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There is considerable debate on the role that humans play in changing global climate through both the burning of fossil fuels and the release of chlorofluorocarbon (CFC) gases. Some argue that human interaction poses less of a threat to our atmosphere than do natural processes, like volcanic eruptions. This places a great deal of importance on understanding the role of volcanic eruptions in affecting global climate change. Whatever the source, it is apparent that compositional changes in the earth's atmosphere generate three principal climatic effects:
Intense sunlight in the stratosphere (above 12 km) produces bluish colored ozone (O3) by naturally breaking down normal oxygen molecules (O2) into two highly reactive oxygen atoms (O). Each oxygen atom then quickly bonds with an oxygen molecule to form ozone. Ozone absorbs UV radiation, and in the process ozone is changed back into an oxygen molecule and an oxygen atom. A balance exists in ozone destruction and production, so that an equilibrium concentration exists in the stratosphere. This equilibrium has probably existed throughout much of geologic time. Recently, however, an ozone hole has been detected in the stratosphere over Antarctica, presumably due to the atmospheric build up of ozone-destroying CFCs by humans. Ozone depletion has resulted in a greater penetration of ultraviolet radiation on the earth's surface, which is harmful to life on earth because it damages cellular DNA. The ozone effect does not appear to have a direct influence on global temperatures.
Certain gases, called greenhouse gases (primarily carbon dioxide and water vapor; but also methane, N2O, and CFCs), allow short wavelength radiation from the sun (UV and visible light) to penetrate through the lower atmosphere to the earth's surface. These same gases, however, absorb long wavelength radiation (infrared), which is the energy the earth reradiates back into space. The trapping of this infrared heat energy by these greenhouse gases results in global warming. Global warming has been evident since the beginning of the Industrial Revolution. Most scientists attribute global warming to the release of greenhouse gases through the burning of fossil fuels.
Suspended particles, such as dust and ash, can block out the earth's sunlight, thus reducing solar radiation and lowering mean global temperatures. The haze effect often generates exceptionally red sunsets due to the scattering of red wavelengths by submicron-size particles in the stratosphere and upper troposphere.
PINATUBO (1991) -- Mt. Pinatubo erupted in the Philippines on June 15, 1991, and one month later Mt. Hudson in southern Chile also erupted. The Pinatubo eruption produced the largest sulfur oxide cloud this century. The combined aerosol plume of Mt. Pinatubo and Mt. Hudson diffused around the globe in a matter of months. The data collected after these eruptions show that mean world temperatures decreased by about 1 degree Centigrade over the subsequent two years. This cooling effect was welcomed by many scientists who saw it as a counter-balance to global warming.