How has the global climate changed over the past 100 years

Averaged over all land and ocean surfaces, temperatures have warmed roughly 1.33°F (0.74ºC) over the last century, according to the Intergovernmental Panel on Climate Change (see page 2 of the IPCC's Climate Change 2007: Synthesis Report Summary for Policymakers (PDF)). More than half of this warming—about 0.72°F (0.4°C)—has occurred since 1979. Because oceans tend to warm and cool more slowly than land areas, continents have warmed the most (about 1.26°F or 0.7ºC since 1979), especially over the Northern Hemisphere.

The year 1998 was the warmest on record for the contiguous United States, followed closely by 2006 and 1934, according to the National Climatic Data Center. In 2008, the U.S. saw its coolest year in more than a decade. It was the first time since 1997 that the nation has been close to its 100-year average temperature (though 2008 was still slightly above that norm). The United States was actually one of the least-warm spots on Earth in 2008 when compared to local averages. The globe as a whole had its coolest year since 2000, but the global average for 2008 was still warmer than any year from 1880 to 1996, according to NCDC.

There are slight differences in global records between groups at NCDC, NASA, and the University of East Anglia. Each group calculates global temperature year by year, using slightly different techniques. However, analyses from all three groups point to the decade between 1998 and 2008 as the hottest since 1850.

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