Global warming is the increase in the average temperature of the planet 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) between the start and the end of the 20th century.[1][A] The Intergovernmental Panel on Climate Change (IPCC) concludes that most of the observed temperature increase since the middle of the 20th century was caused by increasing concentrations of greenhouse gases resulting from human activity such as fossil fuel burning and deforestation.[1] The IPCC also concludes that variations in natural phenomena such as solar radiation and volcanism produced most of the warming from pre-industrial times to 1950 and had a small cooling effect afterward.[2][3] These basic conclusions have been endorsed by more than 40 scientific societies and academies of science,[B] including all of the national academies of science of the major industrialized countries.[4]
The effects of global warming and climate change[2] are of concern both for the environment and human life. Evidence of observed climate change includes the instrumental temperature record, rising sea levels, and decreased snow cover in the Northern Hemisphere.[3] According to the IPCC Fourth Assessment Report, "[most] of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in [human greenhouse gas] concentrations". It is predicted that future climate changes will include further global warming (i.e., an upward trend in global mean temperature), sea level rise, and a probable increase in the frequency of some extreme weather events. Ecosystems are seen as being particularly vulnerable to climate change. Human systems are seen as being variable in their capacity to adapt to future climate change.[4] To reduce the risk of large changes in future climate, many countries have implemented policies designed to reduce their emissions of greenhouse gases.
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