Although Earth has undergone many periods of significant environmen-tal 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 boundariesTo meet the challenge of maintaining the Holocene state, we propose a framework based on ‘planetary boundaries’. These A safe operating space for humanityIdentifying and quantifying planetary boundaries that must not be transgressed could help prevent human activities from causing unacceptable environmental change, argue Johan RockstrÖm 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.AtmosphericBiodiversity lossChange in land useGlobalPhosphorus Nitrogen(biogeochemicalStratosphericOcean acidificationClimate changeChemical pollution(not yet quantified)aerosol loading(not yet quantified)ozone depletionfreshwater useflow boundary)cyclecycleSUMMARY● 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 oversteppedboundaries 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 472Vol 461|24 September 2009FEATURE472-475 Opinion Planetary Boundaries MH AU.indd 472472-475 Opinion Planetary Boundaries MH AU.indd 472 18/9/09 11:12:3918/9/09 11:12:39© 2009 Macmillan Publishers Limited. All rights reservedevidence 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 changeAnthropogenic 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