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close this bookClimate Protection and the National Interest (WRI, 1997, 56 pages)
close this folder2. THE CLIMATE CHANGE PROBLEM
View the document(introduction...)
View the documentThe Cooling Effect of Air Pollution
View the documentSearching for the Signal of Global Warming
View the documentExpected Impacts of Global Warming
View the documentCoping with Climate Change

The Cooling Effect of Air Pollution

In addition to the CFC destruction of ozone (see the sidebar on halogenated compounds), a second cooling effect offsets some global warming. Small atmospheric particles (called aerosols) are formed from sulfur dioxide air pollution, biomass burning, and other sources. These aerosols shield the earth - mostly in regions down-wind of industrialized areas - from some of the incoming sunlight and cause cooling both directly, by scattering sunlight, and indirectly, by helping to form reflective clouds13. According to the Intergovernmental Panel on Climate Change (IPCC), these small particles have offset some of the expected global warming over the past several decades. The predicted warming, with and without the aerosol effect, is shown in Figure 5. The cooling effect of the aerosols brings the calculated rise in temperature into closer agreement with the observed changes of the past few decades. Unlike most greenhouse gases, aerosols remain in the air for only a matter of days. As a result, as pollution emissions are reduced, so too will the cooling effect of the aerosols. Given the continued need to cut air pollution emissions to protect health and reduce acid deposition, the task of cutting greenhouse gas emissions becomes increasingly important.



The principal sources of nitrous oxide from human activities are the application of nitrogen fertilizers to agricultural lands, the burning of biomass and fuels, and industrial chemical production. Human sources are about a third of total global emissions. Nitrous oxide is a very stable molecule (lifetime of 120 years) and also contributes to stratospheric ozone depletion.9 This gas accounts for about 5 percent of the human sources of greenhouse warming.

BOX 1 - THE 12 HOTTEST YEARS IN RECORDED HISTORY ( in descending order )

1995,1990,1991,1981,1996, 1988, 1987,1994,1983, 1980, 1989, 1993

Source: Goddard Institute for Space Studies, NASA.


Halogenated compounds contain fluorine, chlorine, bromine, or iodine, and many such compounds are strong greenhouse gases. Halocarbons containing carbon and either chlorine or bromine, such as the CFCs, halons, and HCFCs, also cause stratospheric ozone depletion. CFCs are made for use as aerosol propellants, blowing agents for plastic foams, refrigerants, and solvents. They are particularly damaging as greenhouse gases because of their long lives in the atmosphere and their effectiveness in trapping heat, approximately 20,000-30,000 times that of CO2. Hydrofluorocarbons (HFCs) - substitutes for some CFC applications - are also potent greenhouse gases.

Estimates of the net warming effect of CFCs have recently been reduced. This reduction stems from the cooling effect from the CFC destruction of ozone (a greenhouse gas) in the lower stratosphere.11 CFCs and some other ozone-depleting compounds have been phased out in the industrialized countries by the Montreal Protocol on Substances that Deplete the Ozone Layer but production for essential uses and to meet developing-country needs continues.

Also included in the halo-carbon family are perfluorocarbons (PFCs) and sulfur hexafluoride (SF6), which have lifetimes measured in thousands of years. PFCs are a byproduct of aluminum making and are also manufactured for use in the semiconductor industry. They are exceedingly powerful greenhouse gases. Though presently low, concentrations of the PFCs and SF6 are increasing. Because of their long lifetimes, these compounds represent an essentially irreversible threat to the climate.12 The halocarbons contribute about 10 percent of the warming from human sources.



The problem of climate change is being addressed internationally through the UN Framework Convention on Climate Change (UNFCCC). Important milestones include


166 governments endorse the UNFCCC at the Earth Summit in Rio de Janeiro. The United States ratifies the treaty on October 15, 1992.


By November, 50 nations have ratified the treaty.


UNFCCC enters into force in March.


First meeting of the Conference of the Parties (COP1), in Berlin. "Berlin Mandate" adopted to guide negotiations of a protocol requiring further action for post-2000.


At COP2, in Geneva, United States takes strong position in favor of binding targets and timetables for reducing emissions.


COP3 in Kyoto in December. The goal is to adopt a protocol that will required reductions in greenhouse gas emissions after the year 2000.


Ozone (O3) in the upper atmosphere (stratosphere) acts as a crucial filter that protects life on the planet from many of the harmful effects of ultraviolet radiation. However, ozone in the lower atmosphere (troposphere) is both a greenhouse gas and, in many polluted urban areas, a threat to public health. Nitrogen oxides (NOx) and volatile organic compounds (VOCs) are both necessary for ozone formation. VOCs and NOx have both natural as well as man-made sources, but in urban areas, which are prone to ozone formation, combustion is a major source of both.10 As NOx are formed, they combine with VOCs in the presence of sunlight to form tropospheric ozone. Background tropospheric ozone concentrations, the evidence suggests, have approximately doubled in the northern hemisphere since preindustrial times.