Table 1. Proposed planetary boundaries. Earth System process Control variable Threshold avoided or influenced by slow variable Planetary Boundary (zone of uncertainty) State of knowledge* Climate change Atmospheric CO2 concentration, ppm; Energy imbalance at Earth’s surface, W m-2 Loss of polar ice sheets. Regional climate disruptions. Loss of glacial freshwater supplies. Weakening of carbon sinks. Atmospheric CO2 concentration: 350 ppm (350–550 ppm) Energy imbalance:+1 W m-2 (+1.0–+1.5 W m-2) 1. Ample scientific evidence. 2. Multiple sub-system thresholds. 3. Debate on position of boundary. Ocean acidification Carbonate ion concentration, average global surface ocean saturation state with respect to aragonite (Ωarag) Conversion of coral reefs to algal-dominated systems. Regional elimination of some aragonite- and high-magnesium calcite-forming marine biota Slow variable affecting marine carbon sink. Sustain ≥80% of the pre-industrial aragonite saturation state of mean surface ocean, including natural diel and seasonal variability (≥80%–≥70%) 1. Geophysical processes well known. 2. Threshold likely. 3. Boundary position uncertain due to unclear ecosystem response. Stratospheric ozone depletion Stratospheric O3 concentration, DU Severe and irreversible UV-B radiation effects on human health and ecosystems. <5% reduction from pre-industrial level of 290 DU (5%–10%) 1. Ample scientific evidence. 2. Threshold well established. 3. Boundary position implicitly agreed and respected. Atmospheric aerosol loading Overall particulate concentration in the atmosphere, on a regional basis Disruption of monsoon systems. Human-health effects. Interacts with climate change and freshwater boundaries. To be determined 1. Ample scientific evidence. 2. Global threshold behavior unknown. 3. Unable to suggest boundary yet. Biogeo-chemical flows: interference with P and N cycles P: inflow of phosphorus to ocean, increase compared with natural background weathering N: amount of N2 removed from atmosphere for human use, Mt N yr-1 P: avoid a major oceanic anoxic event (including regional), with impacts on marine ecosystems. N: slow variable affecting overall resilience of ecosystems via acidification of terrestrial ecosystems and eutrophication of coastal and freshwater systems. P: < 10× (10× - 100×) N: Limit industrial and agricultural fixation of N2 to 35 Mt N yr-1, which is ~ 25% of the total amount of N2 fixed per annum naturally by terrestrial ecosystems (25%–35%) P: (1) Limited knowledge on ecosystem responses; (2) High probability of threshold but timing is very uncertain; (3) Boundary position highly uncertain. N: (1) Some ecosystem responses known; (2) Acts as a slow variable, existence of global thresholds unknown; (3) Boundary position highly uncertain. Global freshwater use Consumptive blue water use, km3 yr-1 Could affect regional climate patterns (e.g., monsoon behavior). Primarily slow variable affecting moisture feedback, biomass production, carbon uptake by terrestrial systems and reducing biodiversity <4000 km3 yr-1 (4000–6000 km3 yr-1) 1. Scientific evidence of ecosystem response but incomplete and fragmented. 2. Slow variable, regional or subsystem thresholds exist. 3. Proposed boundary value is a global aggregate, spatial distribution determines regional thresholds Land-system change Percentage of global land cover converted to cropland Trigger of irreversible and widespread conversion of biomes to undesired states. Primarily acts as a slow variable affecting carbon storage and resilience via changes in biodiversity and landscape heterogeneity ≤15% of global ice-free land surface converted to cropland (15%–20%) 1. Ample scientific evidence of impacts of land-cover change on ecosystems, largely local and regional. 2. Slow variable, global threshold unlikely but regional thresholds likely. 3. Boundary is a global aggregate with high uncertainty, regional distribution of land-system change is critical. Rate of biodiversity loss Extinction rate, extinctions per million species per year (E/MSY) Slow variable affecting ecosystem functioning at continental and ocean basin scales. Impact on many other boundaries—C storage, freshwater, N and P cycles, land systems. Massive loss of biodiversity unacceptable for ethical reasons. <10 E/MSY (10–100 E/MSY) 1. Incomplete knowledge on the role of biodiversity for ecosystem functioning across scales. 2. Thresholds likely at local and regional scales. 3. Boundary position highly uncertain. Chemical pollution For example, emissions, concentrations, or effects on ecosystem and Earth System functioning of persistent organic pollutants (POPs), plastics, endocrine disruptors, heavy metals, and nuclear wastes. Thresholds leading to unacceptable impacts on human health and ecosystem functioning possible but largely unknown. May act as a slow variable undermining resilience and increase risk of crossing other thresholds. To be determined 1. Ample scientific evidence on individual chemicals but lacks an aggregate, global-level analysis. 2. Slow variable, large-scale thresholds unknown. 3. Unable to suggest boundary yet.