Safe and just Earth system boundaries

Nature | www.nature.com | 1 Article Safe and just Earth system boundaries Johan Rockström1,2,3 ✉, Joyeeta Gupta4,5, Dahe Qin6,7,8, Steven J. Lade3,9,10 ✉, Jesse F. Abrams11, Lauren S. Andersen1, David I. Armstrong McKay3,11,12, Xuemei Bai10, Govindasamy Bala13, Stuart E. Bunn14, Daniel Ciobanu3, Fabrice DeClerck15,16, Kristie Ebi17, Lauren Gifford18, Christopher Gordon19, Syezlin Hasan14, Norichika Kanie20, Timothy M. Lenton11, Sina Loriani1, Diana M. Liverman18, Awaz Mohamed21, Nebojsa Nakicenovic22, David Obura23, Daniel Ospina9, Klaudia Prodani4, Crelis Rammelt4, Boris Sakschewski1, Joeri Scholtens4, Ben Stewart-Koster14, Thejna Tharammal24, Detlef van Vuuren25,26, Peter H. Verburg27,28, Ricarda Winkelmann1,29, Caroline Zimm22, Elena M. Bennett30,31, Stefan Bringezu32, Wendy Broadgate9, Pamela A. Green33, Lei Huang34, Lisa Jacobson9, Christopher Ndehedehe14,35, Simona Pedde9,36, Juan Rocha3,9, Marten Scheffer37, Lena Schulte-Uebbing25,38, Wim de Vries38, Cunde Xiao6,39, Chi Xu40, Xinwu Xu7,8, Noelia Zafra-Calvo41 & Xin Zhang42 The stability and resilience of the Earth system and human well-being are inseparably linked1–3, yet their interdependencies are generally under-recognized; consequently, they are often treated independently4,5. Here, we use modelling and literature assessment to quantify safe and just Earth system boundaries (ESBs) for climate, the biosphere, water and nutrient cycles, and aerosols at global and subglobal scales. We propose ESBs for maintaining the resilience and stability of the Earth system (safe ESBs) and minimizing exposure to significant harm to humans from Earth system change (a necessary but not sufficient condition for justice)4. The stricter of the safe or just boundaries sets the integrated safe and just ESB. Our findings show that justice considerations constrain the integrated ESBs more than safety considerations for climate and atmospheric aerosol loading. Seven of eight globally quantified safe and just ESBs and at least two regional safe and just ESBs in over half of global land area are already exceeded. We propose that our assessment provides a quantitative foundation for safeguarding the global commons for all people now and into the future. Humanity is well into the Anthropocene6, the proposed new geological epoch where human pressures have put the Earth system on a trajectory moving rapidly away from the stable Holocene state of the past 12,000 years, which is the only state of the Earth system we have evidence of being able to support the world as we know it7,8. These rapid changes to the Earth system undermine critical life-support systems 1,9,10 , with significant societal impacts already felt 1,3 , and they could lead to triggering tipping points that irreversibly destabilize the Earth system 7,11,12 . These changes are mostly driven by social and economic systems run on unsustainable resource extraction and consumption. Contributions to Earth system change and the consequences of its impacts vary greatly among social groups and countries. Given these interdependencies between inclusive human development and a stable and resilient Earth system1–3,13, an assessment of safe and just https://doi.org/10.1038/s41586-023-06083-8 Received: 23 June 2022 Accepted: 14 April 2023 Published online: xx xx xxxx Open access Check for updates 1Potsdam Institute for Climate Impact Research (PIK), Member of the Leibniz Association, Potsdam, Germany. 2Institute of Environmental Science and Geography, University of Potsdam, Potsdam, Germany. 3Stockholm Resilience Centre, Stockholm University, Stockholm, Sweden. 4Amsterdam Institute for Social Science Research, University of Amsterdam, Amsterdam, The Netherlands. 5IHE Delft Institute for Water Education, Delft, The Netherlands. 6State Key Laboratory of Cryospheric Science, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, China. 7China Meteorological Administration, Beijing, China. 8University of Chinese Academy of Sciences, Beijing, China. 9Future Earth Secretariat, Stockholm, Sweden. 10Fenner School of Environment & Society, Australian National University, Canberra, Australia. 11Global Systems Institute, University of Exeter, Exeter, UK. 12Georesilience Analytics, Leatherhead, UK. 13Center for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bengaluru, India. 14Australian Rivers Institute, Griffith University, Brisbane, Australia. 15EAT, Oslo, Norway. 16Alliance of Bioversity International and CIAT of the CGIAR, Montpellier, France. 17Center for Health & the Global Environment, University of Washington, Seattle, WA, USA. 18School of Geography, Development and Environment, University of Arizona, Tucson, AZ, USA. 19Institute for Environment and Sanitation Studies, University of Ghana, Legon, Ghana. 20Graduate School of Media and Governance, Keio University, Fujisawa, Japan. 21Functional Forest Ecology, Universität Hamburg, Barsbüttel, Germany. 22International Institute for Applied Systems Analysis, Laxenburg, Austria. 23CORDIO East Africa, Mombasa, Kenya. 24Interdisciplinary Center for Water Research, Indian Institute of Science, Bengaluru, India. 25Copernicus Institute of Sustainable Development, Utrecht University, Utrecht, The Netherlands. 26PBL Netherlands Environmental Assessment Agency, The Hague, The Netherlands. 27Swiss Federal Institute for Forest, Snow and Landscape Research, Birmensdorf, Switzerland. 28Institute for Environmental Studies, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands. 29Institute of Physics and Astronomy, University of Potsdam, Potsdam, Germany. 30Bieler School of Environment, McGill University, Montreal, Canada. 31Department of Natural Resource Sciences, McGill University, Montreal, Canada. 32Center for Environmental Systems Research, Kassel University, Kassel, Germany. 33Environmental Sciences Initiative, Advanced Science Research Center at the Graduate Center, City University of New York, New York, NY, USA. 34National Climate Center, Beijing, China. 35School of Environment & Science, Griffith University, Nathan, Australia. 36Soil Geography and Landscape Group, Wageningen University & Research, Wageningen, The Netherlands. 37Department of Environmental Sciences, Wageningen University & Research, Wageningen, The Netherlands. 38Environmental Systems Analysis Group, Wageningen University & Research, Wageningen, The Netherlands. 39State Key Laboratory of Earth Surface Processes and Resource Ecology, Beijing Normal University, Beijing, China. 40School of Life Sciences, Nanjing University, Nanjing, China. 41Basque Centre for Climate Change bc3, Scientific Campus of the University of the Basque Country, Biscay, Spain. 42Appalachian Laboratory, University of Maryland Center for Environmental Science, Frostburg, MD, USA. ✉e-mail: [email protected]; steven.lade@ futureearth.org 2 | Nature | www.nature.com Article boundaries is required that accounts for Earth system resilience and human well-being in an integrated framework4,5. We propose a set of safe and just Earth system boundaries (ESBs) for climate, the biosphere, fresh water, nutrients and air pollution at global and subglobal scales. These domains were chosen for the following reasons. They span the major components of the Earth system (atmosphere, hydrosphere, geosphere, biosphere and cryosphere) and their interlinked processes (carbon, water and nutrient cycles), the ‘global commons’14 that underpin the planet’s life-support systems and, thereby, human well-being on Earth; they have impacts on policy-relevant timescales; they are threatened by human activities; and they could affect Earth system stability and future development globally. Our proposed ESBs are based on existing scholarship, expert judgement and widely shared norms, such as Agenda 2030. They are meant as a transparent proposal for further debate and refinement by scholars and wider society. First, we identify ‘safe’ boundaries at subglobal and global scales for “maintain[ing] and enhanc[ing] the stability and resilience of the Earth system over time, thereby safeguarding its functions and ability to support humans and all other living organisms” 4 . To determine safe boundaries, we use assessments of tipping point risks among local and regional tipping elements, evidence on declines in Earth system functions, analyses of historical variability and expert judgement. We assess the uncertainty in and confidence of these ESBs. Tipping elements are those components or processes that regulate the functioning and state of the planet and that show evidence of having thresholds at which small additional perturbations can trigger self-reinforcing changes that undermine Earth system resilience15,16. We do not exclusively rely on tipping points for setting safe ESBs, however, and the ESBs should not be interpreted as representing tipping points. As a reference state for human life support on Earth, we use an interglacial Holocene-like Earth system functioning dominated by balancing feedbacks that cope with, buffer and dampen disturbances. Methods and Supplementary Information have details on how safe boundaries are determined. Second, we use three criteria to assess whether adhering to the safe ESBs could protect people from significant harm (Box1): ‘interspecies justice and Earth system stability’ (I1)17; ‘intergenerational justice’18 between past and present generations (I2a) and present and future generations (I2b); and ‘intragenerational justice’ (I3) between countries19, communities and individuals through an intersectional lens20. These criteria sit within a wider Earth system justice framework that goes beyond planetary and issue-related justice to take a multi-level transformative justice approach focusing on ends (boundaries and access levels) and means 21,22 . Methods and Supplementary Information have more detailed discussions of the justice approach applied in this paper. We define harm as negative impacts on humans, communities and countries from Earth system change in addition to background rates. The most recent Intergovernmental Panel on Climate Change (IPCC) report identifies ‘severe’ risks and ‘high’ reasons for concern when tens to hundreds of millions of people are exposed to changes in climate, such as increases in temperature and extreme events 23 . In this paper, we define significant harm as widespread severe existential or irreversible negative impacts on countries, communities and individuals from Earth system change, such as loss of lives, livelihoods or incomes; displacement; loss of food, water or nutritional security; and chronic disease, injury or malnutrition (a glossary is in theSupplementary Methods). Third, we combine these justice criteria with historical analyses, international health standards, Earth system modelling and expert judgement to quantify safe and just ESBs that minimize human exposure to significant harm (no significant harm (NSH)) from Earth system change. Minimizing significant harm is a cornerstone of national and international law and corrective justice24,25. We focus on assessing the levels of Earth system change leading to widespread exposure to significant harm, which will lead to greater impacts when vulnerable populations are exposed3. Methods and Supplementary Information have details on how just boundaries are determined. The just (NSH) boundaries described here are necessary but not sufficient conditions for Earth system justice, which must also enable access to resources for all 26 and distributional and procedural fairness22. A foundation that enables minimum access to water, food, energy and infrastructure for all humans alongside a safe and just (NSH) ESB ceiling of maximum allowed human pressure on biophysical domains could constitute a safe and just ‘corridor’ over time4,22 (Fig.1). Our assessment builds upon and advances beyond previous research and science-based political consensus, such as the Planetary Boundaries (PBs) framework27, doughnut economics28 and the Sustainable Development Goals29 in the following ways. (1) We define just ESBs for avoiding significant harm using the same units as the safe ESBs for the same domains and propose that actors use the stricter of the safe and just boundaries to inform target setting. The PBs identify only safe biophysical boundaries. The social goals related to access to or harm from natural resources adopted in Agenda 2030, doughnut economics and other approaches 28,30–32 are not quantified in comparable units or examine only the consequences of human activities on the Earth system, not Box 1 The ‘3I’ justice criteria used to analyse safe ESBs Further explanation is in Gupta etal.22. Discussion of the caveats related to the justice approach applied in this paper is inMethods and Supplementary Information. Interspecies justice and Earth system stability (I1) Interspecies justice aims to protect humans, other species and ecosystems, rejecting human exceptionalism. In many domains, interspecies justice could be achieved by maintaining Earth system stability within safe ESBs. Intergenerational justice (I2a and I2b) Intergenerational justice examines relationships and obligations between generations, such as the legacy of greenhouse gas emissions or ecosystem destruction for youth and future people. Achieving intergenerational justice requires recognizing the potential long-term consequences of short-term actions and associated trade-offs and synergies across time. We define two types of intergenerational justice: (between past and present; I2a) whether actions of past generations have minimized significant harm to current generations and (between present and future; I2b) the responsibility of current generations to minimize significant harm to future generations. Intragenerational justice: between countries, communities and individuals (I3) Intragenerational justice includes relationships between present individuals, between states (international), among people of different states (global) and between community members or citizens (communitarian or nationalist). Intersectional justice considers multiple and overlapping social identities and categories (for example, gender, race, age, class and health) that underpin inequality, vulnerability and the capacity to respond. Achieving intragenerational justice means minimizing significant harm caused by one country to another, one community to another and one individual to another. Nature | www.nature.com | 3 harm to humans from Earth system change. Articulating sociopolitical notions, such as Earth system justice, and converting their implications into biophysical units can enable a better understanding of the space within which humans can function. (2) We define global and subglobal ESBs in most domains. The PBs’ emphasis on the global scale can be inappropriate for the assessment and management of domains such as the biosphere33 and fresh water34–37. (3) We set boundaries at multiple likelihood levels for Earth system states. (4) Tipping element assessments in climate, biosphere and other Earth system domains are key, although not exclusive, evidence for our ESBs. Recent PB assessments instead emphasize risks related to the departure from Holocene ranges of Earth system variability38. Quantifying ESBs For each Earth system domain, we first quantify safe boundaries for maintaining Earth system resilience, with multiple levels of likelihood reflecting uncertainty or variability in the exact position of the boundary. Adhering to these safe boundaries implements our ‘interspecies justice and Earth system stability’ criterion (I1 in Box1) and will safeguard future generations against significant harm from Earth system change (intergenerational justice; I2b in Box1), but it may not avoid significant harm to current generations, particularly vulnerable populations (I2a and I3 in Box1). Hence, (1) we propose that some boundaries be made more stringent to protect present generations and ecosystems; (2) we complement safe boundaries with local-level standards to protect present generations and ecosystems; and (3) if the boundary is likely to cause considerable difficulties for present generations, we propose that it is complemented with policies that account for distributive justice. We also assess the current state of the Earth system with respect to each safe and just ESB. Climate We identify safe ESBs for warming (Fig.1 and Table1) based on minimizing likelihoods of triggering climate tipping elements; maintaining biosphere and cryosphere functions; and accounting for Holocene (<0.5–1.0 °C) and previous interglacial (<1.5–2 °C) climate variability (Supplementary Methods). Some climate tipping points, such as circulation collapse or Amazon dieback, have high uncertainty or low confidence in their dynamics and potential warming thresholds 16 , but the complementary palaeoclimate and biosphere analyses independently support the safe climate ESB assessment. Cryosphere function includes maintaining permafrost in the northern high latitudes, permanent polar ice sheets and mountain glaciers and minimizing sea ice loss. We find that global warming beyond 1.0 °C above pre-industrial levels, which has already been exceeded9, carries a moderate likelihood of triggering tipping elements, such as the collapse of the Greenland ice sheet or localized abrupt thawing of the boreal permafrost 16 . One-degree Celsius global warming is consistent with the safe limit proposed in 1990 39 and the PB of 350 ppm CO 2 (ref. 27). Above 1.5 °C or 2.0 °C warming, the likelihood of triggering tipping points increases to high or very high, respectively (high confidence in Extended Data Table1). Biosphere damage and the risk of global carbon sinks becoming carbon sources, potentially triggering further climate feedbacks, increase substantially 40 . We conclude that stabilizing at or below a safe ESB of 1.5 °C warming avoids the most severe climate impacts on humans and other species, reinforcing the 1.5 °C guardrail set in the Paris Agreement on Climate Change. Assessment of significant harm from climate change suggests the need for a stricter just (NSH) boundary. At 1.0 °C global warming, tens of millions of people were exposed to wet bulb temperature extremes (Fig.2), raising concerns of interand intragenerational justice. At 1.5 °C Current Safe Just Safe and just align Climate Functional integrity Aerosols (subglobal) Phosphorus Nitrogen Groundwater Surface water Natural ecosystem area A c c e s s f o u n d a t i o n S a f e a n d j u s t c o r r i d o r E a r t h s y s t e m b o u n d a r i e s Fig. 1 | Proposed safe and just (NSH) ESBs. Visualization of safe ESBs (dark red), just (NSH) ESBs (blue), cases where safe and just (NSH) boundaries align (green) and current global states (Earth icons). Radial axes are normalized to safe ESBs. Headline or central estimate global boundaries (Table1) are plotted to support comparison with the current global state, but we emphasize that we have also defined subglobal boundaries and multiple likelihood levels for many domains (Table1). For aerosols, however, we display the subglobal boundaries to compare safe and just boundaries. For nitrogen, we plot with a dashed blue line the boundary quantification for harm from nitrate in groundwater while noting that the just boundary must also incorporate safe considerations via eutrophication, leading to a more stringent safe and just boundary. Minimum access to water, food, energy and infrastructure for all humans (dotted green line) could constitute the foundation of a safe and just ‘corridor’ (green filled area), but we do not quantify this foundation here. Alternative visualizations are presented in Extended Data Fig.1. 4 | Nature | www.nature.com Article warming, more than 200 million people, disproportionately those already vulnerable, poor and marginalized (intragenerational injustice), could be exposed to unprecedented mean annual temperatures 41 , and more than 500 million could be exposed to long-term sea-level rise (Fig.2 and Methods). These numbers of people harmed vastly exceed the widely accepted ‘leave no one behind’ principle29 and undermine most of the Sustainable Development Goals. Moreover, past emissions have already led to significant harm, including extreme weather events, loss of habitat by Indigenous communities in the Arctic, loss of land area by low-lying states and sea-level rise or reduced groundwater recharge from changing glacial melt systems 3 . Irreversible impacts from cryosphere and biosphere tipping elements that are committed by anthropogenic greenhouse gas emissions in the coming decades but which unfold over centuries or millennia also threaten intergenerational justice (Supplementary Methods). We conclude that if exposure of tens of millions of people to significant harm is to be avoided, the just (NSH) boundary should be set at or below 1.0 °C. Since returning within this boundary may not be achievable in the foreseeable future, adaptations and compensations to reduce sensitivity to harm and vulnerability will be necessary. During the2022 United Nations Climate Change Conference (COP-27), developing countries indeed focused actively on issues of adaptation, loss and damage. Biosphere For the biosphere, we identify safe ESBs for two complementary measures of biodiversity: (1) the area of largely intact natural ecosystems and (2) the functional integrity of all ecosystems, including urban and agricultural ecosystems (Table1). Maintaining areas of largely intact natural ecosystems is necessary for securing the Earth system functions on which all humans, other species (I1 in Box1) and Earth system stability depend, including stocks and flows of carbon, water and nutrients and halting species extinction (Earth system nature’s contribution to people (NCP) via Earth system functions). Based on climate, water and species conservation model outcomes, we propose a safe ESB of 50–60% (medium confidence in Extended Data Table1) of global land surface covered by largely intact natural areas to maintain Earth system NCP (Table1 and Supplementary Methods). This range uses the current area of natural land cover as a minimum value while indicating Table 1 | Proposed safe and just (NSH) ESBs (visualized in Fig.1) Domain: state variable Relevant Earth system change Safe ESB subglobal (local/regional) Safe ESB globally aggregated Just (NSH) ESB Safe and just ESB Current global state Climate: global mean surface temperature change since pre-industrial (1850–1900) Climate tipping points; exceed interglacial range; biosphere functioning Global climate boundary set to avoid regional tipping points and biome degradation Likelihood of passing tipping points: low, 0.5–1.0 °C; moderate, >1.0 °C; high, >1.5 °C; very high, >2.0 °C Exposure to additional significant harm: moderate, 0.5–1 °C; high, 1–1.5 °C; very high, >1.5 °C 1.0 °C at high exposure to significant harm 1.2 °C Biosphere: natural ecosystem area Loss of climate, water, biodiversity NCP Critical natural ecosystems need to be preserved or restored >50–60% natural ecosystem area (depending on spatial distribution) Align with safe boundary plus ensure distributional justice >50–60% (upper end) depending on distribution 45–50% natural ecosystem area Biosphere: functional integrity Loss of multiple local NCP >20–25% of each 1 km2 under (semi-) natural vegetation; >50% in vulnerable landscapes; at <10%, few NCP remain 100% of land area satisfies local boundary Align with safe boundary >20–25% of each 1 km2 under (semi-) natural vegetation One third (31–36%)of human-dominatedland area satisfies ESB Water: surface water flows Collapse of freshwater ecosystems <20% magnitude monthly surface flow alteration 100% of land area satisfies local boundary (sums to 7,630 km3 per year global flow alteration budget) Align with safe plus World Health Organization and United Nations Environment Programme quality standards Regional and global safe ESBs 66% of global land area satisfies ESB annually (3,553 km3 per year global alterations) Water: groundwater levels Collapse of groundwaterdependent ecosystems Annual drawdown does not exceed average annual recharge 100% of land area satisfies local boundary (sums to 15,800 km3 per year global drawdown) Align with safe plus World Health Organization and United Nations Environment Programme quality standards Safe ESB (and ensure recovery) 53% of global land area satisfies ESB (15,700 km3 per year annual drawdown) Green water38 (previous assessment) Not assessed Monthly root-zone soil moisture deviates from Holocene variability <10% of ice-free land area exceeds boundary Not assessed Not assessed 18% Nutrient cycles: nitrogen Surface water and terrestrial ecosystem eutrophication <2.5 (1–4) mg N l−1 in surface water; <5–20 kg N ha−1 per year in terrestrial ecosystems (biome dependent) Surplus, <61 (35–84) Tg N per year; total input, <143 (87–189) Tg N per year Align with local safe plus drinking water (<11.3 (10–11.3) mg NO3–N l−1; globally, <117 (111–117) Tg N per year) and any available air pollution (for example, NH3) standards Local ESBs; and global surplus, 57 (34–74) Tg N per year Surplus, 119 Tg N per year; total input, 232 Tg N per year Nutrient cycles: phosphorus Surface water eutrophication <50–100 mg P per m3 Surplus, <4.5–9 Tg P per year; mined input, <16 (8–17) Tg P per year Align with local safe boundary to avoid eutrophication Local and global safe ESBs Surplus, ~10 Tg P per year; mined input, ~17 Tg P per year Atmosphere: aerosol loading Monsoon systems <0.25–0.50 AOD Annual mean interhemispheric AOD difference: <0.15 Align with safe plus <15 μg per m3 mean annual PM2.5; other levels of exposure to significant harm in Supplementary Table11 <15 μg per m3 PM2.5 plus regional and global safe ESBs 0.05 annual mean interhemispheric AOD difference Nature | www.nature.com | 5 the need to restore largely intact natural areas. The exact safe boundary depends strongly on the demand for specific ecological functions (which in turn depend, for example, on the remaining carbon emissions to be sequestered) and on the spatial distribution of the largely intact natural area across ecoregions and ecosystems. Studies generally indicate that up to 60% of the terrestrial earth surface area may be needed, with some extending up to 80% (Supplementary Methods). Natural ecosystem areas comparable with the 50–60% terrestrial ESB are needed in the ocean to maintain carbon sequestration and minimize additional marine species extinction 42 . Biome-scale boundaries may be more stringent: for example, to protect tropical forest biomes due to their contribution to climate stability and moisture recycling. If allocation and coordination of restoration efforts are less than opti - mal, the required minimum area will be larger. If these boundaries are transgressed, tipping points involving loss of biome-scale functional integrity and associated NCP may be triggered, including increases in species extinction rates. Adherence to our proposed safe ESB for the area of largely intact natural ecosystems should minimize harm to future generations (I2b in Box1) by securing biosphere contributions to all life support through a stable and resilient Earth system and localized NCP provided by largely intact nature. However, achieving justice for current generations (I2a and I3 in Box1) may require a stricter boundary because the safe ESB does not account for the current uneven distribution of largely intact natural ecosystems needed to support local livelihoods 43 , especially in poor or Indigenous communities 44,45 . Some people and countries may directly benefit from policies to maintain or increase natural ecosystem area 46 , while others may face opportunity costs 47 . Hence, to ensure just distribution of largely intact natural ecosystems, a just (NSH) boundary may need to be set at the upper end of the 50–60% safe range, as allocation will be less than optimal for achieving the functions the lower boundary was optimized for. We emphasize that natural ecosystem area includes all largely intact natural areas and not only those currently requiring conservation attention; it does not imply protection that excludes human habitation and sustainable use. Functional integrity is the capacity of urban, agricultural or other human-modified ecosystems to provide ecological functions and their contributions to people at landscape scale, complementing the Earth system NCP provided by large-scale intact natural ecosystem areas. We analyse what minimum amount, quality and distance of natural habitat and seminatural habitat are needed to maintain local terrestrial NCP provision, including pollination, pest and disease control, water quality regulation, soil protection, natural hazards mitigation and recreation. We identify that at least 20–25% diverse seminatural habitat including native species in each square kilometre in human-modified lands is needed to support the provisioning of multiple local NCP48. The exact amount and quality required differ based on landscape type, climate and topography; the amount can range up to 50% in some landscapes vulnerable to natural hazards, such as steep slopes or highly erodible soils. This boundary applies to fine scales, currently proposed as 1 km2, because NCP are not transferable (for example, erosion or landslide can only be avoided by natural cover on the same slope) and are often provided or supported by non-mobile or limited mobility species (for example, foraging ranges of pollinating or pest-regulating insects are limited to a few hundred metres). About two thirds of human-dominatedland area (approximately 40% of total land area)has insufficient functional integrity (Supplementary Methods), and large areas are showing symptoms of resilience loss 49 , requiring regenerative practices to restore local and Earth system functions. The safe boundary for functional integrity reduces future exposure to significant harm (intergenerational justice). Loss of functional integrity in agricultural ecosystems and cities below the safe boundary would reduce food productivity, ecosystem capacity to mitigate natural hazards, pollution and nutrient losses and increase reliance on harmful pesticides and biocides and capacity to choose alternate land uses (intragenerational justice). The dependence on these services is 50 75 100 125 150 175 1.00 0 500 1,000 1,500 2,000 0123 Exposed population (millions of people) Mean annual temperature Wet bulb temperature Sea-level rise (2100) Sea-level rise (multicentury) 2.001.751.501.25 Global mean surface temperature change (°C) Fig. 2 | Exposure to significant harm from climate change at different levels of warming. We examine the exposure of the 2010 global population to mean annual temperatures above 29 °C (purple; linear fit, P < 0.01), wet bulb temperatures of 35 °C for an average of at least 1 day per year (orange; quadratic fit, P < 0.01) and future sea-level rise (blue; linear interpolation). Sea-level rise is calculated for 2100 (blue solid) and multi-centennial (blue dashed; linear interpolation) responses to a given temperature stabilization by 2100, representing near-term impacts and long-term equilibria, respectively. The inset shows the magnification of wet bulb temperature in the range 1–2 °C. Shading indicates one s.e. 6 | Nature | www.nature.com Article often higher in regions with more vulnerable communities. Specific interventions that secure functional integrity are highly local and are best implemented under local authority, knowledge and leadership 50 , with policy interventions often needed to ensure that marginalized groups are not further disempowered but are given the space to use their knowledge and approaches to participate in such processes51. Water For fresh water, we propose two spatially defined safe ESBs based on subglobal boundaries that can be aggregated to the global scale: (1) a flow alteration ESB for surface water and (2) a drawdown ESB for groundwater (Table1). Flow alteration in rivers is one of the key drivers of freshwater biodiversity loss52, leading to declines in freshwater biodiversity that outpace those of terrestrial and marine systems 53 and in large-scale NCP, such as coastal and inland fisheries, on which millions of people depend 54,55 . Local-scale flow-ecology analyses are often used to establish environmental flow needs to define safe levels of flow alteration for individual watersheds 56 . These local-scale assessments could provide the basis for spatially explicit safe boundaries but are absent across most of the world57. In their absence, we propose that a presumptive subglobal safe ESB of 20% alteration (increase or decrease) of monthly surface water flows compared with the prevailing natural flow regime be met in all rivers globally (medium confidence in Extended Data Table1). This ESB leaves 80% of flows unaltered to meet environmental needs 58,59 , assuming that required water quality standards are also met. The ESB is supported by empirical studies showing that flow alterations within 20% support native fish species and flow alteration beyond this level strongly affects biodiversity and ecosystem structure and function60,61 (Supplementary Methods has additional references supporting the use of this threshold). The global ESB for surface water is that 100% of all land area meets the subglobal boundary by limiting alterations of flows by 20% in all rivers in the world. Meeting the global ESB sums to a global alteration budget of 7,630 km3per year (Supplementary Methods; with high confidence in Extended Data Table1). Globally aggregated river flow alterations are currently less than this figure; however, we are outside the global ESB because the subglobal safe ESB is only met for 66% of land area (Table1) and less than half of the global population (Supplementary Methods). These results are consistent with recent analyses of water scarcity, which highlight the challenge of meeting environmental flow requirements to support ecosystem services, such as fisheries production, while ensuring there is sufficient water for human needs57,62. Groundwater aquifers contribute to base flows in many river systems and directly sustain wetlands and terrestrial vegetation. Unsafe levels of groundwater extraction occur when drawdown exceeds replenishment rates, impacting groundwater-dependent ecosystems and in some instances, leading to land subsidence and irreversible aquifer loss12,63,64. Given the temporal nature of groundwater recharge and discharge and a lack of widespread consistent data on historical aquifer levels, we propose that the safe ESB for annual groundwater drawdown for all aquifers be the average annual recharge, with groundwater considered safe if drawdown is less than recharge. The subglobal safe ESB is met for a given aquifer when local drawdown does not exceed average annual recharge. The global ESB for groundwater is that the subglobal ESB is met for all aquifers around the world. For the 2003–2016 period, the global sum of average annual recharge is approximately 16,000 km 3 per year (Table1 and Supplementary Methods; with high confidence in Extended Data Table1). The groundwater extraction that may safely occur within this boundary naturally varies across the planet and, where possible, should be defined based on local-scale monitoring, although broad trends can also be determined via satellite remote sensing 65 . We estimate that we are currently outside the global ESB because groundwater levels in 47% of basins are currently in decline (Table1). Our justice analysis of the safe ESBs for surface and groundwater highlights the challenges of (1) multi-level distribution, (2) water insecurity and (3) water quality. The regional surface and groundwater ESBs are generally in the long-term interests of surrounding communities, as they conserve future fresh water (intergenerational justice: I2b in Box1). Where depleted aquifers have already caused significant environmental impacts 66 , groundwater extraction should urgently be reduced, and recharge areas should be protected to restore aquifers to safe levels (NSH to present generations: I2a and I3 in Box1). Minimizing significant harm to current generations also requires the following. (1) Accounting for multi-level distribution indicates the allocation of allowed alterations between communities, sectors or nations sharing the water body, whether directly or indirectly via virtual water. This allocation is particularly challenging where the safe ESB requires drastic reductions in water use. (2) Minimizing exposure to significant harm should account for water insecurity in different regions of the world. For example, harm associated with poor water sanitation and hygiene conditions disproportionately impacts the health of young children in low-income countries67, particularly in Sub-Saharan Africa and South Asia 68 . (3) Minimizing exposure to significant harm implies addressing surface water quality guidelines for human use69, not just an allocation of water quantity. At a minimum, water needs to be safe for consumption and irrigation, meaning that acceptable standards for faecal coliforms and salinity must be met. We align our just (NSH) ESBs for water with the safe ESBs while noting that adhering to the boundaries would considerably restrict current use and will require policies to ensure distributive justice. These proposed surface and groundwater ESBs are independent of green water stocks. Green water stocks are critical for maintaining the atmospheric water cycle, which regulates seasonal precipitation levels 34 ; can support a significant proportion of global agricultural production 70 with less impact on aquatic ecosystems than blue water use71; and are closely related to the biosphere ESBs. A recent assessment 38 proposed a spatially explicit green water boundary to ensure hydrological regulation of terrestrial ecosystems, climate and biogeochemical processes by defining a maximum allowed deviation (drying or wetting) of soil moisture levels from mid-Holocene conditions. The state variable for green water is defined as the percentage of ice-free land area that in any month has root-zone soil moisture levels outside the 95th percentile of the local baseline variability. The boundary value is set at 10%, corresponding to the median departure level from mid-Holocene conditions. We include this green water boundary in our set of safe ESBs (Table1), but we limit our interand intragenerational justice analysis (I2 and I3 in Box1) to surface and ground blue water. Nutrients We set safe ESBs for agricultural nitrogen (N) and phosphorus (P) surpluses for minimizing eutrophication of surface water and terrestrial ecosystems due to runoff, leaching and atmospheric N deposition via ammonia and nitrogen oxide emissions (Table1). We propose safe global-scale ESBs of 61 (35–84) Tg N per year for agricultural nitrogen surplus72 and 4.5–9.0 Tg P per year for cropland soil phosphorus surplus73,74 (medium confidence in Extended Data Table1). These ESBs are based on recent papers 72,74 calculating subglobal and global agricultural nutrient losses, surpluses and inputs from critical N and P concentrations in water and air beyond which eutrophication occurs (Methods, Table1 and Supplementary Methods). These ESBs primarily relate to agriculture, which accounts for approximately 90% of anthropogenic N/P inputs to the Earth system72,75. Our ESBs are based on agricultural surpluses and losses 72,74 , although for comparison with previous PB quantifications (Supplementary Methods), we also provide corresponding global inputs assuming current N/P use efficiency. These recent studies also account for non-agricultural sources, assuming they remain at current levels, and the redistribution of nutrients from over-fertilized to under-fertilized regions (Supplementary Methods). Nature | www.nature.com | 7 Elevated N and P concentrations cause harm through the consequences of eutrophication on ecosystems and their services, such as fishery collapse, toxic compounds released by algal blooms 72,76 and the health impacts of air pollution from ammonia-derived aerosols77. Harm can also occur from drinking surface or groundwater with elevated nitrate concentrations78 but at a higher level than the safe N concentration for surface water eutrophication. We therefore align the just (NSH) ESBs for subglobal N and subglobal and global P with their safe boundaries, as human harm from nutrient cycle disruption is primarily driven by environmental degradation. Accounting for significant harm from groundwater nitrate tightens the global N boundary slightly to 57 (34–74) Tg N per year (Supplementary Methods). These ESBs should be complemented by standards for local air and water pollution for N and water pollution for P. Additional justice considerations include lack of access to N and P fertilizers, which can threaten food security especially for low-income communities and countries 76 , and extraction of phosphate rock, which is a limited resource currently underpinning food production but exposes poor and marginalized communities to mining waste, destroyed land and human rights abuses76,79. Aerosol pollution For aerosols, we propose a safe ESB defined by the interhemispheric difference in aerosol optical depth (AOD) (Table1) based on evidence that a rising North/South Hemisphere difference can trigger regional-scale tipping points and cause substantial adverse effects on regional hydrological cycles, in addition to the existing PB of 0.25–0.50 AOD based on regional considerations 27 . We consider AOD differences and their potential impacts arising from natural emissions, anthropogenic emissions and stratospheric aerosol injection (solar geoengineering). Observational data for the West African monsoon rainfall80 and climate modelling studies for the Indian monsoon81 have identified potential shifts in the location of the Intertropical Convergence Zone triggered by differences in sulfate AOD between the Northern and Southern Hemispheres 81 . Observational studies on the impacts of interhemispheric AOD difference on the Indian monsoon are lacking, but observations based on past volcanic eruptions and climate modelling studies show that an increased concentration of reflecting aerosols in one hemisphere leads to precipitation decreasing in the same hemisphere’s tropical monsoon regions while increasing in the opposite hemisphere80,82,83. Observed changes in the South Asian monsoon have well-understood mechanisms (Supplementary Information) that are consistent with the effects of interhemispheric AOD difference84. The volcanic eruptions of El Chichon in the 1980s (AOD difference of 0.07) and Katmai (AOD difference of 0.08) provide empirical examples80, while model-simulated AOD differences of 0.1 and approximately 0.2 lead to declining precipitation in tropical monsoon regions85. Interhemispheric AOD difference and its impact on shifts in tropical precipitation are sensitive to the aerosol particle size and the latitudinal and altitudinal distribution of reflecting aerosols 86 . Considering this and the range of these studies (approximately 0.05–0.20 of additional AOD difference), we assess that these shifts may become disruptive if the interhemispheric AOD difference, currently approximately 0.0587 on average and approximately 0.1 in the boreal spring and summer 87 , exceeds 0.15 (low confidence in Extended Data Table1) due to air pollution 85 or geoengineering-related aerosol asymmetries 81,85 (Supplementary Methods). Significant harm to human health from exposure to aerosols, such as particulate matter (PM), suggests a more stringent just (NSH) boundary based on local air pollution standards 88 . PM and other aerosols are associated with respiratory illnesses and premature deaths as well as heart problems and debilitating asthma89. We select a just (NSH) boundary of 15 μg per m 3 mean annual exposure to PM 2.5 to avoid a high likelihood of significant harm from aerosols (Table1 and Supporting Information) based on World Health Organization Number of boundaries transgressed 0 1 2 3 4 5 6 7 Fig. 3 | Hotspots of current ESB transgressions. The number of subglobal climate (two local exposure boundaries), functional integrity, surface water, groundwater, nitrogen, phosphorus and aerosol safe and just ESBs currently transgressed by location. No more than seven of these eight metrics have their ESBs transgressed in any one pixel. Since climate is a globally defined ESB, we use wet bulb temperatures of over 35 °C for at least 1 day per year and lowelevation coastal zones (<5 m) exposed to sea-level rise as proxies for local climate transgression while acknowledging that the impacts of climate change are far more diverse. We also emphasize that exposure of a location does not necessarily imply responsibility for causing or addressing these environmental impacts. We invite the reader to investigate the consequences of different boundary values using the code in the code availability information. 8 | Nature | www.nature.com Article 202188 guidelines (Table1) and European Union and US Environmental Protection Agency air quality standards 90,91 . Such local and regional guidance is needed because PM 2.5 characteristics, such as toxicity, are highly place and source specific. Eighty-five percent of the world population is currently exposed to PM 2.5 concentrations beyond this boundary92, and exposure to ambient PM2.5 is estimated to cause 4.2 million deaths annually89, with vulnerable groups being affected disproportionately more while polluting less93. Air pollution scenarios based on globally successful stringent mitigation and pollution control show reductions in affected populations, but areas of high air pollution might remain 94 . A 15 μg per m 3 PM 2.5 concentration translates 95,96 to an AOD of approximately 0.17, indicating that the just (NSH) boundary for aerosols is more stringent than the safe regional boundary (0.25–0.50) (Table1). Novel entities and other pollutants We acknowledge the risks to Earth system stability and human wellbeing from other air and water pollutants, for which there are already well-accepted guidelines 88 , and the emerging threats from novel entities, new forms of existing substances and modified life forms that are geologically or evolutionarily novel and could have large-scale unwanted geophysical or biological impacts on the Earth system 27,97 . Evidence on the diverse risk potentials of novel entities, such as microplastics, ‘forever chemicals’, antibiotics, radioactive waste, heavy metals or other emerging contaminants, for Earth system function and human health and food security is increasing, but knowledge gaps on the scale and scope of potential impacts remain 98 . Persson etal. 97 reported that humanity has crossed the PB for novel entities, although data limitations and quantification are challenging even for the known novel entities. The differentiated impacts of novel entities already witnessed today across different populations and the long lifetimes of these substances raise clear intragenerational and intergenerational justice concerns97,98. Current state Seven of the eight global-scale safe and just ESBs that we quantified have already been crossed (Fig.1 and Table1). Transgression of ESBs is spatially widespread, with two or more safe and just ESBs transgressed for 52% of the world’s land surface, affecting 86% of the global population (Fig.3). Some communities experience many ESB transgressions, with four or more ESBs transgressed for 28% of global population but only 5% of global land surface (Fig.3). Spatial hotspot transgressions are therefore concentrated in regions of higher population density, raising major intragenerational justice concerns. Toward a safe and just future We defined and quantified safe and just (NSH) ESBs for sustaining the global commons that regulate the state of the planet, protect other species, generate NCP, reduce significant harm to humans and support inclusive human development (Fig.1 and Table1). Because exceeding safe boundaries results in widespread significant harm, our just and safe ESBs align for surface water, groundwater, functional integrity, natural ecosystem area, phosphorus and nitrogen. Meeting these boundaries without transformation, however, could significantly harm current generations. In two cases, aerosols and climate, the just boundaries are more stringent than the safe boundaries, which indicates that people experience significant harm before that Earth system domain is destabilized. We identified subglobal ESBs, which, in many domains, are the relevant scale for action to avoid loss of Earth system stability and minimize exposure to significant harm, and global ESBs, which are reference points for monitoring human impacts at the Earth system scale. Nations, cities, businesses and other key actors need to set and achieve science-based targets for reducing their environmental impacts based on translation of the safe and just ESBs to actor fair shares 99 . Climate is the only ESB that has a relatively well-established and implemented methodology100,101, with methodologies for other domains under development101,102. We emphasize that our ESBs complement, not over-ride, environmental restrictions for specific local settings: for example, stricter biosphere boundaries for carbon-dense ecosystems or targeted conservation efforts for protecting endangered or emblematic species. We also acknowledge that other actors may choose to implement targets based on other likelihood levels than those we have highlighted (Fig.1 and Table1): for example, a lower risk tolerance than the high risk of passing tipping points associated with a 1.5 °C safe boundary. We offer our ESBs as an integration of social and natural sciences for further refinement, in the spirit that the PBs were proposed over a decade ago 103 . Seven of the eight globally quantified ESBs have been crossed and at least two local ESBs in much of the world have been crossed, putting human livelihoods for current and future generations at risk. Nothing less than a just global transformation across all ESBs is required to ensure human well-being. 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