Safe and just Earth system boundaries

Humanity is well into the Anthropocene, 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 it. These rapid changes to the Earth system undermine critical life-support systems, with significant societal impacts already felt, and they could lead to triggering tipping points that irreversibly destabilize the Earth system. 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 system, an assessment of safe and just boundaries is required that accounts for Earth system resilience and human well-being in an integrated framework.

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’ 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”. 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 resilience. 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); ‘intergenerational justice’ between past and present generations (I2a) and present and future generations (I2b); and ‘intragenerational justice’ (I3) between countries, communities and individuals through an intersectional lens. 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. 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. 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 justice. 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 exposed. 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 and distributional and procedural fairness. 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 time (Fig. 1).

Our assessment builds upon and advances beyond previous research and science-based political consensus, such as the Planetary Boundaries (PBs) framework, doughnut economics and the Sustainable Development Goals 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 are not quantified in comparable units or examine only the consequences of human activities on the Earth system, not 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 biosphere and fresh water. (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 variability.

Further explanation is in Gupta et al.. 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.