Safe and just Earth system boundaries

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. Climate is the only ESB that has a relatively well-established and implemented methodology, with methodologies for other domains under development. 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. 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. Such transformations must be systemic across energy, food, urban and other sectors, addressing the economic, technological, political and other drivers of Earth system change, and ensure access for the poor through reductions and reallocation of resource use. All evidence suggests this will not be a linear journey; it requires a leap in our understanding of how justice, economics, technology and global cooperation can be furthered in the service of a safe and just future.