SUMMARY
Global warming is the rise in the average temperature of Earth's atmosphere and oceans since the late 19th century and its projected continuation. Since the early 20th century, Earth's mean surface temperature has increased by about 0.8 C (1.4 F), Warming of the climate system is unequivocal, and scientists are more than 90% certain that it is primarily caused by increasing concentrations of greenhouse gases produced by human activities such as the burning of fossil fuels and deformation. Climate model projections were summarized in 21st Century Fourth Assessment Report (AR4) by the Intergovernmental Panel on Climate Change (IPCC). They indicated that during the 21st century the global surface temperature is likely to rise a further 1.1 to 2.9 C (2 to 5.2 F) for their lowest emission scenario and 2.4 to 6.4 C (4.3 to 11.5 F) for their highest. The ranges of these estimates arise from the use of models with differing sensitivity to greenhouse gas concentrations. The Earth's average surface temperature rose by .74 0.18 C over the period 19062005.Temperature changes vary over the globe. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 C per decade against 0.13 C per decade).Proposed policy responses to global warming include mitigation by emissions reduction, adaptation to its effects, and possible future geo engineering. Most countries are parties to the United Nations Framework Convention on Climate Change (UNFCCC), whose ultimate objective is to prevent dangerous anthropogenic climate change. Reports published by the United Nations Environment Programme (UNEP) and the International Energy Agency suggest that efforts as of the early 21st century to reduce emissions may be inadequate to meet the UNFCCC's 2 C target. Future of 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 a probable expansion of subtropical deserts. Warming is expected to be strongest in the Arctic and would be associated with the continuing retreat of glaciers, permafrost and sea ice. Effects significant to humans include the threat to food security from decreasing crop yields and the loss of habitat from inundation.