Safe and just Earth system boundaries

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. 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 rainfall and climate modelling studies for the Indian monsoon have identified potential shifts in the location of the Intertropical Convergence Zone triggered by differences in sulfate AOD between the Northern and Southern Hemispheres. 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 hemisphere. Observed changes in the South Asian monsoon have well-understood mechanisms (Supplementary Information) that are consistent with the effects of interhemispheric AOD difference. The volcanic eruptions of El Chichon in the 1980s (AOD difference of 0.07) and Katmai (AOD difference of 0.08) provide empirical examples, while model-simulated AOD differences of 0.1 and approximately 0.2 lead to declining precipitation in tropical monsoon regions. 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. 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 0.05 on average and approximately 0.1 in the boreal spring and summer, exceeds 0.15 (low confidence in Extended Data Table 1) due to air pollution or geoengineering-related aerosol asymmetries (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. PM and other aerosols are associated with respiratory illnesses and premature deaths as well as heart problems and debilitating asthma. We select a just (NSH) boundary of 15 μg per m³ mean annual exposure to PM2.5 to avoid a high likelihood of significant harm from aerosols (Table 1 and Supporting Information) based on World Health Organization 2021 guidelines (Table 1) and European Union and US Environmental Protection Agency air quality standards. Such local and regional guidance is needed because PM2.5 characteristics, such as toxicity, are highly place and source specific. Eighty-five percent of the world population is currently exposed to PM2.5 concentrations beyond this boundary, and exposure to ambient PM2.5 is estimated to cause 4.2 million deaths annually, with vulnerable groups being affected disproportionately more while polluting less. 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. A 15 μg per m³ PM2.5 concentration translates 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 well-being from other air and water pollutants, for which there are already well-accepted guidelines, 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. 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. Persson et al. 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 concerns.

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.