Impact of Spatial Inhomogeneity of Atmospheric CO₂ Concentration on Surface Air Temperature Variations

Atmospheric CO₂ concentration is characterized by spatial inhomogeneity and seasonal variability, which affect the variation of surface air temperature (SAT). How does SAT respond to the inhomogeneity of CO₂ concentration across the globe and East Asia remains elusive. In this paper, the climate model BCC-CSM2-MR is used to investigate the differences of global surface air temperature (SAT) in response to spatial inhomogeneous distributions of CO₂ concentration, based on three historical experiments (Hist_1dCO2, Hist_2dCO2, Hist_3dCO2) separately under the forcing of globally homogeneous, zonally homogeneous, and whole spatially inhomogeneous CO₂ concentrations from 1850 to 2014, which are derived from 12 Earth System Models (ESMs) participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6). The simulation results show that the evolution trends of global mean SAT in the 20th century under the three CO₂ concentration distributions are similar to each other, and the simulated historical SATs considering meridional inhomogeneity of CO₂ concentration in Hist_2dCO2 and whole spatial inhomogeneity in Hist_3dCO2 are more consistent with the observations. Compared with Hist_1dCO2, the SAT in Hist_2dCO2 is warmer over land in the mid–high latitudes of the Northern Hemisphere (NH) than in other land areas. Further considering the zonal inhomogeneous CO₂ concentration in Hist_3dCO2 produces generally colder SAT over the NH mid–high latitude ocean than over land at the same latitude, and even the zonal mean SAT in the NH becomes slightly colder than that in Hist_2dCO2. The difference is ascribed to the uneven distribution of CO₂ concentration along the same latitude/longitude in the NH in Hist_3dCO2, which leads to strong large-scale fluctuations of the atmospheric circulation. Eurasia is the region with the highest CO₂ concentration, causing remarkable SAT warming there due to more downward longwave radiation. The warmer winter SATs in Eurasia further strengthen the northwest winds in East Asia, resulting in an increase of sea ice and strengthened cold SAT anomalies in the northern North Pacific. The simulated varied responses of the atmospheric circulation and SAT to the inhomogeneous CO₂ forcing demonstrate an imperative need for refined representation of inhomogeneous CO₂ distribution in climate models, so as to more accurately assess the climate change.