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and black carbon (soot), absorb both incoming solar energy and the outgoing infrared energy emitted by the Earth. These aerosols tend to warm the atmosphere, offsetting some (but not all) of the cooling associated with the direct and indirect effects. Black carbon particles that settle on snow and ice surfaces can also accelerate melting; however, this positive forcing is typically included in estimates of the forcing associated with land use change, which is discussed below.

It is worth noting the sources of a few key types of aerosols to illustrate their diversity: Dust and some organic aerosols arise from natural processes, but some human activities such as land use change also lead to changes in the abundance of these species. Black carbon particles are produced from the burning of both fossil fuels and vegetation. Sulfate (SO4) aerosols—which are a major contributor to the aerosol direct and indirect effects—have three notable sources: fossil fuel burning, marine phytoplankton, and volcanoes. The composition and size of each of these aerosol species affect how they absorb or scatter radiation, how much water vapor they absorb, how effectively they act to form cloud droplets, and how long they reside in the atmosphere—although in general most aerosols only remain in the atmosphere for a few weeks on average.

In addition to their role in global climate forcing, aerosols also have a number of other important environmental effects. The same industrial emissions that give rise to SO4 aerosols also contribute to acid rain, which has a major detrimental effect on certain ecosystems. One of the major objectives—and successes—of the 1990 Clean Air Act (P.L. 101-549) was to reduce the amount of sulfur emissions in the United States. Similar laws in Europe have also been successful in reducing SO4 aerosol concentrations (Saltman et al., 2005). The relationship between aerosols and cloud formation also means that changes in aerosols play an important role in modulating precipitation processes (see Chapters 8 and 15). Also, many aerosols are associated with negative impacts on public health, as discussed in further detail in Chapter 11.

Finally, aerosol emissions represent an important dilemma facing policy makers trying to limit the magnitude of future climate change: If aerosol emissions are reduced for health reasons, or as a result of actions taken to reduce GHG emissions, the net negative climate forcing associated with aerosols would decline much more rapidly than the positive forcing associated with GHGs due to the much shorter atmospheric lifetime of aerosols, and this could potentially lead to a rapid acceleration of global warming (see, e.g., Arneth et al., 2009). Understanding the many and diverse effects of aerosols is also important for helping policy makers evaluate proposals to artificially increase the amount of aerosols in the stratosphere in an attempt to offset global warming (see Chapter 15).

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