Global warming
Global mean surface temperatures 1856 to 2004
Mean temperature anomalies during the period 1995 to 2004 with respect to the average temperatures from 1940 to 1980Global warming is an increase in the average temperature of the Earth's atmosphere and oceans. The term is also used for the more specific scientific theory of anthropogenic global warming, which states that much of the recent observed (and projected) global warming is human-induced and the result of a strengthened greenhouse effect caused by man-made increases in carbon dioxide (through the burning of fossil fuels and deforestation) and other greenhouse gases. The natural greenhouse effect keeps the Earth 30 °C warmer than it otherwise would be; adding carbon dioxide to an atmosphere, with no other changes, will make a planet's surface warmer. Current research tries to uncover more details, e.g. about positive and negative feedback mechanisms, to allow a more precise quantification of the effects of global warming.
Temperature change is just one aspect of the broader subject of (human-induced) climate change. The scientific opinion on climate change, as expressed by the UN Intergovernmental Panel on Climate Change (IPCC) and explicitly endorsed by the national science academies of the G8 nations, is that the average global temperature has risen 0.6 ± 0.2 °C since the late 19th century, and that "most of the warming observed over the last 50 years is attributable to human activities". A small minority of qualified scientists contest the view that humanity's actions have played a significant role in increasing recent temperatures. Uncertainties do exist regarding how much climate change should be expected in the future, and a hotly contested political and public debate exists over what actions, if any, should be taken in light of global warming.
Based on basic science, observational sensitivity studies, and the climate models referenced by the IPCC, temperatures may increase by 1.4 to 5.8 °C between 1990 and 2100. This is expected to result in other climate changes including rises in sea level and changes in the amount and pattern of precipitation. Such changes may increase extreme weather events such as floods, droughts, heat waves, and hurricanes, change agricultural yields, or contribute to biological extinctions. Although warming is expected to affect the frequency and magnitude of these events, it is very difficult to connect any particular event to global warming.