Global warming is the increase in the average temperature of Earths near-surface air and oceans since the mid-20th century and its projected continuation. The scientific consensus is that global warming is occurring and is mostly the result of human activity. This finding is recognized by the national science academies of all the major industrialized countries and is not rejected by any scientific body of national or international standing.[2][3][4][B]

During the 20th century, global surface temperature increased by 0.74 ?± 0.18 ?°C (1.33 ?± 0.32 ?°F).[5][A] Most of the observed temperature increase since the middle of the 20th century has been caused by increasing concentrations of greenhouse gases, which result from human activities such as the burning of fossil fuel and deforestation.[6][7]

Regarding future warming, the 2007 Fourth Assessment Report by the Intergovernmental Panel on Climate Change (IPCC) used climate model projections to assess six possible greenhouse-gas emission scenarios. Depending on which scenario most closely matches actual overall greenhouse gas emissions, global surface temperature is likely to rise a further1.1 to 6.4 ?°C (2.0 to 11.5 ?°F) by 2100,[5][8] and the upper limit of that range does not include any warming from the potential release of certain carbon-cycle feedbacks.[9] By 2010, more recent observations of emissions made the A1FI scenario the “business as usual” case[10], and confirmed that “the worst-case IPCC scenario trajectories or even worse are being realised”.[11] Including of some of the carbon-cycle feedbacks in the A1FI scenario increases the best estimate to a temperature rise of 4?°C in the 2070s and 5?°C by 2090, with a potential range between 3-7?°C.[9], and even that estimate appears conservative because it does not include feedbacks from permafrost[12], which could be extreme.

In even a 4?°C world, the limits for human adaptation are likely to be exceeded in many parts of the world, while the limits for adaptation for natural systems would largely be exceeded throughout the world. Hence, the ecosystem services upon which human livelihoods depend would not be preserved.[13] For these reasons, virtually all climatologists are now convinced that global warming poses a clear and present danger to civilization. [14]

The uncertainty in IPCCs estimates arises from (1) the use of models with differing sensitivity to greenhouse gas concentrations, (2) the use of differing estimates of humanities future greenhouse gas emissions, and (3) any additional emissions from climate feedbacks that were not included in the models IPCC used to prepare its report, i.e., greenhouse gas releases from permafrost[15]

An increase in global temperature will cause sea levels to rise and will change the amount and pattern of precipitation, probably including expansion of subtropical deserts.[16] Warming is expected to be strongest in the Arctic and would be associated with continuing retreat of glaciers, permafrost and sea ice. Other likely effects of the warming include more frequent occurrence of extreme weather events including heatwaves, droughts and heavy rainfall events, species extinctions due to shifting temperature regimes, and changes in agricultural yields. Warming and related changes will vary from region to region around the globe, though the nature of these regional changes is uncertain.[17] Warming of around 6.7 ?°C would cause some areas of the world to surpass the wet-bulb temperature limit of human survivability, and warming of 11.7 ?°C would put half of the worlds population in an uninhabitable environment.[18][19]

According to a recent Gallup poll, people in most countries are more likely to attribute global warming to human activities than to natural causes. The major exception is the U.S., where nearly half the US population attributes global warming to natural causes despite overwhelming scientific opinion to the contrary.[20]

The Kyoto Protocol is aimed at stabilizing greenhouse gas concentration to prevent a “dangerous anthropogenic interference”.[21] As of May 2010, 192 states had ratified the protocol.[22] The only members of the UNFCCC that were asked to sign the treaty but have not yet ratified it are the USA and Afghanistan. Proposed responses to global warming include mitigation to reduce emissions, adaptation to the effects of global warming, and geoengineering to remove greenhouse gases from the atmosphere or reflect incoming solar radiation back to space.