On the fever chart of rising temperatures, 2006 was the warmest year on record for the 48 contiguous states, pairing a summer heat wave with a mild winter - in some places, daffodils bloomed out of season and bears forgot to hibernate - government climate experts reported Tuesday.
Based on an analysis of readings from 1,200 weather stations, the average annual temperature in the 48 states last year was 2.2 degrees warmer than the mean temperature for the 20th century and fractionally warmer than 1998, which held the previous temperature record, the researchers reported.
Seven months last year were much warmer than average, concluded the scientists at the National Climatic Data Center in Asheville, N.C., a branch of the National Oceanic and Atmospheric Administration.
Indeed, last January was the warmest on record in the U.S., and December was the fourth warmest since record-keeping began in 1895. In five states, December temperatures set records: Minnesota, New York, Connecticut, Vermont and New Hampshire.
"We are breaking warm records all over the place," said climate scientist Gavin Schmidt at NASA's Goddard Institute for Space Studies in New York, who was not involved in the NOAA analysis.
Each of the past nine years have been among the 25 warmest years on record nationally - an unprecedented hot streak historically, the scientists said.
Overall, annual temperatures in the U.S. and around the world are 1 degree warmer than a century ago, and the rate of warming has accelerated threefold in recent decades. Eight of the past 10 years are the warmest on record worldwide. Climate experts at the British Meteorological Office last week predicted that this coming year could become the warmest year globally on record.
The warming temperature trends of recent years appear to closely track the general predictions of computerized climate models analyzing the effects of greenhouse gases on global climate patterns, several scientists said.
"It looks pretty much like what the climate models of global warming expected," said Penn State climate analyst Richard Alley, who was not part of the research effort. "We have turned up Earth's thermostat."
Rising levels of carbon dioxide and other greenhouse gases, which can trap heat in the atmosphere, were a key factor in the warming trend, the climate center researchers acknowledged. The rate at which carbon dioxide is being released into the atmosphere has doubled since than 1990s, Australian researchers recently reported, with 7.85 billion tons of carbon dioxide released into the atmosphere in 2005 alone.
But other influences, such as a mild El NiÃ±o current stirring in the Pacific Ocean, also played a role in blocking Arctic air that might normally chill the country, as did warmer conditions in the Indian Ocean, they said.
In 1998, record high temperatures were driven by an unusually powerful El NiÃ±o current that disrupted weather patterns worldwide. The current El NiÃ±o, a periodic warming current that took shape last summer, is far weaker and has had only a moderate effective on global climate, several experts said.
"What we are seeing is much more than El NiÃ±o," said climate analyst Kevin Trenberth at the National Center for Atmospheric Research in Boulder, Colo. "The overall pattern is consistent with our concepts of global warming."