Aerosols have played a large role in climate change over the 20th century and have partially offset greenhouse gas warming, leading to a net warming that is smaller than a purely GHG-forced 20th century climate change. They affect climate though their direct ability to reflect shortwave radiation, but also through their indirect influence on cloud cover. A recent study (in press) by some folks at GISS using ModelE have published on aerosol effects from 1890-1995 and I’ll summarize some of their work:
— Aerosols cool the surface but have the potential to warm at the area of the atmosphere in which they reside, and thus may have a positive RF at the TOA but negative at the surface.
— Increasing Cloud Condensation Nuclei or Ice Nuclei can modfy the microphysical properties of clouds. Aerosols increase cloud droplet number concentrations thereby increasing their albedo, the so-called “first indirect effect (or Twomey Effect) and resulting in an overall cooling effect. The microphysically-induced effect on the liquid water content, cloud height, and lifetime of clouds is the “second indirect effect.”
— “Dark” aerosols such as Black Carbon (BC) which are deposited on the surface will lower the surface albedo, promote melting, and provide an overall warming effect.
— In fixed GHG experiments global surface air temperature changed -0.2, -1.0 and +0.2 degrees C for the direct, indirect, and BC albedo effects, respectively. Global cloud cover increased roughly half a percent because of the indirect effect.
— Direct-Effect only simulations reveal 1890 RF’s of -0.4, +0.2, and -0.2 W/m2 for Sulphate Aerosols, Black Carbon, and Organic Matter respectively at the TOA. 1890 RF’s are -0.4, -0.3, and -0.5 W/m2 at the surface. 1995 TOA RF’s are -0.7, +0.5, and -0.3 W/m2 (same order) and all are -0.7 W/m2 at the surface.
— Simulation with Indirect Effect has increased low-level clouds over mid-latitudes (especially in the Northern Hemisphere), while GHG warming decreases cloud cover in either hemisphere. The impact of the indirect is primarily on is primarily on low-level clouds.
— Global mean sulfate and BC annual aerosol burden increase by a factor of two over the century, especially in the Northern Hemisphere. In the two big industrializers (UK and U.S.A) BC concentrations and emissions were higher in 1890 however.
— The anthropogenic cloud forcing (human component in the instantaneous change in net shortwave plus longwave TOA radiative flux with and without clouds) is largely negative in polluted regions where aerosol load is greatest, particularly in
— Albedo generally decreases with GHG and BC increase, with the cyrosphere being effected the most. The net effect of direct, indirect, and BC albedo effects is to increase snow/ice cover by about 8% of the original cover value.
— Black Carbon is removed more effectively by liquid clouds than ice clouds, and the transition into a warmer, higher-GHG world promotes a lower ice/liquid ratio in clouds, and a lower deposition rate near areas far away from the source. This slightly reduces the impact of BC on ice-albedo in a warmer world.
— Aerosols can continue to cool the polar latitudes even in winter when incident shortwave radiation is absent, due to persistent effects from the summer/autumn months, and perhaps non-local forcing changes. Sea level pressure and longwave forcing changes also show up large in the cold months.