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Global warming refers to the increase in the average temperature of the Earth’s near-surface air and oceans in recent decades and its projected continuation. The term “global warming” is a specific example of the broader term climate change, which can also refer to global cooling. In common usage the term refers to recent warming and implies a human influence. The United Nations Framework Convention on Climate Change (UNFCCC) uses the term “climate change” for human-caused change, and “climate variability” for other changes. The term “anthropogenic climate change” is sometimes used when focusing on human-induced changes.
Global average air temperature near the Earth’s surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the past century. The Intergovernmental Panel on Climate Change (IPCC) concludes, “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,” which leads to warming of the surface and lower atmosphere by increasing the greenhouse effect. Natural phenomena such as solar variation combined with volcanoes have probably had a small warming effect from pre-industrial times to 1950, but a small cooling effect since 1950. These basic 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 American Association of Petroleum Geologists is the only scientific society that rejects these conclusions. A few individual scientists disagree with some of the main conclusions of the IPCC.
Climate models referenced by the IPCC project that global surface temperatures are likely to increase by 1.1 to 6.4 °C (2.0 to 11.5 °F) between 1990 and 2100. The range of values reflects the use of differing scenarios of future greenhouse gas emissions and results of models with differences in climate sensitivity. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a millennium even if greenhouse gas levels are stabilized. This reflects the large heat capacity of the oceans.
Remaining scientific uncertainties include the exact degree of climate change expected in the future, and how changes will vary from region to region around the globe. There is ongoing political and public debate on a world scale regarding what, if any, action should be taken to reduce or reverse future warming or to adapt to its expected consequences. Most national governments have signed and ratified the Kyoto Protocol, aimed at combating greenhouse gas emissions.
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