Global Warming

Global warming is the increase in the average temperature of Earth's 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 2070's 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...