With 2002 on track to be the second-warmest year on record, scientists said Monday they are confident that human activity, rather than a natural variation in Earth's climate, is responsible for the unprecedented warming trend of the past two decades.

Global data from NASA's Goddard Institute for Space Studies shows that the first 11 months of 2002 averaged 58.37 degrees F, just under the record high of 58.44 degrees set in 1998. Goddard director James Hansen said he is confident that the trend will hold true through the end of this month as well.

The 16 warmest years since record-keeping began in 1880 have all occurred since 1980. Last year was the second-warmest, but now drops to third.

Indeed, studies show that 1998 and 2002 are the two warmest years in the Northern Hemisphere in at least the last 1,000 years, said University of Virginia climatologist Michael Mann.

The estimate is based on both human record-keeping and physical records like ice cores, tree rings and coral, Mann said. A similar estimate has not been made for the Southern Hemisphere due to gaps in available data, he said.

Scientists had thought the record temperatures of the past two decades might be attributable to a natural, temporary warm spell. But the persistent increase in warming has "now taken us to a level that is well outside natural variability," Hansen said. "It is now clear that human (activity) has become dominant over the natural variability."

The human activity chiefly responsible for warming is the emission of carbon dioxide through the burning of fossil fuels like oil, coal and gas, scientists believe. Concentrations of carbon dioxide in the atmosphere have increased more than 30 percent since the start of the industrial revolution 200 years ago, with most of the increase occurring in the last 50 years.

Some scientists had argued that the warming trend of recent decades was comparable to a period of unusual warmth in the 1930s and 1940s, and thus might be the result primarily of natural fluctuations in Earth's climate. But the record warmth of the 1990s and early 21st century "go well beyond" previous periods of warm temperatures, Hansen said.

The average global temperature for January 2002 was 58.49 degrees, the highest of any January on record. March temperatures were the highest of any March on record. And from April through November, each month was either the second-, third- or fourth-warmest for its month on record.

Some scientists have questioned whether 1998's record warmth can be attributed primarily to global warming since it occurred in an El Nino year. El Nino is a disruption of the ocean-atmosphere system in the tropical Pacific associated with increased rain and warm temperatures around the globe.

But there is no El Nino this year and 2002 temperatures are nearly as high as those of 1998, Mann noted.

"It seems obvious just to the lay person that if we're getting these record breakers almost every year something unusual is going on that doesn't just result from a random rolling of the dice," Mann said. "It suggests we're moving well beyond the envelope of variability of climate that we've seen in the past into levels of warmth that just doesn't happen naturally."

On the Net: Goddard Institute for Space Studies - www.giss.nasa.gov/research/observe/surftemp/

© 2002 The E.W. Scripps Co.

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