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The Greenhouse Effect
Air pollution affects global warming through what is known as the enhanced greenhouse effect. Shortwave solar radiation, mostly visible light, passes through the atmosphere and warms the earth's surface. The surface then radiates this heat back into the atmosphere as long-wave infrared radiation. However, certain gases, known as greenhouse gases, absorb this heat and reflect a portion of it back to the surface rather than allowing it to escape into space. This effect is necessary to maintain a livable climate on earth. However, if levels of greenhouse gases are artificially raised, such as through air pollution, then temperatures on earth can rise too much. This danger is called the enhanced greenhouse effect.
Greenhouse Gases
The primary greenhouse gases are carbon dioxide, methane, nitrous oxide, hydrofluorocarbons and chlorofluorocarbons. Each of these air pollutants impacts global warming differently. Methane is 25 times as powerful as carbon dioxide, nitrous oxide is 298 times as powerful as carbon dioxide and chlorofluorocarbons are 10,000 times as powerful as carbon dioxide. However, each is also present in the atmosphere in different concentrations. Because carbon dioxide is present in much higher concentrations, it has the greatest impact on global warming. Specifically, carbon dioxide accounts for 63 percent of the change in intensity of the greenhouse effect, while methane accounts for 18 percent, chlorofluorocarbons account for 10 percent and nitrous oxide accounts for 6 percent.
Air Pollution Levels
All the major greenhouse gases have increased in concentration since the start of the Industrial Revolution in the early 1700s. Carbon dioxide levels have increased from 278 parts per million, prior to 1700, to 390 parts per million, in 2006: an increase of 36 percent. Methane levels have increased from 715 parts per billion, prior to 1700, to 1,774 parts per billion, in 2005: an increase of 152 percent. Nitrous oxide was not measured until 1978, but increased from 270 to 324 parts per billion in just 32 years, an 18 percent increase. While chlorofluorocarbons are being phased out due to their negative affect on ozone, their substitutes, hydrofluorocarbons, are still extremely potent greenhouse gases.
Air Pollution Sources
Approximately 65 percent of carbon dioxide is produced by the burning of fossil fuels. The remainder is the result of deforestation, solid waste and industrial applications such as cement manufacturing. Methane is produced by the decay of organic materials in landfills, agricultural activities such as rice cultivation, the digestive tracts of grazing animals, the mining of coal and the extraction of natural gas and oil. Nitrous oxide is released by fertilizers, industrial applications and the burning of fossil fuels. All these sources of air pollution combine to increase levels of greenhouse gases in the atmosphere, increasing the enhanced greenhouse effect and global temperatures.