Global warming is the increase in the average temperature 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 100 years ending in 2005. The Intergovernmental Panel on Climate Change (IPCC) concludes that most of the temperature increase since the mid-twentieth century is "very likely" due to the increase in anthropogenic greenhouse gas concentrations. Natural phenomena such as solar variation and 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. While a small minority have voiced disagreement with these findings, the overwhelming majority of scientists working on climate change agree with the IPCC's main conclusions. However, there is significant evidence that the climate models currently in use are unduly optimistic, as they fail generally to include non-linear effects such as the clathrate gun, which may lead to runaway climate change.

Climate model projections indicate that global surface temperature will likely rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the twenty-first century. The uncertainty in this estimate arises from use of differing estimates of future greenhouse gas emissions and from use of models with differing climate sensitivity. Another uncertainty is how warming and related changes will vary from region to region around the globe. Although most studies focus on the period up to 2100, warming is expected to continue for more than a thousand years even if greenhouse gas levels are stabilized unless geo engineering is used. This results from the large heat capacity of the oceans.