Global dimming, a gradual reduction in the amount of global direct irradiance at the Earths surface, has partially counteracted global warming from 1960 to the present.[31] The main cause of this dimming is aerosols produced by volcanoes and pollutants. These aerosols exert a cooling effect by increasing the reflection of incoming sunlight. James Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion—CO2 and aerosols—have largely offset one another in recent decades, so that net warming has been driven mainly by non-CO2 greenhouse gases.[32]
In addition to their direct effect by scattering and absorbing solar radiation, aerosols have indirect effects on the radiation budget.[33] Sulfate aerosols act as cloud condensation nuclei and thus lead to clouds that have more and smaller cloud droplets. These clouds reflect solar radiation more efficiently than clouds with fewer and larger droplets.[34] This effect also causes droplets to be of more uniform size, which reduces growth of raindrops and makes the cloud more reflective to incoming sunlight.[35]
Soot may cool or warm, depending on whether it is airborne or deposited. Atmospheric soot aerosols directly absorb solar radiation, which heats the atmosphere and cools the surface. Regionally (but not globally), as much as 50% of surface warming due to greenhouse gases may be masked by atmospheric brown clouds.[36] When deposited, especially on glaciers or on ice in arctic regions, the lower surface albedo can also directly heat the surface.[37] The influences of aerosols, including black carbon, are most pronounced in the tropics and sub-tropics, particularly in Asia, while the effects of greenhouse gases are dominant in the extratropics and southern hemisphere.[38]
