Safe and just Earth system boundaries

Abstract

The stability and resilience of the Earth system and human well-being are inseparably linked^1-3, yet their interdependencies are generally under-recognized; consequently, they are often treated independently^4,5. 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.

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.

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. Source Data

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 low-elevation 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.

Extended Data Fig. 1. Alternative visualizations of safe and just Earth system boundaries (Fig. 1).

Concentric (a) and parallel (b) visualizations of global (a, inner circle; b, left circle) and sub-global (a, outer circle; b, right circle) safe and just ESBs. Colours are as in Fig. 1. Global rings (a, inner circle; b, left circle) show current global states; a single current state cannot be defined sub-globally. Short concentric lines (that extend across less than the full width of a wedge) represent alternative likelihood levels (safe) or levels of exposure (just NSH) (Table 1).