NOAA 98-88

CONTACT:  Patricia Viets, NOAA              FOR IMMEDIATE RELEASE
          Barbara McGehan, NOAA             12/7/98  
          John Leslie, NOAA        


Paleoclimatologists, using a compilation of available data from around the Northern Hemisphere, have confirmed that 20th century global warming is unprecedented relative to the last 1200 years. Jonathan Overpeck, head of NOAA's Paleoclimatology Program in Boulder, Colo., says that research has failed to identify any known natural climate-forcing mechanism that could have generated all of the unprecedented warming that has led to 1998 being, most likely, the warmest year in at least 1200 years.

Overpeck also said that the so-called Medieval Warm Period, a period from the 9th to 14th centuries that is commonly thought to be as warm or warmer than today, may not have been what it seemed after all. He reported his findings today at the fall meeting of the American Geophysical Union in San Francisco. He presented a talk on "How Unprecedented is Recent Arctic Warming: A Look Back to the Medieval Warm Period."

Overpeck's work, building on that of others, suggests that there was no global Medieval Warm period, and that the patterns of climate change during that time indicate that changes in North Atlantic circulation might have been the cause of observed regional warming.

"Over the past decade, methods were developed and refined to extract paleotemperature estimates from a wide variety of natural 'proxy' sources, such as ocean and lake sediments, glacier ice cores, tree rings, and historical documents," he said. "The use of these multiple sources is enabling paleoclimatologists to construct a network of records that cover much of the Northern Hemisphere, and also to avoid biases inherent in using any one source."

"Over the past year, a number of studies have shown that 20th century Arctic and hemispheric warming are unprecedented relative to the last six centuries," said Overpeck. "Now, high-resolution paleoclimate records stretching back 1200 years confirm that the so-called Medieval Warm Period did not exist in the form of a globally synchronous period as warm, or warmer, than today. Thus, recent record high hemispheric temperatures are probably unprecedented in at least 1200 years. In addition, our study of the Medieval Warm Period supports the likelihood that no known natural phenomenon can explain the record 20th century warmth. Twentieth Century global warming is a reality and should be taken seriously."


NOTE: Dr. Overpeck's presentation will be given on Monday, Dec. 7, 1998, at 3:45 p.m. in Room MC 131 of the Moscone Center.