In 2009, around 30 international scientists led by Johan Rockström defined nine planetary boundaries that set out the Earth's capacity to absorb strain in key environmental processes. If these boundaries are crossed, the risk of large-scale, abrupt or irreversible environmental change rises.

In 2009 only seven boundaries could be quantified. In 2023, all nine processes were measured for the first time. Results from the Planetary Health Check of 2025 show that seven of the nine critical boundaries have now been crossed. Ocean acidification has newly entered the danger zone. The main cause is the burning of fossil fuels, amplified by deforestation and land-use change.

Only two boundaries remain in the safe zone: aerosol loading and the ozone layer, which has largely recovered. The example of the ozone layer shows that international action and political measures can bring about improvement.

In 2009 a group of around 30 international scientists led by Johan Rockström described nine planetary boundaries. Each names an environmental process whose resilience can be estimated: climate change, biosphere integrity, land-system change, freshwater use, the biogeochemical flows of nitrogen and phosphorus, novel entities such as chemicals and plastics, ocean acidification, stratospheric ozone depletion and atmospheric aerosol loading. In 2009 only seven of these processes could be quantified; in 2023 all nine were measured for the first time.

The Planetary Health Check 2025 concludes that seven of the nine boundaries have been crossed. Ocean acidification was the most recent addition. Only two remain in the safe zone: atmospheric aerosol loading and stratospheric ozone depletion, since the ozone layer has largely recovered. The report names the burning of fossil fuels as the main cause of the crossings, amplified by deforestation and land-use change.

A crossed boundary does not mean sudden collapse. It describes an increased risk of large-scale, abrupt or irreversible environmental change. For most of the nine processes it is not precisely known where the thresholds lie at which a system tips; this is the subject of ongoing research.

The boundaries are not independent of one another. Crossing one often worsens the situation for others: climate change intensifies both ocean acidification and freshwater stress. Conversely, the ozone layer shows that targeted international action can reduce damage and bring a system back into the safe zone.