Heatwave Differences Between Overshoot and Non-Overshoot Conditions Under Identical Emissions and Warming

We analyze regional warming and extreme heat under a CO₂-overshoot pathway using eight CESM2 ensemble members forced by an emission-based experiment. The experiment prescribes a rise and decline in anthropogenic CO₂ emissions, allowing the selection of two climate states—one during overshoot and one outside overshoot—with nearly identical global emissions and global-mean surface temperature (GMST). This design provides a controlled framework to assess whether regional climate responses depend solely on the global mean state or also on the temporal sequence of forcing.

Despite matching GMST, the spatial distribution of near-surface warming differs substantially between the two states. During the overshoot period, temperatures are lower across most Northern Hemisphere land areas and higher over portions of the Southern Hemisphere compared with the non-overshoot state, producing net cooling across most global land regions. These differences are reflected in the behavior of extreme heat is generally reduced during overshoot relative to the non-overshoot state, consistent with the altered surface warming pattern.

Analysis of energy-budget components indicates that these spatial contrasts arise from asymmetric sea-ice responses between the Arctic and Antarctic. Differences in ice-sheet and sea-ice behavior modify ocean heat uptake and lead to distinct regional warming patterns under otherwise similar global forcing levels.

These results highlight that overshoot and non-overshoot climates with identical emissions and GMST can yield different regional warming and extreme-heat responses, indicating limited reversibility of regional climate impacts along overshoot pathways.