Effects could be severe, federal researchers warn.
Scientists still debate if man or nature is to blame.
Such a steep warming rate was not expected to occur until well into the 21st century, said Tom Karl, the National Oceanic and Atmospheric Administration climatologist who led the study. It could be the beginning of a new increase in temperatures. Historical and geological records show that the Earth warms and cools in fits and starts, not at a constant rate. During the 1900s, most warming occurred between 1910 and 1940 and then after 1970. In contrast, warming since 1976 occurred at a rate of nearly four degrees per century.