Contact: Harvey Leifert
American Geophysical Union

WASHINGTON — Researchers at NOAA’s National Climate Data Center (NCDC) have found evidence that the rate of global warming is accelerating and that in the past 25 years it achieved the rate of two degrees Celsius (four degrees Fahrenheit) per century. This rate had previously been predicted for the 21st Century.

Writing in the March 1 issue of Geophysical Research Letters, Dr. Thomas R. Karl, Director of NCDC, and colleagues analyze recent temperature data. They focus particularly on the years 1997 and 1998, during which a string of 16 consecutive months saw record high global mean average temperatures. This, Karl notes, was unprecedented since instruments began systematically recording temperature in the 19th Century. During much of 1998, records set just the previous year were broken.

Karl and his colleagues conclude that there is only a one-in-20 chance that the string of record high temperatures in 1997-1998 was simply an unusual event, rather than a change point, the start of a new and faster ongoing trend. Since completing the research, the data for 1999 have been compiled. They found that 1999 was the fifth warmest year on record, although as a La Nina year it would normally be cooler. Outside the band between 20 degrees north latitude and 20 degrees south latitude, 1999 was the second warmest year of the 20th Century, just behind 1998, an El Nino year.

The researchers at NCDC, part of the National Oceanic and Atmospheric Administration (NOAA), based in Asheville, North Carolina, analyzed data from land based and satellite instruments for their study. They conclude that the rate of warming since 1976 is clearly greater than the average rate over the late 19th and 20th Centuries. To account for the string of record setting temperatures, the average rate of global temperature increase since 1976 would have to be three degrees Celsius (five degrees Fahrenheit) per century.

In its Second Assessment Report in 1995, the Intergovernmental Panel on Climate Change (IPCC) projected the rate of warming for the 21st Century to be between one and 3.5 degrees Celsius (two and six degrees Fahrenheit). Karl and his colleagues have already observed over the past 25 years a rate that is between two and three degrees Celsius (four and five degrees Fahrenheit) per century. The IPCC study used a “business as usual” scenario with regard to manmade influences on climate, such as carbon dioxide and other atmospheric constituents.

Karl and his colleagues are not ready to say with certainty that the rate of global warming has suddenly increased, because they recognize that unusual events sometimes happen. There is strong evidence, they say, that the faster rate of climate change since 1976 is human-induced. Given the steady increase in atmospheric greenhouse gases and the length of time, ranging from decades to centuries, that they remain in the atmosphere, they urge that studies be conducted to enable society to minimize the risks of climate change and prepare for more, and perhaps even more rapid, changes to come.