Heatwave response in quasi-equilibrium versus transient climate scenarios

Future changes in mean climate and extremes have been extensively assessed using model simulations of the 21st century under varying levels of anthropogenic greenhouse gas (GHG) forcing. Here, we examine the long-term climate legacy of an idealized abrupt stabilization of present-day and near-future GHG concentrations, with a focus on summer heatwaves across the Northern Hemisphere. Our analysis is based on multicentennial simulations performed with the EC-Earth3 model, in which external forcing is held fixed in time. After several centuries of internal adjustment, the climate system approaches a quasi-equilibrium state characterized by a stable level of global warming that depends strongly on the timing of forcing stabilization. Crucially, far-future quasi-equilibrium conditions can differ substantially from those that would arise if the same warming levels were reached by the end of the century, reflecting the distinct roles of fast and slow components of the Earth system. A key feature of the quasi-equilibrium response is a partial recovery of the Atlantic Meridional Overturning Circulation relative to transient simulations, which influences regional climate and leads to a pronounced amplification of heatwave frequency and intensity over the North Atlantic sector. Conversely, many land areas ultimately experience less severe heatwaves than in transient scenarios, owing to the slower warming rates in the stabilization experiments. Results show that the long-term response of extremes is shaped by the magnitude of global warming, as well as the pathway and timescale over which that warming is realized, highlighting the need for equilibrium-focused experiments in future climate risk assessments.