Irreversibility and hysteresis in regional temperature extreme frequency under net-negative CO2 emissions

Currently implemented emission reduction policies are projected to result in anthropogenic warming of the climate system beyond the 1.5 ºC and 2ºC temperature targets of the Paris Agreement. Consequently, achieving the Agreement objectives may only be possible following a period of ‘overshoot’, where warming temporarily exceeds either of these targets, before later declining and stabilising below them through net-negative CO₂ emissions. Whilst previous studies have illustrated that global mean surface temperature rise is reversible under net-negative CO₂, it remains unclear whether other human-induced climate impacts will exhibit the same degree of reversibility. This study assesses the irreversibility and hysteresis behaviour of regional temperature extreme frequencies under net-negative CO₂ emissions. We analyse the results of eight Earth System Models that have completed the Carbon Dioxide Removal Model Intercomparison Project (CDRMIP) Tier 1 experiment, in which atmospheric CO₂ concentrations follow a symmetric 1% per year rise and decline between their preindustrial level and up to quadruple this value. For equivalent global warming levels reached through periods of positive and negative emissions respectively, we observe a high degree of hysteresis and short-term irreversibility in extreme temperature frequency across most land and ocean regions, with the sign of such changes displaying a general hemispheric asymmetry. Whilst much of this behaviour can be attributed to ongoing thermal inertia, our results suggest non-linearities in large-scale climate components, such as the Atlantic Meridional Overturning Circulation and El Niño-Southern Oscillation, may contribute to centennial-scale, irreversible changes in regional temperature extreme frequency under net negative CO₂ emissions.