FEATURE

A safe operating space for humanityIdentifying and quantifying planetary boundaries that must not be transgressed could help prevent humanactivities from causing unacceptable environmental change, argue Johan Rockstrom and colleagues.

Although Earth has undergone manyperiods of significant environmen-tal change, the planet's environment

has been unusually stable for the past 10,000years1-3. This period of stability - known togeologists as the Holocene - has seen humancivilizations arise, develop and thrive. Suchstability may now be under threat. Since theIndustrial Revolution, a new era has arisen,the Anthropocene", in which human actionshave become the main driver of global envi-ronmental change5. This could see humanactivities push the Earth system outside thestable environmental state of the Holocene,with consequences that are detrimental oreven catastrophic for large parts of the world.

During the Holocene, environmentalchange occurred naturally and Earth's regu-latory capacity maintained the conditionsthat enabled human development. Regulartemperatures, freshwater availability andbiogeochemical flows all stayed within a rela-tively narrow range. Now, largely because ofa rapidly growing reliance on fossil fuels and

industrialized forms of agriculture, humanactivities have reached a level that could dam-age the systems that keep Earth in the desirableHolocene state. The result could be irrevers-ible and, in some cases, abrupt environmentalchange, leading to a state less conducive tohuman development6. Without pressure fromhumans, the Holocene is expected to continuefor at least several thousands of years7.

Planetary boundariesTo meet the challenge of maintaining theHolocene state, we propose a frameworkbased on 'planetary boundaries'. These

boundaries define the safe operating spacefor humanity with respect to the Earth systemand are associated with the planet's bio-physical subsystems or processes. AlthoughEarth's complex systems sometimes respondsmoothly to changing pressures, it seems thatthis will prove to be the exception rather thanthe rule. Many subsystems of Earth react ina nonlinear, often abrupt, way, and are par-ticularly sensitive around threshold levels ofcertain key variables. If these thresholds arecrossed, then important subsystems, such as amonsoon system, could shift into a new state,often with deleterious or potentially evendisastrous consequences for humans8,9.

Most of these thresholds can be defined bya critical value for one or more control vari-ables, such as carbon dioxide concentration.Not all processes or subsystems on Earth havewell-defined thresholds, although humanactions that undermine the resilience of suchprocesses or subsystems - for example, landand water degradation - can increase the riskthat thresholds will also be crossed in otherprocesses, such as the climate system.

We have tried to identify the Earth -systemprocesses and associated thresholds which, ifcrossed, could generate unacceptable envi-ronmental change. We have found nine suchprocesses for which we believe it is neces-sary to define planetary boundaries: climatechange; rate of biodiversity loss (terrestrialand marine); interference with the nitrogenand phosphorus cycles; stratospheric ozonedepletion; ocean acidification; global fresh-water use; change in land use; chemical pol-lution; and atmospheric aerosol loading (seeFig. 1 and Table).

In general, planetary boundaries are valuesfor control variables that are either at a 'safe'distance from thresholds - for processeswith evidence of threshold behaviour - orat dangerous levels - for processes without

evidence of thresholds. Determining a safedistance involves normative judgements ofhow societies choose to deal with risk anduncertainty. We have taken a conservative,risk-averse approach to quantifying our plan-etary boundaries, taking into account the largeuncertainties that surround the true positionof many thresholds. (A detailed descriptionof the boundaries - and the analyses behindthem - is given in ref. 10.)

Humanity may soon be approaching theboundaries for global freshwater use, changein land use, ocean acidification and interfer-ence with the global phosphorous cycle (seeFig. 1). Our analysis suggests that three oftheEarth-system processes - climate change, rateof biodiversity loss and interference with thenitrogen cycle - have already transgressedtheir boundaries. For the latter two of these,the control variables are the rate of species lossand the rate at which N2 is removed from theatmosphere and converted to reactive nitrogenfor human use, respectively. These are rates ofchange that cannot continue without signifi-cantly eroding the resilience of major compo-nents of Earth -system functioning. Here wedescribe these three processes.

Climate changeAnthropogenic climate change is now beyonddispute, and in the run-up to the climatenegotiations in Copenhagen this December,the international discussions on targets forclimate mitigation have intensified. There isa growing convergence towards a '2°C guard-rail' approach, that is, containing the rise inglobal mean temperature to no more than 2°Cabove the pre-industrial level.

Our proposed climate boundary is basedon two critical thresholds that separate quali-tatively different climate-system states. It hastwo parameters: atmospheric concentrationof carbon dioxide and radiative forcing (therate of energy change per unit area of theglobe as measured at the top of the atmos-phere). We propose that human changes toatmospheric CO2 concentrations should notexceed 350 parts per million by volume, andthat radiative forcing should not exceed 1wattper square metre above pre-industrial levels.Transgressing these boundaries will increasethe risk of irreversible climate change, such asthe loss of major ice sheets, accelerated sea-level rise and abrupt shifts in forest and agri-cultural systems. Current CO2 concentrationstands at 387 p.p.m.v. and the change in radia-tiveforcing is 1.5W m-2 (ref. 11).

There are at least three reasons for our pro-posed climate boundary. First, current cli-mate models may significantly underestimatethe severity oflong-term climate change for

a given concentration of greenhouse gases12.Most models11 suggest that a doubling inatmospheric CO2 concentration will lead to aglobal temperature rise of about 3 °C (with aprobable uncertainty range of 2-4.5 °C) oncethe climate has regained equilibrium. But thesemodels do not include long-term reinforcingfeedback processes that further warm the cli-mate, such as decreases in the surface area ofice 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 threatenthe ecological life-support systems that havedeveloped in the late Quaternary environment,and would severely challenge the viability ofcontemporary human societies.

The second consideration is the stability ofthe large polar ice sheets. Palaeoclimate datafrom the past 100 million years show thatCO2 concentrations were a major factor in thelong- term cooling of the past 50 million years.Moreover, the planet was largely ice- free untilCO2 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. aimsto ensure the continued existence of the largepolar ice sheets.

Third, we are beginning to see evidence thatsome of Earth's subsystems are already mov-ing outside their stable Holocene state. Thisincludes the rapid retreat of the summer seaice in the Arctic ocean13, the retreat of moun-tain glaciers around the world11 the loss ofmass from the Greenland and West Antarcticice sheets14 and the accelerating rates of sea-level rise during the past 10-15 years15.

Rate of biodiversity lossSpecies extinction is a natural process, andwould occur without human actions. How-ever, biodiversity loss in the Anthropocene hasaccelerated massively. Species are becomingextinct at a rate that has not been seen sincethe last global mass-extinction event16.

The fossil record shows that the back-ground extinction rate for marine life is 0.1-1extinctions per million species per year; for

mammals it is 0.2-0.5 extinctions per millionspecies per year16. Today, the rate of extinctionof species is estimated to be 100 to 1,000 timesmore than what could be considered natural.As with climate change, human activities arethe main cause of the acceleration. Changesin land use exert the most significant effect.These changes include the conversion of natu-ral ecosystems into agriculture or into urbanareas; changes in frequency, duration or mag-nitude of wildfires and similar disturbances;and the introduction of new species into landand freshwater environments17. The speed ofclimate change will become a more importantdriver of change in biodiversity this century,leading to an accelerating rate of species loss18

.

Up to 30% of all mammal, bird and amphib-ian species will be threatened with extinctionthis century19.

Biodiversity loss occurs at the local toregional level, but it can have pervasive effectson how the Earth system functions, and it inter-acts with several other planetary boundaries.For example, loss of biodiversity can increasethe vulnerability of terrestrial and aquatic eco-systems to changes in climate and ocean acidity,thus reducing the safe boundary levels of theseprocesses. There is growing understanding ofthe importance of functional biodiversity inpreventing ecosystems from tipping into unde-sired states when they are disturbed20. Thismeans that apparent redundancy is required tomaintain an ecosystem's resilience. Ecosystemsthat depend on a few or single species for criti-cal functions are vulnerable to disturbances,such as disease, and at a greater risk of tippinginto undesired states8,21.

From an Earth-system perspective, set-ting a boundary for biodiversity is difficult.Although it is now accepted that a rich mixof species underpins the resilience of ecosys-terns20,21, little is known quantitatively abouthow much and what kinds of biodiversitycan be lost before this resilience is eroded22.This is particularly true at the scale of Earthas a whole, or for major subsystems such asthe Borneo rainforests or the Amazon Basin.Ideally, a planetary boundary should capturethe role of biodiversity in regulating the resil-ience of systems on Earth. Because sciencecannot yet provide such information at anaggregate level, we propose extinction rateas an alternative (but weaker) indicator. As aresult, our suggested planetary boundary forbiodiversity often times the background ratesof extinction is only a very preliminary esti-mate. More research is required to pin downthis boundary with greater certainty. However,we can say with some confidence that Earthcannot sustain the current rate ofloss withoutSignificant erosion of ecosystem resilience.

Nitrogen and phosphorus cyclesModern agriculture is a major cause of envi-ronmental pollution, including large-scalenitrogen- and phosphorus-induced environ-mental change23. At the planetary scale, theadditional amounts of nitrogen and phospho-rus activated by humans are now so large thatthey significantly perturb the global cycles ofthese two important elements24,25.

Human processes - primarily the manu-facture of fertilizer for food production andthe cultivation ofleguminous crops - con-vert around 120 million tonnes of N 2 fromthe atmosphere per year into reactive forms- which is more than the combined effectsfrom all Earth's terrestrial processes. Much ofthis new reactive nitrogen ends up in the envi-ronment, polluting waterways and the coastalzone, accumulating in land systems and add-ing a number of gases to the atmosphere.It slowly erodes the resilience of importantEarth subsystems. Nitrous oxide, for exam-ple, is one of the most important non-CO2,greenhouse gases and thus directly increasesradiative forcing.

Anthropogenic distortion of the nitro-gen cycle and phosphorus flows has shiftedthe state oflake systems from clear to turbidwater26. Marine ecosystems have been subjectto similar shifts, for example, during periodsof anoxia in the Baltic Sea caused by exces-sive nutrients27. These and other nutrient-generated impacts justify the formulationof a planetary boundary for nitrogen andphosphorus flows, which we propose shouldbe kept together as one boundary given theirclose interactions with other Earth-systemprocesses.

Setting a planetary boundary for humanmodification of the nitrogen cycle is notstraightforward. We have defined the bound-ary by considering the human fixation of N2from the atmosphere as a giant 'valve' that con-trols a massive flow of new reactive nitrogeninto Earth. As a first guess, we suggest that thisvalve should contain the flow of new reactivenitrogen to 25% of its current value, or about35 million tonnes of nitrogen per year. Giventhe implications of trying to reach this target,much more research and synthesis of informa-tion is required to determine a more informedboundary.

Unlike nitrogen, phosphorus is a fossil min-erai that accumulates as a result of geologicalprocesses. It is mined from rock and its usesrange from fertilizers to toothpaste. Some 20million tonnes of phosphorus is mined everyyear and around 8.5 million-9.5 milliontonnes of it finds its way into the oceans25,28.This is estimated to be approximately eighttimes the natural background rate of influx.

Records of Earth history show that large-scale ocean anoxic events occur when criticalthresholds of phosphorus inflow to the oceansare crossed. This potentially explains past massextinctions of marine life. Modelling sug-gests that a sustained increase of phosphorusflowing into the oceans exceeding 20% of thenatural background weathering was enough toinduce past ocean anoxic events29.

Our tentative modelling estimates suggestthat if there is a greater than tenfold increasein phosphorus flowing into the oceans (com-pared with pre-industrial levels), then anoxicocean events become more likely within 1,000years. Despite the large uncertainties involved,the state of current science and the presentobservations of abrupt phosphorus-inducedregional anoxic events indicate that no morethan 11 million tonnes of phosphorus per yearshould be allowed to flow into the oceans -ten times the natural background rate. Weestimate that this boundary level will allowhumanity to safely steer away from the risk ofocean anoxic events for more than 1,000 years,acknowledging that current levels alreadyexceed critical thresholds for many estuariesand freshwater systems.

Delicate balanceAlthough the planetary boundaries aredescribed in terms of individual quantitiesand separate processes, the boundaries aretightly coupled. We do not have the luxury ofconcentrating our efforts on anyone of themin isolation from the others. If one boundaryis transgressed, then other boundaries are alsounder serious risk. For instance, significantland-use changes in the Amazon could influ-ence water resources as far away as Tibet30.The climate-change boundary depends onstaying on the safe side of the freshwater, land,aerosol, nitrogen-phosphorus, ocean andstratospheric boundaries. Transgressing thenitrogen-phosphorus boundary can erode theresilience of some marine ecosystems, poten-tially reducing their capacity to absorb CO2

and thus affecting the climate boundary.The boundaries we propose represent a new

approach to defining biophysical precondi-tions for human development. For the firsttime, we are trying to quantify the safe lim-its outside of which the Earth system cannotcontinue to function in a stable, Holocene-likestate.

The approach rests on three branches of sci-entific enquiry. The first addresses the scaleof human action in relation to the capacityof Earth to sustain it. This is a significantfeature of the ecological economics researchagenda31, drawing on knowledge of the essen-tial role of the life-support properties of the

environment for human wellbeing32,33 andthe biophysical constraints for the growth ofthe economy34,35. The second is the work onunderstanding essential Earth processes6,36,37

including human actions23,28, brought togetherin the fields of global change research and sus-tainability science39. The third field of enquiryis research into resilience4o-42 and its links tocomplex dynarnics43,44 and self-regulation ofliving systems45,46, emphasizing thresholds andshifts between states8.

Although we present evidence that threeboundaries have been overstepped, thereremain many gaps in our knowledge. We havetentatively quantified seven boundaries, butsome of the figures are merely our first bestguesses. Furthermore, because many of theboundaries are linked, exceeding one will haveimplications for others in ways that we do notas yet completely understand. There is also sig-nificant uncertainty over how long it takes tocause dangerous environmental change or totrigger other feedbacks that drastically reducethe ability of the Earth system, or importantsubsystems, to return to safe levels,

The evidence so far suggests that, as long asthe thresholds are not crossed, humanity hasthe freedom to pursue long-term social andeconomic development. _

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Editor's note This Feature is an edited summary ofa longer paper available at the Stockholm ResilienceCentre (http://wwwstockholmresilience.org/planetary-boundaries). To facilitate debate anddiscussion, we are Simultaneously publishing anumber of linked Commentaries from independentexperts in some of the disciplines covered by theplanetary boundaries concept. Please note that thisFeature and the Commentaries are not peer-reviewedresearch. This Feature, the full paper and the expertCommentaries can all be accessed from http://tinyurl.com/planet boundaries.

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