Irreversibility of regional climate in overshoot scenarios

As global emission reductions and commitments fall short of what would be required to limit global temperature increases below the targets agreed upon in the 2015 Paris Agreement, so-called overshoot scenarios have gained attention for climate policy and modelling. These pathways describe climate trajectories where global temperatures temporarily exceed the 1.5K or 2K global warming thresholds before being brought back down through massive, delayed carbon removal efforts later this century. It is unclear, however, to what extent the climate states at the same global warming levels before and after overshoot are comparable, and whether the climate is dependent on the trajectory of how it reached a certain global temperature level. To understand this trajectory-dependence of regional climate conditions, and in particular climate extremes, we analysed a set of overshoot scenario simulations that were produced as part of the Coupled Model Intercomparison Project Phase 6 (CMIP6). The results show that the ensemble average of simulations show large-scale hemispheric temperature asymmetries, at the same global average temperature before and after the overshoot. These large-scale asymmetries cause shifts in atmospheric circulation and precipitation patterns, and hysteresis of regional temperature extremes. For example, while hot temperature extremes in the northern hemisphere extratropics overcompensate their pre-overshoot increases (i.e. become slightly cooler than at equivalent pre-overshoot levels) when decreasing atmospheric CO2 concentrations, the temperature extremes in the southern hemisphere extratropics are not fully reversed at same global temperature levels after the overshoot.