In this framework, the planetary boundary for climate change is typically defined as a “stock” — the concentration of CO₂ in the atmosphere — whereas the nitrogen and phosphorus boundaries are defined as “flows,” such as tons of nutrient pollution released each year. New joint research conducted by KAIST Graduate School of Green Growth & Sustainability and the Joint Global Change Research Institute proposes a unified, flow-based definition of the planetary boundary for climate change and shows that, under a consistent method, climate change has already far surpassed its safe boundary, even more than nitrogen and phosphorus pollution.

The planetary boundaries framework has become a widely used tool in Earth system science, highlighting that Earth’s resilience has clear limits. However, the biogeochemical boundaries for climate change, nitrogen, and phosphorus have not been defined on a consistent basis. A new perspective paper published in Nature Sustainability re-examines these three boundaries using a unified, flow-based metric and finds that, when assessed in the same way, the planetary boundary for climate change is the most heavily exceeded.

In existing planetary boundary assessments, climate change is typically expressed as atmospheric CO₂ concentration, radiative forcing, or global temperature rise — all stock-type indicators that reflect the cumulative buildup of greenhouse gases. In contrast, the nitrogen and phosphorus boundaries are expressed as annual flows of reactive nitrogen (about 62–82 Tg N per year) and phosphorus (about 11–100 Tg P per year) that can be safely released to the environment.

This study redefines the planetary boundary for climate change in analogous, flow-based terms: the annual CO₂ emissions compatible with limiting warming to 1.5°C, given the remaining global carbon budget and assumptions about CO₂ removal. Under plausible assumptions, this translates to a safe range of roughly 4–17 Gt CO₂ per year, whereas current human-caused CO₂ emissions are about 37 Gt CO₂ per year. On this consistent scale, current emissions overshoot the planetary boundary for climate change by approximately 3–4 times. Seen on this consistent scale, the climate change problem is found to be far exceeding the safe boundary compared to nitrogen and phosphorus, highlighting the need to prioritize rapid CO₂ reductions.

“The planetary boundaries framework provides a valuable guide for keeping humanity within a safe operating space on Earth,” said Prof. Haewon McJeon of the KAIST Graduate School of Green Growth and Sustainability. “However, the planetary boundary for climate change has not been measured consistently with the nitrogen and phosphorus boundaries. When this inconsistency is corrected, it becomes clear that climate change is already beyond the safe operating space, underscoring the urgent need to accelerate global decarbonization efforts.”

By expressing climate change, nitrogen, and phosphorus on the same flow-based measurements, this study shows that solving climate change is more urgent than previously estimated by the planetary boundary literature. A more consistent methodology can support clearer communication, better-aligned global priorities, and integrated strategies that tackle climate change and biogeochemical pollutions together.

Paper link: https://doi.org/10.1038/s41893-026-01770-6