NASA?s analysis of Earth?s surface temperature found that 2012 ranked as the ninth-warmest year since 1880. NASA scientists at the Goddard Institute for Space Studies (GISS) compare the average global temperature each year to the average from 1951 to 1980. This 30-year period provides a baseline from which to measure the warming Earth has experienced due to increasing atmospheric levels of heat-trapping greenhouse gases. While 2012 was the ninth-warmest year on record, all 10 of the warmest years in the GISS analysis have occurred since 1998, continuing a trend of temperatures well above the mid-20th century average. The record dates back to 1880 because that is when there were enough meteorological stations around the world to provide global temperature data.
Above text from War In Context.
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