The United Nations in December reported that 1998 was the hottest year on record, while scientists with the National Oceanic and Atmospheric Administration said the 20th century was the warmest in at least 1,200 years.

Earth's average temperature in 1998 was projected to be 1 degree above the long-term average of 59 F, said the World Meteorological Association. That is the warmest since recordkeeping began in 1860, according to the U.N. weather agency. It will be the 20th consecutive year in which the Earth's surface temperature has been above normal. Temperature records have been set in each of the past 18 months.

Meanwhile, NOAA paleoclimatologists said a compilation of data from the Northern Hemisphere confirmed that 20th century global warming is unprecedented relative to the last 1,200 years.

Jonathan Overpeck, head of NOAA's Paleoclimatology Program in Boulder, CO, said research has failed to identify any known natural climate-forcing mechanism that could have generated all of the warming that has led to 1998 being, most likely, the warmest year in at least 1,200 years.

Overpeck also said that the so-called Medieval Warm Period, a period from the 9th to 14th centuries that is thought to be as warm as or warmer than today, may not have been what it seemed. He reported his findings at the fall meeting of the American Geophysical Union in San Francisco. His 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from a wide variety of natural 'proxy' sources, such as ocean and lake sediments, glacier ice cores, tree rings, and historical documents," he said.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 covers much of the Northern Hemisphere."

"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," he said. "Now, high-resolution paleoclimate re-cords stretching back 1,200 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. As a result, he said, recent record high hemispheric temperatures "are probably unprecedented in at least 1,200 years. In addition, our study of the Medieval Warm Period supports the likelihood that no known natural phenomenon can explain the 20th century warmth. Twentieth century global warming is a reality and should be taken seriously."