Permafrost carbon release scales linearly with overshoot warming mediated by AMOC tipping.

Abstract

Overshooting global warming targets risks irreversible Earth system changes. However, uncertainties remain in how critical Earth system components such as permafrost and the Atlantic Meridional Overturning Circulation (AMOC) respond to and interact under warming. By using three climate models of varying complexity to assess the permafrost carbon response under idealized climate mitigation and overshoot scenarios, we here show that permafrost loses 11-21 PgC of carbon per 100 degree-years of warming exposure (cumulative warming over 100 years) during temperature overshoot in a robust linear relationship. This relationship also holds true under relative Northern Hemisphere cooling from a temporary AMOC slowdown during temperature overshoot. This relative cooling partially offsets climate change impacts on permafrost, while in itself causing adverse impacts on climate and society. These results underscore the importance of including both destabilizing and stabilizing Earth system feedbacks when assessing overshoot impacts, critical for informing carbon budgets, net-zero planning, and climate change reversibility.