MEMBERs of The scientific community by and large concur that the Earth is undergoing a change in climate and that global warming is occurring at an increasing rate. In fact, scientific modeling suggests that Earth will experience an increase in temperature during the next 100 years at a pace up to four times greater than that in the previous 100 years. To a large extent this acceleration in the late 20th century is attributed to carbon dioxide (CO2) emissions generated by human activity. Carbon dioxide acts like a glass barrier over the Earth, preventing heat from leaking into the environment, and thus creating a greenhouse effect. Human activity has counteracted global warming to a small degree by emitting aerosols in the environment, causing global cooling.

Volcanism is another contributing factor to climate change. Each volcano affects the climate based on its location and the nature and extent of an eruption.

Similar to human activity, volcanism leads to both global warming and cooling. The effect of volcanism on climate depends on the interaction between the sun's heat and the volcanic debris. In essence, scientists believe that ongoing volcanic eruptions have maintained the Earth's temperate climate for millions of years. Circumstantial evidence suggests that volcanism can influence short-term weather patterns in addition to having an effect on long-term climate change.

Volcanic dust blown into the atmosphere can remain for months and produce temporary cooling, the degree to which is dependent on the volume of dust, and the duration of which is dependent on the size of the dust particles. The strength of gases can vary greatly among volcanoes. Water vapor is typically the most abundant volcanic gas, followed by carbon dioxide and sulfur dioxide. Other principal volcanic gases include hydrogen sulfide, hydrogen chloride, and hydrogen fluoride.

Volcanoes that discharge great quantities of sulfur compounds affect the climate more significantly than those that release only dust. In fact, the greatest volcanic effect on the Earth's short-term weather patterns is caused by sulfur dioxide gas. Sulfur dioxide can form sulfuric acid aerosols that reflect the sun's heat and trigger cooling of the Earth's surface for as long as two years. Scientific literature frequently refers to the drop in global temperature after the eruption of Mount Pinatubo, Philippines, in 1991 and the very cold temperatures leading to crop failures and famine in North America and Europe for two years following the eruption of Mt. Tambora, Indonesia, in 1815 as examples of this effect. Furthermore, sulfuric dioxide can form acid rain, a combination of sulfuric acid and nitric acid. Acid rain, which is also caused by the burning of fossil fuels, is a critical environmental problem that can

The Philippine Acid Rain Volcanoes
Yasur volcano In Vanuatu Is one of the world's most active volcanoes, erupting dozens of times a day for about 800 years.

injure lakes, streams, and forests and the inhabitants of these ecosystems.

In addition, volcanoes discharge water and carbon dioxide in large quantities in the form of atmospheric gases, and can absorb and retain in the atmosphere heat radiation emanating from the ground. Estimates suggest that water makes up to 99 percent of gas in volcanic expulsions. This short-term warming of the air leads the water to become rain within a matter of hours or days and the carbon dioxide to dissolve in the ocean or to be absorbed by plants. The majority of the heat energy connected to global warming exists in the ocean. If the oceanic depth at which heat is stored is decreased, then global temperature increases are expected to be greater than predicted.

Volcanic eruptions combined with man-made chlorofluorocarbons (CFCs) also can contribute to ozone depletion. CFCs were developed in the early 1930s and were used in industrial, commercial, and household applications, such as refrigeration units and aerosol propellants, because they were nontoxic and nonflammable and met a number of safety criteria. In February 1992, however, following evidence that CFCs contributed to depletion of the ozone layer, the United States announced the phase-out of the production of CFCs by December 1995. Members of the Montreal Protocol in 1992 followed suit and agreed to an accelerated phase-out by the end of 1995 as well.

The ozone layer, which rests in the stratosphere and begins at 7.5 mi. (12 km.) above the Earth, is a shield that protects living beings from ultraviolet-B (UV-B), the sun's most harmful UV radiation. In high doses, UV-B can lead to cellular damage in plants and animals. Scientists believe that global warming will lead to a weakened ozone layer, because as the Earth's surface temperature rises, the stratosphere will get colder, slowing the natural repairing of the ozone layer. Decreased ozone in the stratosphere results in lower temperatures.

Unlike ozone depletion created by man-made CFCs, which will take decades to repair, theories indicate that repair of damage to the ozone layer caused by volcanoes occurred as volcanic activity diminished. Recent mathematical models intimate that a volcanism in Siberia of great proportion significantly depleted the ozone layer.

Finally, hydrogen fluoride gas can concentrate in rain or on ash particles, contaminating grass, streams, and lakes with excess fluorine. Excess fluorine in grass and water supplies can poison the animals that eat and drink at contaminated sites, eventually causing fluorisis, which destroys bones.

SEE ALSO: Forced Climate Variability; Oceanic Changes; Stratosphere; Sulfur Dioxide.

BIBLIOGRAPHY. Australian Greenhouse Office, Department of the Environment and Heritage, "Is Global Warming Primarily Due to Solar Variability?" www.greenhouse.gov. au/science/hottopics/pubs/topic6.pdf; Robert C. Balling, Environmental Geosciences (v.7/4, 2000); Ryan C. Bay, Nathan Bramall, and P. Buford Price, "Bipolar Correlation of Volcanism With Millennial Climate Change," Proceedings of the National Academy of Sciences of the United

States of America (2007); Center for Educational Technologies, Wheeling Jesuit University, "Exploring the Environment: Volcanoes," www.cotf.edu/ete/modules/volcanoes/ vclimate.html; Richard V. Fisher, "Volcano Information Center," www.volcanology.geol.ucsb.edu/gas.htm; Ter-rence M. Joyce, "Observations on Global Warming," www. whoi.edu/page.do?pid=12457&tid=282&cid=14087; Kenneth A. McGee, Michael P. Doukas, Richard Kessler, and Terrence M. Gerlach, "Impacts of Volcanic Gases on Climate, the Environment, and People," U.S. Geological Survey Open-File Report 97-262, www.pubs.usgs.gov/open-file/of97-262/of97-262.html; Ian Plimer, "The Past is the Key to the Present: Greenhouse and Icehouse Over Time," Institute of Public Affairs Review, (v.55/1, 2003); University of Sheffield, www.shef.ac.uk/mediacentre/2007/; U.S. Geological Survey, "Volcanic Eruptions Temporarily Reduce the Effects of Global Warming," www.hvo.wr.usgs. gov/volcanowatch/2005/05_11_03.html.

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