Regional surface climate irreversibility under temperature overshoot scenarios

With the current rate of climate change, exceeding the remaining global carbon budget for a warming target of 1.5 °C becomes increasingly likely. Hence, attention has been put towards carbon dioxide removal (CDR) strategies that, given the required socio-economic feasibility and sustainability, allow temporary emission overshoots. The balance between the potential viability of overshoot scenarios and the associated risks depends on their characteristics and how the global ecosystems respond during and after the overshoot. Here we investigate the global climate and regional ecosystem responses to emission-driven overshoot scenarios of different magnitude, duration and timing, imposing the potential for irreversible changes. Our analysis focuses on the behavior and reversibility of thermodynamic, hydrological, and biogeochemical processes and their impacts on a regional scale. Based on results from state-of-the-art Earth system models, physically driven mechanisms appear to be mostly reversible for moderate overshoot scenarios considering a response lag in the range of decades. However, feedbacks in high overshoot scenarios and biogeochemical processes show signs of irreversibility on a larger spatial scale – some of which have been brought into connection with the crossing of tipping points for certain elements of the Earth system. Our analysis informs about the reversibility of climate system processes, which allows refining thresholds for global climate change mitigation policies. Furthermore, the aspect of partial reversibility of temperature overshoot scenarios might not be the main concern, but rather the impacts and risks occurring during the periods of elevated temperatures during the overshoot.