Global warming refers to an unequivocal and continuing rise in the average temperature of Earth's climate system.
Since 1971, 90% of the warming has occurred in the oceans.Despite the oceans' dominant role in energy storage, the term "global warming" is also used to refer to increases in average temperature of the air and sea at earth's surface.
Since the early 20th century, the global air and sea surface temperature has increased about 0.8 °C (1.4 °F), with about two-thirds of the increase occurring since 1980.
Each of the last three decades has been successively warmer at Earth's surface than any preceding decade since 1850.
Scientific understanding of the cause of global warming has been increasing.
In its fourth assessment (AR4 2007) of the relevant scientific literature, the International Panel on Climate Change (IPCC) reported that scientists were more than 90% certain that most of global warming was being caused by increasing concentrations of greenhouse gases produced by human activities.
In 2010 that finding was recognized by the national science academies of all major industrialized nations.Affirming these findings in 2013, IPCC says that the largest driver of global warming is carbon dioxide (CO2) emissions from fossil fuel combustion, cement production, and land use changes such as deforestation.
Future climate change and associated impacts will vary from region to region around the globe.
The effects of an increase in global temperature include a rise in sea levels and a change in the amount and pattern of precipitation, as well as a probable expansion of subtropical deserts.
Warming is expected to be strongest in the Arctic, with the continuing retreat of glaciers, permafrost and sea ice.
Other likely effects of the warming include more frequent extreme weather events including heat waves, droughts and heavy rainfall; ocean acidification; and species extinctions due to shifting temperature regimes.
Effects significant to humans include the threat to food security from decreasing crop yields and the loss of habitat from inundation.