Earth System process
Threshold avoided or influenced by slow variable
Planetary boundary (zone of uncertainty)
State of knowledge
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.
Carbonate ion concentration, average global surface ocean saturation state with respect to aragonite (Ωarag).
Conversion of coral reefs to algaldominated systems. Regional elimination of some aragonite- and high-magnesium calcite-forming marine biota Slow variable affecting marine carbon sink.
Sustain ≥ 80 % of the preindustrial 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 preindustrial level of 290 DU (5 - 10 %).
1. Ample scientific evidence. 2. T hreshold 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 behaviour unknown. 3. Unable to suggest boundary yet.
Nitrogen and phosphorus inputs to the biosphere and oceans
P: inflow of phosphorus to ocean, increase compared to 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 behaviour). Primarily slow variable affecting moisture feedback, biomass production, carbon uptake by terrestrial systems and reducing biodiversity
< 4,000 km3 yr-1 (4,000 - 6,000 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 & 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.
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.
For example, emissions, concentrations, or effects on ecosystem and Earth system functioning of persistent organic pollutants (POPs), plastics, endocrine disruptors, heavy metals, and nuclear wastet.
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 threshold.
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.
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