Quantifying Surface Climate Response to Shortwave and Longwave Aerosol Radiative Forcing

Aerosols affect the climate by modulating the incident radiative flux at the surface, atmosphere, and top of the atmosphere; known as radiative forcing (RF). Though RF is a common metric used in climate change studies, the impact of this forcing on the Earth's climate is not linear, with previous studies showing a stronger sensitivity to aerosol RF compared to other forcing agents. In addition, the vertical asymmetry in aerosol RF disproportionately affects the Earth's surface, with the surface sensitivity to RF strongly modulated by heterogeneous surface properties. In this study, we examine the response of the land surface temperature to aerosol-induced direct radiative forcing at the surface (DRF_surf) and quantify the surface sensitivity to aerosol loading from the global to the regional scale. To investigate this, we modify an Intrinsic Biophysical Mechanism normally used to isolate pathways of surface temperature perturbations due to deforestation, for aerosols. This technique is used in conjunction with the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) reanalysis dataset to isolate the degree of surface temperature change due to aerosol-induced shortwave and longwave DRF_surf. The shortwave and longwave DRF_surf show opposing effects, with a mean global shortwave DRF_surf of -13.69 W m^-2 over land, and daytime and nighttime longwave DRF_surf of 0.92 W m^-2 and 1.01 W m^-2, respectively. In response to this forcing, surface temperature is reduced by 0.15 K during the day (-0.19 K due to shortwave; 0.04 K due to longwave) and increases by 0.11 K at night due to longwave DRF_surf. Our analysis shows that even though the longwave DRF_surf is only 7% of the shortwave DRF_surf, the temperature sensitivity to longwave DRF_surf is much higher (roughly 40% of the sensitivity to shortwave DRF_surf). This is because regions with greater aerosol-induced longwave DRF_surf are primarily arid with lower turbulent dissipation of heat. This is further compounded by the longwave DRF_surf at night, when convective cooling is negligible. The asymmetry in the shortwave and longwave radiative effects reduces the global diurnal temperature range, with up to 3 K reductions seen in arid and highly polluted regions.