Irreversible climate changes driven by degree-years of temperature overshoot

It is becoming increasingly likely that meeting the long-term temperature goal of the Paris Agreement will involve a period of temperature overshoot. Understanding the differences in climate outcomes between overshoot and non-overshoot scenarios requires an assessment of the reversibility of climate changes along an overshoot pathway. Using an intermediate-complexity Earth system model, we quantify the reversibility of a suite of climate variables and investigate the factors driving differences in reversibility across an ensemble of 42 pairs of overshoot and non-overshoot scenarios. For highly irreversible climate variables like permafrost and ocean changes, we show that the long-term outcome is linearly related to the time-integrated overshoot magnitude, which we define here as the degree-years of temperature overshoot. Our results show that degree-years of overshoot can be used to predict the changes of a range of irreversible ocean and permafrost variables in overshoot scenarios, therefore offering important insights into the difference in climate outcomes of overshoot compared to non-overshoot scenarios.