Allocation of Warming to Natural and Anthropogenic Factors

The best estimates of global warming or cooling in 2005 arising from the various factors is summarized by the bar graphs in Figure 6-21; the effect of each factor is expressed as a percentage of the total anthropogenic effect. The order of the greenhouse gases in terms of the amount of extra warming they have produced is

The value in Figure 6-21 for CFCs includes the cooling of the stratosphere induced by their destruction of ozone, and that for methane includes the warming of the stratosphere produced from the additional water vapor formed there by its decomposition. The cooling labeled surface albedo is the net result of cooling due to changes in land use minus warming arising from the deposition of sunlight-absorbing black soot on snow and ice. Overall, the cooling from anthropogenic aerosols currently cancels about 40% of the net warming from all greenhouse gases. However, the aerosol effect—which is the sum of that from the direct and indirect effects on cloud albedo—has by far the largest uncertainty of any of the factors in Figure 6-21.

FIGURE 6-21 Contributions to global warming and cooling produced by various factors, expressed as percentages of the total anthropogenic warming. [Data source: Intergovernmental Panel on Climate Change, Climate Change 2007: The Physical Science Basis. Summary for Policymakers (February, 2007).]

bo 100 e

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-100

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CH4 Tropospheric CFCs O,

Surface Aerosols Total Solar albedo anthropogenic

Greenhouse gas emissions from airplanes traveling long distances high in the troposphere are particularly effective in promoting global warming. They emit, into the low-density air in which they fly, the carbon dioxide and water vapor that result from the combustion of their hydrocarbon fuel. Because the air temperature there is so low, the IR absorbed by this C02 and HzO is unlikely to be re-emitted; instead, it warms the surrounding air and therefore enhances the greenhouse effect.

Global Warming: Geography

The year-to-year variations in the average worldwide surface temperature from 1880 to the present are shown in Figure 6-2. The increases in temperature were not evenly spread around the globe, however, as indicated by Figure 6-22, which shows the changes in average global temperatures in the 2001-2005 period, relative to the 1951-1980 average. In Figure 6-22, the darker the shade of green, the greater the increase in temperature; the few areas shown in gray underwent decreases in temperature. In general, air

FIGURE 6-22 Changes, in degrees Celsius, in the mean surface temperature in 2001-2005 relative to the 1951-1980 mean. The dotted regions indicate areas for which data is insufficient. ISource: J. Hansen et al., "Global Temperature Change," Proceedings of the National Academy of Science 103 (2006): 14288.]

-0.8 -0.4 -0.2 +0.2 +0.4 +0.8 ■ +1.2 Temperature change (°C)

-0.8 -0.4 -0.2 +0.2 +0.4 +0.8 ■ +1.2 Temperature change (°C)

temperatures over land areas have increased more than those over the seas. Sulfate aerosols keep the eastern portions of the United States and Canada cooler than they would be otherwise.

The Arctic region has warmed most of all, with the consequence that its sea ice is disappearing. The melting produces a positive feedback effect: Since ice reflects sunlight more efficiently than does liquid water, the increasing amount of sunlight absorbed as the ice is replaced by open water increases surface water and surface air temperatures, thereby inducing even more melting. However, scientists have recently found that evaporation of the open water has produced more cloud cover in the region, which is a negative feedback effect that partially negates the positive one since the clouds reflect sunlight and thereby increase the albedo.

The increased lowering of air temperatures due to higher sulfate aerosol levels does not permanently cancel out all the warming due to greenhouse gases because of the very different atmospheric lifetimes of the particles as compared to the gases. The tropospheric aerosols last only a few days, so they do not accumulate with time and their effect is short term. In contrast, today's emissions of carbon dioxide into the atmosphere will exert effects for decades or centuries to come: C02 emissions are cumulative over the medium term. This is also true for CFCs and nitrous oxide, which are also important greenhouse gases, but it is less so for methane since its half-life is only about one decade. Thus, although sulfate-producing S02 emissions can temporarily cancel the effects of C02 emissions, eventually the cumulative effects of the carbon dioxide and the other greenhouse gases win out.

FIGURE 6-22 Changes, in degrees Celsius, in the mean surface temperature in 2001-2005 relative to the 1951-1980 mean. The dotted regions indicate areas for which data is insufficient. ISource: J. Hansen et al., "Global Temperature Change," Proceedings of the National Academy of Science 103 (2006): 14288.]

In summary, global warming has been experienced by most areas in the last half-century. Most of the warming is due to emissions of carbon dioxide into the atmosphere, with lesser amounts of warming from increased levels of methane, tropospheric ozone, nitrous oxide, and the introduction of CFCs. The increased water vapor in the atmosphere resulting from the warming by these gases has itself produced at least as much additional warming. The increased emissions of sulfur dioxide that accompany fossil-fuel combustion have produced aerosols that cancel out some but not all of the warming produced by the greenhouse gases.

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Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

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