Pattern of climate response to CO2, sulfate and black carbon in CESM1

GHGs are considered the dominating factor in shaping observed 20th century, which has been continuously increasing though the last century and is expected to increase for 21st century. Globally, cooling aerosols (e.g. sulfates (SO4)) are “masking” global warming and their forcing are much more heterogeneous in space. Black carbon (BC) aerosols heat the atmosphere directly while reducing the solar radiation reaching the ground, which posed a forcing mechanism different from either GHG or SO4. With a fully coupled ocean-atmospheric model, we examine the climate response to different forcing mechanisms, including instantaneous increase of forcing of CO2 (260 to 400 ppm), SO4 (preindustrial to present-day and indirect forcing included), and BC (preindustrial to present-day and improved emission inventory.

As noted in earlier studies, the BC (SO4) forcing produced a stronger north-south asymmetry in temperature response: northern hemisphere warming (cooling) is 2-3 times larger than Southern hemisphere warming (cooling). The asymmetric feature in temperature response to CO2 is much smaller (with only 30% stronger warming over North hemisphere) and is probably only due to the land-ocean fraction difference in north-south hemisphere. As for the impact on hydrological cycle, the global change of precipitation in response to BC is slightly increasing but near zero. The negligible global precipitation response is due to the cancellation of two opposing effects of BC forcing: (1) the increasing precipitable water (0.37 mm) from enhanced evaporation due to surface warming and (2) the increase of atmospheric stability due to the atmospheric heating. The global reduction of precipitation in response to SO4 is quite significant (2.1 cm). The precipitation reduction in response to sulfate aerosols are due to reduced solar radiation reaching the surface in addition to the cooling of surface temperature, and therefore, the reduction of precipitation scaled with temperature (hydrological sensitivity, cm/K) is larger in the sulfate cooling (4.3 cm/K) than CO2 warming case (1.5 cm/K).

BC induces shifts of precipitation from southern hemisphere to northern hemisphere. Similarly, the reduction of precipitation in response to SO4 also shows a stronger asymmetry, with north hemispheric reduction 5 times larger than southern hemisphere, consistent with the spatial distribution of emission sources. The precipitation response to BC, E Asia BC and S Asia BC is highly correlated, suggestion for a common mechanism in setting the linearity relationship of the response to forcing magnitude. Additional perturbed runs in atmospheric-only simulations were conducted to understand the role of ocean in setting the pattern correlations.