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Global WarmingFebruary 22, 2000 — NOAA Researchers have found evidence that indicates that the rate of global warming is accelerating and that in the past 25 years it achieved the rate previously predicted for the 21st century (2 degrees C per century).

Writing in the March 1 issue of Geophysical Research Letters, Thomas R. Karl, director of NOAA's National Climatic Data Center in Asheville, N.C., and his 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 colleagues conclude that there is only a small 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 has been compiled. The researchers found that 1999 was the fifth warmest year on record, although as a La Niña year it would normally be cooler. Outside the band 20 degrees north latitude and 20 degrees south latitude, 1999 was the second warmest year of the 20th century, just behind 1998, an El Niño year.

The researchers at NCDC analyzed data from land based and satellite instruments for their study. Using sophisticated mathematical and probabilistic models in a series of experiments they concluded 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 projected the rate of warming for the 21st century to be between 1.0 and 3.5 degrees C. Karl and his colleagues have already observed over the past 25 years a rate that is between 2 and 3 degrees C per century. The IPCC study used a "business as usual" scenario with regard to emissions of carbon dioxide and other atmospheric constituents.

Karl and his colleagues aren't ready to say for certain that the rate of global warming has suddenly increased, because they know unusual events sometimes happen. Given the steady increase in atmospheric greenhouse gases and their decades-to-centuries atmospheric residence time, he urges that studies be conducted to better understand how society can minimize the risks of climate change and prepare for more, and perhaps even more rapid changes to come.

Editor's Note: The paper will be published in Geophysical Research Letters vol. 27, no. 5, March 1, 2000 (page number not yet determined).

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