The global average air temperature near the Earth’s surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the last 100 years. The Intergovernmental Panel on Climate Change (IPCC) concludes, “most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations via the greenhouse effect. Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward. The range of values results from the use of differing scenarios of future greenhouse gas emissions as well as models with differing climate sensitivity. An increase in global temperatures is expected to cause the sea level to rise, increase the intensity of extreme weather events, and change the amount and pattern of precipitation. Other effects of global warming include changes in agricultural yields, glacier retreat, species extinctions and increases in the ranges of disease vectors.

Most national governments have signed and ratified the Kyoto Protocol, aimed at reducing greenhouse gas emissions.

The term “global warming” is a specific example of the broader term climate change, which can also refer to global cooling. The United Nations Framework Convention on Climate Change (UNFCCC) uses the term “climate change” for human-caused change, and “climate variability” for other changes.

Earth’s climate changes in response to external forcing, including variations in its orbit around the sun (orbital forcing), volcanic eruptions, and atmospheric greenhouse gas concentrations. The detailed causes of the recent warming remain an active field of research, but the scientific consensus identifies elevated levels of greenhouse gases due to human activity as the main influence. In contrast to the scientific consensus that recent warming is mainly attributable to elevated levels of greenhouse gases, other hypotheses have been suggested to explain the observed increase in mean global temperature. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.

Greenhouse process is the process by which absorption and emission of infrared radiation by atmospheric gases warms a planet’s atmosphere and surface.

Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F), without which Earth would be uninhabitable. Some other naturally occurring gases contribute very small fractions of the greenhouse effect; one of these, nitrous oxide (N₂O), is increasing in concentration owing to human activity such as agriculture. The atmospheric concentrations of CO₂ and methane have increased by 31% and 149% respectively above pre-industrial levels since 1750. Future CO₂ levels are expected to rise due to ongoing burning of fossil fuels and land-use change. Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used.