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 global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the last 100 years.
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" via the greenhouse effect.
Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward.
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.
However, 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 results from the use of differing scenarios of future greenhouse gas emissions as well as models with differing 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. An increase in global temperatures is expected to cause other changes, including sea level rise, increased intensity of extreme weather events, and changes in the amount and pattern of precipitation.
Other effects of global warming include changes in agricultural yields, glacier retreat, species extinctions and increases in the ranges of disease vectors. Remaining scientific uncertainties include the amount of warming expected in the future, and how warming and related changes will vary from region to region around the globe.
There is ongoing political and public debate worldwide 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 reducing greenhouse gas emissions. Sparse records indicate that glaciers have been retreating since the early 1800s.
In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC. Sparse records indicate that glaciers have been retreating since the early 1800s.
In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC. Though it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause other changes, including glacial retreat and worldwide sea level rise.
Changes in the amount and pattern of precipitation may result in flooding and drought.
There may also be changes in the frequency and intensity of extreme weather events.
Other effects may include changes in agricultural yields, reduced summer streamflows, species extinctions and increases in the range of disease vectors. Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming.
A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, increased intensity and frequency of extreme weather events, are being attributed in part to global warming.
While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming.
Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects from warmer temperatures.
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