Global warming

Is the increase in the average temperature of the Earth's near-surface air and oceans since the mid-20th century and its projected continuation. Global surface temperature increased 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the last century. TheIntergovernmental Panel on Climate Change(IPCC) concludes that most of the observed temperature increase since the middle of the 20th century is caused by increasing concentrations ofgreenhouse gases resulting from human activity such as fossil fuelburning and deforestation.The IPCC also concludes that variations in natural phenomena such as solar radiation and volcanoesproduced most of the warming from pre-industrial times to 1950 and had a small cooling effect afterward. These basic conclusions have beenendorsed by more than 40 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries. A small number of scientists dispute the consensus view.

Climate model projections summarized in the latest IPCC report indicate that the globalsurface temperature will probably rise a further1.1 to 6.4 °C(2.0 to 11.5 °F) during the twenty-first century. The uncertainty in this estimate arises from the use of models with differingsensitivity to greenhouse gas concentrationsand the use of differingestimates of future greenhouse gas emissions. Some otheruncertaintiesinclude how warming and related changes will vary from region to region around the globe. Most studies focus on the period up to the year 2100. However, warming is expected to continue beyond 2100 even if emissions stop, because of the large heat capacity of the oceans and the long lifetime ofcarbon dioxide in the atmosphere.

An increase in global temperature will cause sea levels to rise and will change the amount and pattern ofprecipitation, probably including expansion of subtropicaldeserts. The continuing retreat of glaciers,permafrost and sea ice is expected, with warming being strongest in the Arctic. Other likely effects include increases in the intensity ofextreme weather events,species extinctions, and changes in agricultural yields.

Political and public debatecontinues regarding climate change, and what actions (if any) to take in response. The available options aremitigationto reduce further emissions;adaptation to reduce the damage caused by warming; and, more speculatively,geoengineering to reverse global warming. Most national governments have signed and ratified the Kyoto Protocolaimed at reducing greenhouse gas emissions.