"Global warming is the theory that average temperatures around the world have begun to rise, and will continue to rise, because of an increase of certain gases in Earth's atmosphere. These gases are called ""greenhouse gases"" because they trap heat just like a greenhouse. The most plentiful greenhouse gases are water vapor and carbon dioxide. The increase of carbon dioxide in the atmosphere is believed to be the main reason for global warming. What Causes Global Warming? Approximately two-thirds of the energy earth receives from the sun is absorbed by land masses and oceans and is then released into the atmosphere as warm, long-wave radiation. The atmosphere of earth is full of so-called greenhouse gases such as water vapor, carbon dioxide, ozone, methane, and nitrous oxide that act like a blanket, trapping some of the heat radiating from the land and oceans and preventing too much energy from escaping into space. The gas blanket works in much the same way as the glass panels of a greenhouse, serving to trap energy and keep temperatures at a steady level. The trapped heat keeps earth at a comfortable average temperature of about sixty-three degrees Fahrenheit. This process is known as the greenhouse effect. Point 1 - Global warming Caused by Natural Geologic Trends Human activities such as clear-cutting rain forests, burning fossil fuels, and driving cars contribute toward global warming. But human effects on climate are negligible compared with the effects naturally produced by the Earth, the atmosphere, and the Sun. The term global warming can be misleading or even alarmist. A warming climate is a complex and erratic phenomenon, which may include or even pass over into long or short periods of global or regional cooling. Similarly, periods of global cooling can include or pass over into warming periods. Until fairly recently in geologic time, there was a long period when the Earth was too warm to allow the formation of polar ice caps. Ocean levels naturally fall and rise as the Earth alternately cools and warms, and as the size of the polar ice caps consequently increases and decreases. Probably the greatest single factor in determining global cooling and warming cycles is the fluctuation of the Sun's energy output, over which humans obviously have no control. Humans released twice as much carbon dioxide into the atmosphere in 1990 as in 1958, but very little of this carbon dioxide remained in the atmosphere to increase the overall concentration of greenhouse gases. The main reason that atmospheric carbon dioxide concentration has not kept pace with industrial carbon dioxide emissions is that carbon dioxide is soluble in water. The oceans are far from saturated with carbon dioxide. A more troublesome way that humans increase carbon dioxide levels is by deforestation, which increases the amount of carbon dioxide in the atmosphere by reducing the number of trees and other green plants that use atmospheric carbon dioxide in photosynthesis. Another climatological factor is volcanic activity, a widespread, powerful, and completely natural phenomenon that generally has the effect of lowering atmospheric temperature after each eruption. There is no denying the danger facing such places as Venice, New Orleans, the Netherlands, and Bangladesh, which may all disappear under the sea within the next few hundred years. That danger is quite real, but little or nothing can be done about it, as natural causes far outstrip anthropogenic causes of the current global warming trend. Point 2 - Global Warming Caused by Humans According to the Third Assessment Report of the Inter-governmental Panel on Climate Change (IPCC) released in 2001, ""There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities."" Greenhouse gases accumulating from burning fossil fuels, and other gas-producing activities of industry, are blanketing the Earth and so causing the warming trend. The blanket of greenhouse gases (water vapor, carbon dioxide, methane, nitrous oxide, and tropospheric ozone) keeps the planet about 59F (15C) warmer than it would be without the cover. Water vapor is by far the most abundant greenhouse gas. The greenhouse effect gets its name because a layer in the atmosphere acts like the glass in a greenhouse, letting the Sun's light energy in, but preventing reflected lower energy heat radiation from escaping. All the greenhouse gases that occur naturally are also produced by human activities, thus increasing their levels in the atmosphere at an accelerating pace. In addition to the ""natural"" greenhouse gases are the fluorocarbons, which only appeared in the atmosphere when introduced by humans. Over the millennium before the Industrial Era, the atmospheric concentrations of greenhouse gases remained relatively constant, according to the IPCC report. Chlorofluorocarbons are believed by most scientists to be responsible for depleting the ozone layer that exists at the top of the stratosphere, the second layer in the Earth's atmosphere. The upper stratospheric ozone layer shields the Earth from intense ultraviolet radiation that causes skin cancer."
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