4.1.2.2 Aerosols
Atmospheric aerosols are relatively small solid or liquid particles that are suspended in the atmosphere. They are largely natural: they may be generated by evaporation of sea spray, by wind blowing over dusty regions, by forest and grassland fires, by living vegetation (such as the production of sulphur aerosols by phytoplankton), by volcanoes (see section 4.1.2.4), etc. Human activities also produce aerosols by burning fossil fuels or biomass, and by the modification of natural surface cover that influences the amount of dust carried by the wind. Among the anthropogenic aerosols, those that have received the most attention in climatology are sulphate aerosols and black carbon. Sulphate is mainly produced by the oxidation of sulphur dioxide (SO2) in the aqueous phase, with fossil fuel burning, in particular coal burning, as their main source. Black carbon is the result of incomplete combustion during fossil fuel and biomass burning.
Since the majority of aerosols only remain in the atmosphere for a few days, anthropogenic aerosols are mainly concentrated downwind of industrial areas, close to regions where land-use changes have led to dustier surfaces (desertification) and where slash-and-burn agricultural practices are common. As a consequence, maximum concentrations are found in Eastern America, Europe, and Eastern Asia as well as in some regions of tropical Africa and South America (Fig. 4.3).
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Aerosols affect directly our environment as they are responsible, for instance, for health problems and acid rain. They also have multiple direct, indirect and semi-direct effects on the radiative properties of the atmosphere (Fig. 4.4). The direct effects of aerosols are related to how they absorb and scatter shortwave and longwave radiation. For sulphate aerosols, the main effect is the net and scattering of a significant fraction of the incoming solar radiation back to space (Fig. 4.4). This induces a negative radiative forcing, estimated to be around –0.4 Wm–2 on average across the globe. This distribution is highly heterogeneous because of regional variations in the concentration of aerosols (Fig. 4.3). By contrast, the main effect of black carbon is its strong absorption of solar radiation, which tends to warm the local air mass. The associated positive radiative forcing since 1750 is about +0.2 Wm–2 on average. Furthermore, the deposit of black carbon on snow modifies snow's albedo, generating an additional small positive forcing (~+0.1 Wm–2). Taking all the aerosols (but not the effect of black carbon on albedo) into account, the global average of the total direct aerosol effect is about –0.50 Wm–2 (Fig. 4.2).
The indirect effects of aerosols include their impact on cloud microphysics (which induces changes in clouds' radiative properties, their frequency and their lifetimes). In particular, aerosols act as nuclei on which water condenses. A high concentration of aerosols thus leads to clouds that contain many more, and hence smaller, water droplets than clouds with the same water content formed in cleaner areas. As such clouds are more highly reflecting (i.e. have a higher albedo), this induces a negative radiative forcing which is referred to as the first indirect, the cloud-albedo, or the Twomey effect. The effect of aerosols on clouds' height, lifetime and water content (related to the amount of water required before precipitation occurs) has classically been referred to as the second indirect effect although more explicit formulations such as the 'cloud lifetime effect' are now often preferred. Finally, the absorption of solar radiation by some aerosols modifies the air temperature, its humidity and the vertical stability of the air column. This affects the formation and lifetime of clouds, and is referred to as the semi-direct effect of aerosols.
The most recent estimates of the radiative forcing associated with the cloud-albedo effect are between –0.3 and –1.8 Wm–2, with a best estimate of around –0.7 Wm–2. The cloud lifetime effect also induces a negative radiative forcing but the uncertainties over its magnitude are even larger. For the semi-direct effect, even the sign of the radiative forcing is not well known at present, but its magnitude is probably smaller than those of the indirect effects. This illustrates that the aerosols represent one of the largest uncertainties in our estimates of past and future changes in radiative forcing. This is true for the twentieth century, but aerosols have also played a role, not yet known precisely, in past climate changes. For instance, during the last glacial period (see section 5.4), the drier conditions led to a higher number of aerosols in the atmosphere, producing a negative radiative forcing probably larger than 1 Wm–2 that contributed to amplifying the cold conditions at the time.
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