Aerosols

Global dimming, a gradual reduction in the amount of global direct irradiance at the Earth's surface, may have partially counteracted global warming during the period 1960-1990. Human-caused aerosols probably precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of the warming effect of increasing greenhouse gases.[1] Anthropogenic emissions of other pollutants—notably sulphate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century,[18] though the cooling may also be due in part to natural variability. 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.[19]

Atmospheric soot aerosols directly absorb solar radiation, which heats the atmosphere and cools of the surface. As much as 50% of surface warming due to greenhouse gases may be masked by atmospheric brown clouds.[20] Airborne for several days, often settling on glaciers, or on ice in arctic regions, the lower surface albedo directly heats the surface.[21] The influences of aerosols, including black carbon, will be most pronounced in the tropics and sub-tropics, particularly in Asia, while the effects of greenhouse gases will be dominant in the extratropics and southern hemisphere.
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