Safe and just Earth system boundaries

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 loss, leading to declines in freshwater biodiversity that outpace those of terrestrial and marine systems and in large-scale NCP, such as coastal and inland fisheries, on which millions of people depend. Local-scale flow-ecology analyses are often used to establish environmental flow needs to define safe levels of flow alteration for individual watersheds. These local-scale assessments could provide the basis for spatially explicit safe boundaries but are absent across most of the world. 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, 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 function (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 km³per 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 needs.

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 loss. 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 km³ 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. 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, 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 countries, particularly in Sub-Saharan Africa and South Asia. (3) Minimizing exposure to significant harm implies addressing surface water quality guidelines for human use, 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; can support a significant proportion of global agricultural production with less impact on aquatic ecosystems than blue water use; and are closely related to the biosphere ESBs. A recent assessment 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 inter- and 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 surplus and 4.5–9.0 Tg P per year for cropland soil phosphorus surplus (medium confidence in Extended Data Table 1). These ESBs are based on recent papers 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 system. Our ESBs are based on agricultural surpluses and losses, 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).

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 and the health impacts of air pollution from ammonia-derived aerosols. Harm can also occur from drinking surface or groundwater with elevated nitrate concentrations 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, 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 abuses.