shifts in forest and agri- cultural systems. Current CO2 concentration stands at 387 p.p.m.v. and the change in radia- tive forcing is 1.5 W m���2 (ref. 11). There are at least three reasons for our pro- posed climate boundary. First, current cli- mate models may significantly underestimate the severity of long-term climate change for a given concentration of greenhouse gases12. Most models11 suggest that a doubling in atmospheric CO2 concentration will lead to a global temperature rise of about 3 ��C (with a probable uncertainty range of 2���4.5 ��C) once the climate has regained equilibrium. But these models do not include long-term reinforcing feedback processes that further warm the cli- mate, such as decreases in the surface area of ice cover or changes in the distribution of veg- etation. If these slow feedbacks are included, doubling CO2 levels gives an eventual tempera- ture increase of 6 ��C (with a probable uncer- tainty range of 4���8 ��C). This would threaten the ecological life-support systems that have developed in the late Quaternary environment, and would severely challenge the viability of contemporary human societies. The second consideration is the stability of the large polar ice sheets. Palaeo climate data from the past 100 million years show that CO2 concentrations were a major factor in the long-term cooling of the past 50 million years. Moreover, the planet was largely ice-free until CO2 concentrations fell below 450 p.p.m.v. (��100 p.p.m.v.), suggesting that there is a crit- ical threshold between 350 and 550 p.p.m.v. (ref. 12). Our boundary of 350 p.p.m.v. aims to ensure the continued existence of the large polar ice sheets. Third, we are beginning to see evidence that some of Earth���s subsystems are already mov- ing outside their stable Holocene state. This includes the rapid retreat of the summer sea ice in the Arctic ocean13, the retreat of moun- tain glaciers around the world11, the loss of mass from the Greenland and West Antarctic ice sheets14 and the accelerating rates of sea- level rise during the past 10���15 years15. Rate of biodiversity loss Species extinction is a natural process, and would occur without human actions. How- ever, biodiversity loss in the Anthropocene has accelerated massively. Species are becoming extinct at a rate that has not been seen since the last global mass-extinction event16. The fossil record shows that the back- ground extinction rate for marine life is 0.1���1 extinctions per million species per year for PLANETARY BOUNDARIES Earth-system process Parameters Proposed boundary Current status Pre-industrial value Climate change (i) Atmospheric carbon dioxide concentration (parts per million by volume) 350 387 280 (ii) Change in radiative forcing (watts per metre squared) 1 1.5 0 Rate of biodiversity loss Extinction rate (number of species per million species per year) 10 100 0.1���1 Nitrogen cycle (part of a boundary with the phosphorus cycle) Amount of N2 removed from the atmosphere for human use (millions of tonnes per year) 35 121 0 Phosphorus cycle (part of a boundary with the nitrogen cycle) Quantity of P flowing into the oceans (millions of tonnes per year) 11 8.5���9.5 ~1 Stratospheric ozone depletion Concentration of ozone (Dobson unit) 276 283 290 Ocean acidification Global mean saturation state of aragonite in surface sea water 2.75 2.90 3.44 Global freshwater use Consumption of freshwater by humans (km3 per year) 4,000 2,600 415 Change in land use Percentage of global land cover converted to cropland 15 11.7 Low Atmospheric aerosol loading Overall particulate concentration in the atmosphere, on a regional basis To be determined Chemical pollution For example, amount emitted to, or concentration of persistent organic pollutants, plastics, endocrine disrupters, heavy metals and nuclear waste in, the global environment, or the effects on ecosystem and functioning of Earth system thereof To be determined Boundaries for processes in red have been crossed. Data sources: ref. 10 and supplementary information 473 NATURE|Vol 461|24 September 2009 FEATURE �� 2009 Macmillan Publishers Limited. All rights reserved