Just how much warmer Earth will become as a result of greenhouse-gas emissions — and how much it has warmed since preindustrial times — is much debated. In a 2007 report, the Intergovernmental Panel on Climate Change, an agency formed by the United Nations to assess climate change, said that the planet’s average surface temperature will rise by between 2 and 11.5 degrees Fahrenheit by 2100, with a best estimate at between 3.2 to 7.2 degrees F. However, the IPCC’s computer models have a record of overestimating warming: If the IPCC models were right, the planet should now be hotter than it is.
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