Hemispheric contrast in summer season duration responses to CO2 removal

As global warming accelerates, questions arise about whether the climate system can revert to its original state as CO2 concentrations decrease. Delayed summer season recovery has important implications for heatwaves, wildfires, and socio-environmental risks. Using multiple climate models with CO2 ramp-up and ramp-down simulations, here we show that summer season duration in extratropical lands exhibits hysteretic responses, with longer summers during CO2 decrease than increase. Much stronger hysteresis is found in the Southern Hemisphere than the Northern Hemisphere and the associated mechanisms are identified using an inter-model correlation analysis. Delayed Southern Ocean sea surface cooling is a dominant factor controlling Southern Hemisphere summer season hysteresis. In contrast, Northern Hemisphere hysteresis is affected by the Atlantic Meridional Overturning Circulation strength and Southern Ocean sea surface temperature with comparable magnitudes. A similar hemispheric contrast in extreme hot day frequencies reflects global-scale Southern Ocean impact and Northern Hemisphere-confined Atlantic Meridional Overturning Circulation influence. Summer lasts longer during periods of declining CO2 than rising CO2, with this hysteresis effect especially pronounced in the Southern Hemisphere, according to climate model simulation analysis.