Aerosols and Global Warming

The cooling effect of the sulfate aerosol is concentrated almost entirely in the Northern Hemisphere because most industrial activity takes place in that half of the globe, so it is there that most emissions occur. The relatively short lifetime of such sulfate aerosols precludes their spreading to the Southern Hemisphere; consequently, the concentration of sulfate particulates is much higher over the Northern Hemisphere. The short lifetime of the sulfate particles can be understood by considering the processes of their removal from air. The average diameter of the tropospheric sulfate aerosol particles is about 0.4 /xm, and their average altitude is about 0.5 km. For particles of this size and altitude, the expected atmospheric lifetime before gravitational settling to the surface is several years. However, since the sulfate aerosol droplets are also removed efficiently by rain, their actual lifetime in the lower troposphere is of the order of days rather than years.

The increase of global S02 emissions from fossil-fuel combustion over the last century and a half is shown in Figure 6-19. Up to 20% more sulfur dioxide is emitted by smelting, etc. Presumably the trend in anthropogenic sulfate aerosol production has approximately followed the pattern of S02 emissions in Figure 6-19. The initial approximately linear time increase of the sulfur dioxide emission rate changed to one with a much steeper slope after World War II, a behavior we saw previously for C02 and CH4. However, the ratio of global S02 to C02 emission rates, expressed as a percentage of sulfur in the carbon of the fuel, fell from about 2.2% in the 1930s and 1940s to a constant 1.1% in recent decades, due presumably to the gradual replacement of coal by oil and natural gas.

As illustrated by the contour diagrams in Figure 6-20, the bulk of the anthropogenically produced aerosols in North America is centered above the Ohio Valley and directly reflects sunlight mostly above that area. Equal or even larger effects are observed over southern Europe and portions of China. Indeed, according to some calculations, the cooling effect from aerosols

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FIGURE 6-19 Estimated historical emissions of sulfur dioxide from anthropogenic sources. [Source: Adapted from S. J. Smith, H. Pitcher, and T. M. L. Wigley, "Global and Regional Anthropogenic Sulfur Dioxide Emissions/' Global and Planetary Change 29 (2001): 99.]

FIGURE 6-19 Estimated historical emissions of sulfur dioxide from anthropogenic sources. [Source: Adapted from S. J. Smith, H. Pitcher, and T. M. L. Wigley, "Global and Regional Anthropogenic Sulfur Dioxide Emissions/' Global and Planetary Change 29 (2001): 99.]

FIGURE 6-20 The amount of sunlight reflected into space by anthropogenic aerosols by the direct mechanism, in units of watts per square meter of the Earth's surface. [Source: J. T. Houghton et al., Climate Change 1994 — Radiative Forcing of Climate Change (Intergovernmental Panel on Climate Change) (Cambridge: Cambridge University Press, 1995).]

FIGURE 6-20 The amount of sunlight reflected into space by anthropogenic aerosols by the direct mechanism, in units of watts per square meter of the Earth's surface. [Source: J. T. Houghton et al., Climate Change 1994 — Radiative Forcing of Climate Change (Intergovernmental Panel on Climate Change) (Cambridge: Cambridge University Press, 1995).]

BOX 6-4

Cooling over China from Haze

Measurements of the amount of sunshine reaching the surface in China indicate a significant decrease over the last halfcentury (Figure 1). The blockage of solar intensity results from the aerosols in the air above the region produced mainly from the sulfur dioxide emitted by burning coal. As a consequence of the increasing presence of the aerosols, maximum summer temperatures in heavily polluted eastern China have fallen by about 0.6°C per decade. Similar effects are observed in the Brazilian Amazon region, due to soot and ash emitted into the local air from forest and grass fires lit to clear land, and in the African country of Zambia, due to grass fires.

FIGURE 1 Change in the amount of sunlight reaching China relative to the average, over 50 years. [Source: F. Pearce, "Pollution Is Plunging Us into Darkness/' New Scientist (14 December 2002):6.|

outweighs the heating effect due to greenhouse gases for some regions in these areas.

It is not clear how the amount of tropospheric sulfate aerosol will change in the future. Emissions of sulfur dioxide from power production in North America and western Europe are now more tightly controlled in order to combat acid rain, so the S02/C02 ratio in emissions from these areas should decline. However, the anthropogenic sulfate aerosol concentrations over southern Europe and parts of Russia and China are considerably higher than the current maximum values in North America and will not be affected by these legislative controls (see Box 6-4). The only substantial domestic energy source currently available to China for its rapid industrialization is coal, so the SO, emissions from this source and from India may well continue to rise (Chapter 3).

Aerosols also result from the oxidation of the gas dimethyl sulfide (DMS), (CH3)2S, which is produced by marine phytoplankton and subsequently released into the air over oceans. Once in the troposphere, DMS undergoes oxidation, some of it to S02, which then can oxidize to sulfuric acid, and some to methanesulfonic acid, CH3SO3H. Both of these acids form aerosol particles, which in turn lead to the formation of water droplets and hence clouds over the oceans. The particles and droplets both deflect incoming light from the Sun. Some scientists believe that increased emissions of dimethyl sulfide from the oceans will occur when seawater warms as a result of the enhancement of the greenhouse effect and that this negative feedback will temper global warming.

Although the sulfate aerosol has a short lifetime, new supplies of it are constantly being formed from the sulfur dioxide pollution that pours into the atmosphere on a daily basis. Consequently, there is a steady-state amount of the aerosol in the troposphere; sulfur dioxide emissions keep postponing the full effects of global warming induced by the rise in greenhouse gas concentrations.

Global Warming to Date

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