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Greenhouse Gases

Many chemical compounds found in Earth's atmosphere act as greenhouse gases (GHGs). These gases allow sunlight to pass through the atmosphere; however, when the sunlight is reradiated back toward space as infrared radiation, GHGs absorb the infrared radiation and trap the heat in the atmosphere and at Earth's surface. GHGs allow Earth to be habitable—without them, the average temperature of the planet would be −18 °C instead of 15 °C. However, human actions are increasing the atmospheric concentrations of GHGs, resulting in an enhanced greenhouse effect that is warming Earth's climate and increasing the incidence of extreme weather events.

The most abundant GHGs are naturally occurring: water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and ozone (O3). There are also many other, highly potent, anthropogenic GHGs. Three of these also deplete the stratosopheric ozone layer: chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and bromofluorocarbons (i.e., halons). These ozone-depleting substances (ODSs) are controlled under the Montreal Protocol on Substances That Deplete the Ozone Layer of 1987, and as a result, their impact on both ozone and the greenhouse effect has been greatly reduced. Other anthropogenic fluorine-containing halogenated substances do not deplete stratospheric ozone but are potent GHGs. The most important of these are collectively called the F-gases and include hydrofluoro-carbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6). Numerous other minor trace gases complete the inventory of GHGs.

The contribution of a gas to the greenhouse effect is a result of both its characteristics and its atmospheric concentration. For example, on a molecule-by-molecule basis, CH4 is a much stronger greenhouse gas than CO2, but it is present in much lower concentrations, so that its total contribution is smaller. When these gases are ranked by their contribution to the greenhouse effect, the most important ones (excluding ODSs) are water vapor, CO2, CH4, and O3, followed by N2O and the three F-gases.

Institutional Framework

In 1992, 157 countries signed the United Nations Framework Convention on Climate Change (UNFCCC) and agreed to a common objective: “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.” As of 2009, 192 parties, including the European Union, have ratified the UNFCCC. This treaty focused attention on “anthropogenic” GHGs, that is, those GHGs that are either the direct result of human activities or the result of natural processes that have been affected by human activities.

The Kyoto Protocol (Japan) to the UNFCCC was negotiated in 1997; it focused on six GHGs (CO2, CH4, N2O, SF6, HFCs, and PFCs). This treaty established legally binding commitments for the reduction of these six gases by the industrialized nations, as well as general commitments for all member countries. The Kyoto Protocol went into force in 2005, and as of 2009, 181 parties, including the European Union, had ratified it. Under the Kyoto Protocol, the industrialized countries agreed to reduce their collective emissions of the six GHGs by 5.2% relative to the year 1990. Because of its dominant influence in global warming and climate change, the rest of this entry focuses on CO2.

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