Satellite data indicate a slight cooling in the climate in the last 18 years. These satellites use advanced technology and are not subject to the "heat island" effect around major cities that alters ground-based thermometers.
Projections of future climate changes are uncertain. Although some computer models predict warming in the next century, these models are very limited. The effects of cloud formations, precipitation, the role of the oceans, or the sun, are still not well known and often inadequately represented in the climate models --- although all play a major role in determining our climate. Scientists who work on these models are quick to point out that they are far from perfect representations of reality, and are probably not advanced enough for direct use in policy implementation. Interestingly, as the computer climate models have become more sophisticated in recent years, the predicted increase in temperature has been lowered.
Are humans causing the climate to change?
98% of total global greenhouse gas emissions are natural (mostly water vapour); only 2% are from man-made sources.
By most accounts, man-made emissions have had no more than a minuscule impact on the climate. Although the climate has warmed slightly in the last 100 years, 70% percent of that warming occurred prior to 1940, before the upsurge in greenhouse gas emissions from industrial processes. (Dr. Robert C. Balling, Arizona State University)
A Gallup survey indicated that only 17% of the members of the American Meteorological Society and the American Geophysical Society thought the warming of the 20th century was the result of an increase in greenhouse gas emissions.
If global warming occurs, will it be harmful?
The idea that global warming would melt the ice caps and flood coastal cities seems to be mere science fiction. A slight increase in temperature -- whether natural or mankind induced -- is not likely to lead to a massive melting of the earth ice caps, as sometimes claimed in the media. Also, sea-level rises over the centuries relate more to warmer and thus expanding oceans, not to melting ice caps.
Contrary to some groups' fear mongering about the threat of diseases, temperature changes are likely to have little effect on the spread of diseases. Experts say that deterioration in public health practices such as rapid urbanization without adequate infrastructure, forced large scale resettlement of people, increased drug resistance, higher mobility through air travel, and lack of insect-control programs have the greatest impact on the spread of vector-borne diseases.
Larger quantities of CO2 in the atmosphere and warmer climates would likely lead to an increase in vegetation. During warm periods in history vegetation flourished, at one point allowing the Vikings to farm in now frozen Greenland.
Pollution
Smog is an often-visible haze made up of thousands of constituents, the most abundant, but not necessarily the most toxic, of which is ozone. Ozone -- made up of three oxygen atoms -- is produced when volatile organic compounds (VOCs) and nitrogen oxides released from natural and human sources combine in the presence of sunlight. Ozone at elevated levels can create breathing problems, particularly in the young, the old, and persons with existing health problems. Exposure to excess ozone reduces lung function by increasing sensitivity to asthma and aging lungs. It may irritate eyes, cause nasal congestion, and reduce resistance to colds and other infections. Ozone at elevated levels is toxic to other living organisms as well; it damages plants and trees by affecting the stability of cell walls.
Particulate matter, which is made up of ash, smoke soot, dust, fibers, and liquid droplets, can be produced by the burning of wood, diesel, and other fuels; agricultural activities, mining, and industrial processes; and traffic on unpaved roads. Particulate matter produces a haze that can cause visibility problems. It also dirties and damages buildings and clothes. Smaller particulates can be inhaled deeply and, with elevated concentrations over an extended period of time, can cause lung damage and bronchitis. Recent studies have linked exposure to these smallest particles to a greater risk of premature death.
Nitrogen oxides result from the burning of fossil fuels, such as coal, natural gas, gasoline, and oil. At present, automobiles are the main source of nitrogen oxide emissions in urban areas. Nitrogen oxides, at elevated concentrations, can damage the respiratory system. They can also be a key ingredient in the formation of both ozone and acid rain.
Sulphur dioxide is released when sulfur-containing fuels, such as coal and oil, are burned. Common sources of sulfur dioxide emissions are electric utilities and certain industrial processes, such as copper smelting. Sulfur dioxide at elevated concentrations can cause respiratory problems and also affects plant and crop production. It also can be a contributing component of acid rain.
Toxic chemicals, which include VOCs such as benzene, toluene and 1,1,1 dichloromethane, when inhaled in elevated concentrations over time, can cause birth defects, cancer, and various other health problems. Sources of toxic emission include industrial processes, such as refineries and chemical manufacturers, and small businesses, such as print shops and dry cleaners. VOCs also contribute directly to the formation of ozone, another air pollutant.
Carbon monoxide (CO) is formed when fuel does not burn completely. Cars and trucks, as well as power plants, are the main contributors. Carbon monoxide interferes with the blood's ability to transport oxygen to cells and tissues. Exposure to elevated levels of carbon monoxide can cause drowsiness, headaches, and sometimes death. Carbon monoxide is particularly hazardous to those who have heart disease or pre-existing lung conditions.
Acid rain forms when the sulfur dioxide, nitrogen oxides, and carbon dioxide emitted from fossil-fuel-burning industrial plants and other combustion processes combine with rain, fog, or snow in the atmosphere. Over time, with conducive geologic conditions, acid rain can increase the acidities of lakes, streams, and soils, disturbing or destroying local environments. Wind can carry pollutants far away from where they originated, creating problems in other states or countries.
Lead is a heavy metal that persists in the environment for decades. The former use of leaded gasoline, which is no longer commercially available in most of the United States, the manufacture of lead-based paint, and lead-acid battery reclamation operations are sources of lead in the air. Emissions from metal smelters are another source. Exposure to lead at high levels and over time can cause brain and other nervous system damage, particularly in children. Excess exposure to lead can also harm wildlife and is known to cause cancer in animals.
Carbon dioxide, a compound naturally found in the atmosphere, is produced by humans and animals and utilized by plants in the photosynthesis process. Electric utilities, cars, petroleum refineries, the burning of wood, also produce carbon dioxide, however, and many other sources. The elevated presence of carbon dioxide, as well as other gases such as methane and nitrous oxide, is believed to influence global climate change by preventing heat from escaping out of the atmosphere in a phenomenon known as global warming. The implications of global warming over the long term are serious, including catastrophic floods and droughts.
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