Asymmetric Responses of Temperature to Increasing vs Decreasing CO2 Concentrations

Temperature responds asymmetrically to increases versus decreases in atmospheric CO₂ concentrations. Understanding this asymmetry is important for our fundamental knowledge of the climate system and for projecting temperature responses to negative emission scenarios. Here we use CESM2, a fully coupled ocean-atmosphere Earth system model, to simulate the response of temperature to a period of increasing CO₂ concentrations followed by a period of prescribed decreasing concentrations. CESM2 exhibits a pronounced hemispheric contrast in temperature reversibility, with persistent warming in the Southern Hemisphere and an over-recovery of temperature in the Northern Hemisphere following CO₂ removal. The Southern Hemisphere response is broadly consistent with CDRMIP simulations from other models, which similarly show that temperatures remain elevated after a reduction in CO₂ concentrations. In contrast, models disagree on the sign and magnitude of temperature reversibility in the Northern Hemisphere, particularly in the high northern latitudes. This work investigates the mechanisms responsible for persistent Southern Hemisphere warming and explores the sources of inter-model disagreement in Northern Hemisphere temperature recovery. This work will help clarify the reversibility of forced temperature changes and assist in setting expectations for carbon dioxide removal strategies.