Global warming is the increase in the average temperature (Instrumental temperature record) of Earths near-surface air and oceans since the mid-20th century and its projected continuation. According to a report by the Intergovernmental Panel on Climate Change (IPCC) global surface temperature increased 0.74 � 0.18 �C (Celsius) (1.33 � 0.32 �F (Fahrenheit)) between the start and the end of the 20th century.Cref2A The climatology literature uniformly states that most of the observed temperature increase since the middle of the 20th century was caused (attribution of recent climate change) by increasing concentrations of greenhouse gases, which results from human activity (anthropogenic) such as fossil fuel burning and deforestation.

Climate model projections summarized in the latest IPCC report indicate that the global surface temperature is likely to rise a further nowrap1.1 to 6.4 �C nowrap(2.0 to 11.5 �F) during the 21st century. The uncertainty in this estimate arises from the use of models with differing sensitivity to greenhouse gas concentrations (climate sensitivity) and the use of differing estimates of future greenhouse gas emissions (Special Report on Emissions Scenarios). An increase in global temperature will cause sea levels to rise (sea level rise) and will change the amount and pattern of precipitation (precipitation (meteorology)), probably including expansion of subtropical (Subtropics) deserts. Warming is expected to be strongest in the Arctic (Arctic shrinkage) and would be associated with continuing retreat of glaciers (retreat of glaciers since 1850), permafrost and sea ice. Other likely effects include changes in the frequency and intensity of extreme weather events, species extinctions (extinction risk from climate change), 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.

The overwhelming scientific consensus is that human-made global warming is real. Nevertheless, political (Politics of global warming) and public (global warming controversy) debate continues. Some oil companies have funded public relations campaigns and deeply flawed research studies intended to discredit the global scientific consensus (Scientific opinion on climate change).