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. Global surface temperature increased 0.74°C between the start and the end of the 20th century. The Intergovernmental Panel on Climate Change (IPCC) concludes that most of the observed temperature increase since the middle of the 20th century was very likely caused by increasing concentrations of greenhouse gases resulting from human activity such as fossil fuel burning and deforestation. Climate model projections summarised in the latest IPCC report indicate that the global surface temperature is likely to rise a further 1.1°C to 6.4°C during the 21st century.
An increase in global temperature will cause sea levels to rise and will change the amount and pattern of precipitation, probably including expansion of subtropical deserts. Warming is expected to be strongest in the Arctic and would be associated with continuing retreat of glaciers, permafrost and sea ice. Other likely effects include changes in the frequency and intensity of extreme weather events, species extinctions and changes in agricultural yields. Warming and related changes will vary from region to region around the globe, though the nature of these regional variations is uncertain.
Human activity since the Industrial Revolution has increased the amount of greenhouse gases in the atmosphere. The concentrations of carbon dioxide and methane have increased by 36% and 148% respectively since 1750. These levels are much higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. Less direct geological evidence indicates that carbon dioxide values higher than this were last seen about 20 million years ago. Fossil fuel burning has produced about three-quarters of the increase in carbon dioxide from human activity over the past 20 years. Most of the rest is due to land-use change, particularly deforestation.