The dominant mechanisms to which recent global warming and climate change have been attributed all result from human activity, such as increasing atmospheric concentrations of greenhouse gases, or aerosols. These attributions are supported by observations from the Intergovernmental Panel on Climate Change (IPCC). The IPCC concluded that most of the observed increase in globally averaged temperatures since the mid-20th century is "very likely" due to the observed increase in anthropogenic greenhouse gas concentrations via the greenhouse effect. It also concluded that it is "extremely likely" that human activities have exerted a substantial net warming influence on climate since 1700. These conclusions have probabilities greater than 90 and 95 percent, respectively.
Since the 1700s, the CO2 content of the atmosphere has been increasing as a result of air pollution emitted by human activities. Pollution has also caused a significant increase in secondary greenhouse gases, namely methane from animal digestion and bacteria, ozone and nitrogen oxides from urban air pollution, and chlorofluorocarbons from spray cans and fugitive refrigerants. The increase in CO2 and the four secondary greenhouse gases is a global trend. Since 1760, the atmospheric concentrations of CO2 and methane have increased by 32 percent and 151 percent, respectively, compared to the pre-industry period. Therefore, the biggest root cause is worldwide population growth.
According to the IPCC, the projected increase of global surface temperatures is likely to be 2.1-11.7 degrees F (1.2-6.5 degrees C) between 1995 and 2100, based on the different scenarios of future greenhouse gas emissions and climate sensitivity. These conclusions have been endorsed by at least 30 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries.
The argument related to global warming focuses on how much the greenhouse effect will be increased with the growing atmospheric concentrations of some greenhouse gases by human activities. Naturally occurring greenhouse gases consist of about 3670 percent water vapor (not including clouds), 9-26 percent CO2, 4-9 percent methane (CH4), and 3-7 percent ozone. Comparatively small fractions of the greenhouse effect are caused by naturally occurring gases such as nitrous oxide (N2O). Concentrations of N2O are increasing because of human activity such as agriculture. CO2 is produced mainly by natural processes such as volcanoes and rock weathering, and is recycled through the atmosphere by the biological processes of respiration and photosynthesis.
Over geologic time, atmospheric CO2 has varied appreciably, causing shifts in the Earth's heat balance. Over periods of thousands of years, however, the CO2 cycle can keep a fairly stable balance, thereby helping maintain a relatively stable global temperature regime. CO2 from human sources, however, represents a surcharge on the natural system.
Human sources of CO2 fall into roughly two major classes: fossil fuel combustion and open fires. Use of fossil fuels by humans contributes to three-quarters of the increase in CO2 over the past 25 years. The fossil fuel sources are industry, power plants, automobiles, and households; the developed countries are the principal contributors. The open-fire sources are associated with the clearing of tropical forests, forest fires in general, grassland fires, household cooking and heating, and wars. The rate of increase in atmospheric carbon dioxide approximates that of global economic growth, about 2 percent annually. Current carbon dioxide levels are about 30 percent above the 18th century (pre-industrial) level.
It is crucial to scientifically determine mechanisms responsible for the observed warming of the Earth, and many efforts have been made to prove changes observed during the last 50 years, when human activity has grown fastest and observations of the upper atmosphere have become available. There are measurable signs that warming is underway, because the global atmospheric temperature has increased at least 0.9 degrees F (0.5 degrees C) in the last 100 years. This rise corresponds to the measured increase in atmospheric CO2. Even though short-term temperature trends on the order of 50 or 100 years generally have little meaning with respect to global-scale warming, it is curious to note that studies of global temperature records dating from the late 1800s indicate that the 1980s and 1990s were the warmest two decades on record.
The IPCC has played an important role in providing the attribution of global warming, however, there are still several uncertainties that have to be resolved, including the exact degree of global warming expected in the future, and how changes will vary from region to region around the globe. One reason for scientists' uncertainty about how much global warming will take place is related to the rate of removal of CO2 from the atmosphere. CO2 is removed from the atmosphere by two main absorbing agents, vegetation and the oceans.
Under natural circumstances, it can be assumed that the oceans annually extracted 90 billion tons of CO2 from the atmosphere, an amount equal to the quantity they put into the atmosphere yearly. However, with the rise of global air pollution, the oceans appear to have increased their CO2 intake and now absorb a large part of the 7 billion tons of excess CO2 put into the atmosphere annually from human sources. The amount may be as great as three billion additional tons a year. Therefore, the net amount of CO2 left to accumulate in the atmosphere is about 4 billion tons a year.
At this rate, coupled with allowances for population growth, atmospheric CO2 is expected to double by 2050. Whether the oceans and land vegetation will increase or decrease their intake of CO2 in the future is difficult to claim at this point. Recent researchers concluded that the estimate of 3 billion tons of annual ocean intake may be high. The oceans may actually be taking in closer to 2 billion tons a year, and some other agency is responsible for extracting the remaining 1 billion tons of CO2. Perhaps the world's forests might be a good candidate to increase the rate of intake. Some studies suggest that air pollutants such as CO2 and sulfur dioxide may actually increase forest growth and intake CO2 in some regions. The annual rate of rainforest destruction, as much as 65,000 sq. mi. (168,349 sq. km.) a year, is a considerable loss, especially when forests have very high annual CO2 intake capacities.
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