Level: SL and HL Subtopic 1.3 Sustainability. The planetary boundaries model identifies nine Earth-system processes and proposes limits to human disturbance that define a safe operating space for humanity. According to the 2025 Planetary Health Check, seven of the nine boundaries have been crossed.
The model uses scientific control variables, such as atmospheric carbon dioxide concentration, to assess pressure on each Earth-system process. Remaining within a boundary is associated with the relatively stable conditions of the Holocene epoch, during which human societies developed.
Crossing a boundary increases the risk of abrupt, large-scale, or irreversible environmental change. The boundaries interact as a system: for example, climate change can weaken biosphere integrity, alter freshwater systems, and increase ocean acidification.
| Planetary boundary | Current status in the 2025 assessment |
|---|---|
| Climate change | Crossed |
| Biosphere integrity | Crossed |
| Land-system change | Crossed |
| Freshwater change | Crossed |
| Biogeochemical flows, especially nitrogen and phosphorus | Crossed |
| Novel entities, including synthetic chemicals and plastics | Crossed |
| Ocean acidification | Crossed |
| Atmospheric aerosol loading | Within the global boundary, although regional limits may be exceeded |
| Stratospheric ozone depletion | Within the boundary |
A common misconception is that crossing a boundary means an immediate ecological collapse. Instead, it means entering a zone of increasing risk, where Earth-system resilience declines and tipping points become more likely. The boundaries are also not independent or equally uncertain.
The recovery of stratospheric ozone following the Montreal Protocol demonstrates that coordinated international action can reduce pressure on a boundary. However, responsibility and vulnerability are unequal, creating an environmental justice issue because societies contributing least to environmental disruption may experience severe consequences.
Exam technique: Define the model, identify its nine boundaries, and explain that crossing a boundary increases risk rather than guaranteeing immediate collapse. For an “evaluate” question, discuss both the model’s value as a clear sustainability framework and its limitations as a simplified global model, then give a supported conclusion.