Nine processes regulate the stability, resilience, and life-support functions of the Earth system. Seven have now been breached, ocean acidification for the first time in 2025.
Nine Earth system processes regulate planetary stability, resilience, and life-support functions, each tracked by one or two control variables. Wedge length shows how far each has moved beyond the safe operating space; softened edges mark variables whose current value or risk zone is not precisely quantified. Values from the Planetary Health Check 2025, Potsdam Institute for Climate Impact Research. DOI 10.48485/pik.2025.017, CC BY 4.0.
The planetary boundaries framework, first proposed in 2009, identifies the Earth system processes that have kept the planet in the unusually stable conditions of the Holocene — the eleven thousand years during which agriculture and civilisation developed.
For each process it defines a boundary: a value beyond which the risk of destabilising change rises materially. The boundaries are set conservatively, at the lower end of scientific uncertainty, precisely because the consequences of crossing them are poorly reversible.
Crossing a boundary does not mean catastrophe has occurred. It means the system has left the range in which its behaviour is well understood, and the probability of abrupt or irreversible change has increased. The framework is a risk assessment, not a prediction.
Climate change — Atmospheric CO₂ has reached 423 ppm against a boundary of 350. Radiative forcing is at +2.97 W/m², well beyond the +1.5 high-risk threshold.
Change in biosphere integrity — Extinction rates run at 100 to 1,000 times the background rate. Human appropriation of net primary production stands near 30%, against a boundary of 10%.
Modification of biogeochemical flows — The most severely transgressed boundary. Industrial nitrogen fixation runs at 165 Tg per year against a boundary of 62; phosphorus at 18.2 against 6.2.
Land-system change — Forest cover stands at 59% of its original extent, below the 75% boundary.
Freshwater change — Both blue water, in rivers and reservoirs, and green water, in soil moisture, show disturbance across roughly 22% of global land area.
Ocean acidification — Crossed for the first time in 2025. Aragonite saturation has fallen to 2.84 against a boundary of 2.86, with shell damage already recorded in polar and coastal waters.
Introduction of novel entities — Synthetic chemicals, plastics, and engineered materials released faster than their effects can be assessed. Transgressed, though the risk zone is not yet quantifiable.
Atmospheric aerosol loading — Within the safe operating space, and improving as air quality regulation takes effect.
Stratospheric ozone depletion — Within the safe operating space and recovering, under the Montreal Protocol.
Stratospheric ozone is the framework’s proof of concept. A planetary boundary was approached, the mechanism was identified, an international agreement was reached in 1987, production of the responsible chemicals was phased out, and the trend reversed.
It took two decades from first observation to binding agreement, and recovery will take decades more. But it happened. Any argument that planetary-scale environmental problems are inherently unsolvable has to account for the ozone layer.
My work engages most directly with the boundaries on climate, freshwater, and biogeochemical flows: