climate forcing occurs when the global energy balance of the Earth is changed. There are a number of mechanisms that can force climate change. The Earth's global climate is a dynamic system that is in equilibrium. On a planetary scale, it is steered by the amount of energy available in the system for use by its various ecosystems. If the amount of energy stored or received in the climate system changes, then climate changes occur as well. The global climate is affected by the Sun, which provides much of the energy in the global energy balance. If the energy from the Sun increases or decreases, climatic changes will likely occur. On the other hand, if the amount of energy kept by the Earth from sunlight increases, then climate changes will also occur.
The flows of energy occurring in the planetary climate system are important to the global climate system. The global climate system includes the heated core of the Earth and the parts of the Earth that receive the Sun's energy. The parts of the Earth's climate system include the atmosphere, the oceans, the cryosphere (ice caps at the poles and the alpine glaciers), the geosphere (for example, the reflective desert sands), the dense foliage of the tropics, the vast forests of the temperate and boreal zones, and other parts of the biosphere. All of these play a role in the convection system on the planet that transfers heat around the globe.
The atmosphere is central to the Earth's climate, which is a mixture of gases and aerosols (suspended liquids and solid articles). Commonly known as the air, the atmosphere is mostly nitrogen and oxygen. These two gases are available in amounts of about 78 percent nitrogen, 21 percent oxygen. They total about 99 percent of the air that surrounds the planet. The remaining gases and aerosols are present in only trace amounts; however, the greenhouse gases (such as carbon dioxide, methane, and nitrous oxide) play an extremely important part in the energy dynamics of the planet's global climate.
Greenhouse gases regulate the amount of heat that is present in the lower atmosphere. The gases capture radiant infrared energy and bounce it back to Earth. The gases act as a thin thermal blanket and have a natural greenhouse effect. As a result of greenhouse gases, the temperature of the Earth is 33 degrees C warmer than it would be otherwise. However, since the Industrial Revolution began about 200 years ago, the volume of human-made greenhouse gas emissions has increased enough to affect the greenhouse gas effect and to cause global warming.
Convection is the mechanism through which many energy transfers in the system are exchanged. The oceans are heated by the energy of sunlight. The sunlight is a system input that dynamically affects the global climate on a planetary scale. The output in the system is that most of the Earth's sunlight warming occurs in the tropics on either side of the equator. The warm seawater evaporates and rises, forming a lower pressure over the oceans and seas of the tropics and the land, as well. The rising warmer air is replaced by cooler, denser air flowing in from the polar regions. Movements in the atmosphere are also heat transfers. The atmosphere also stores energy. About 70 percent of the Earth's surface is covered by water. The oceans are energy storage centers that contain, in their top 656 ft. (200 m.) of ocean water, 30 times the amount of energy that is stored by the atmosphere.
The continental and alpine glaciers of the high mountains contain most of the Earth's cryosphere. Filled with ice and snow, and frozen by subzero temperatures for much of the year, the white surfaces reflect vast quantities of energy that would otherwise be absorbed by the Earth. The global climate system would then have a major energy source that would affect the dynamics of the interrelated parts of the system, and thereby invoke major changes in its output.
The plant portion of the Earth's biosphere (all living organisms, including humans) uses carbon dioxide and sunlight to photosynthesize food for growth. The oceans play an important role in this process because plankton, while microscopic, consume vast quantities of carbon dioxide. The gas is locked away in carbonate shells, which sink to the ocean depths. In the planetary climate system, this portion of the biological part of the system reduces the amount of carbon dioxide in the atmosphere and thereby weakens the greenhouse gas effect. Both the sunlight reflection of the cryosphere and the biosphere reduce the energy in the global climate system, thereby cooling the Earth. Clouds and the upper levels of the atmosphere also play a role in the global climate system. Clouds reflect vast volumes of sunlight into outer space. Energy in the form of x-rays, gamma rays, or ultraviolet energy is also reflected back into outer space.
The global energy balance can be affected by changes on Earth, such as an increase in volcanism. Active volcanoes send huge clouds of gases and ash into the atmosphere. If the volume is sufficiently large, the input of gases and ash to the system can affect the planetary climate system by blocking the amount of the available energy and temporarily forcing cooling of the planet. The major question is whether rising levels of anthropogenic carbon dioxide are forcing global warming.
SEE ALSO: Anthropogenic Forcing; Carbon Sinks; Cloud Feedback; Greenhouse Effect; Ocean Component of Models; Sunlight; Volcanism.
BIBLIOGRApHY. D.V. Hoyt and K.H. Shatten, Role of the Sun in Climate Change (Oxford University Press, 1996); William Kininmonth, Climate Change: A Natural Hazard (Multi-Science Publishing Co., Ltd., 2004); Staff of National Research Council, Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties (National Academies Press, 2005); D.G. Victor, Collapse of the Kyoto Protocol and the Struggle to Slow Global Warming (Princeton University Press, 2004).
Andrew J. Waskey Dalton State College
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