2012: Hottest year on record in U.S.

The numbers are in: 2012, the year of a surreal March heat wave, a severe drought in the Corn Belt and a massive storm that caused broad devastation in the mid-Atlantic states, turns out to have been the hottest year recorded in the contiguous United States.

How hot was it? The temperature differences between years are usually measured in fractions of a degree, but last year's 55.3 degree average demolished the previous record, set in 1998, by a full degree Fahrenheit.

If that does not sound sufficiently impressive, consider that 34,008 daily high records were set at U.S. weather stations, compared with only 6,664 record lows, according to a count maintained by Weather Channel meteorologist Guy Walton, using federal temperature records.

That ratio, which was roughly in balance as recently as the 1970s, has been out of whack for decades as the country has warmed, but never by as much as it was last year.

"The heat was remarkable," said Jake Crouch, a scientist with the National Climatic Data Center in Asheville, N.C., which released the official climate compilation Tuesday. "It was prolonged. That we beat the record by one degree is quite a big deal."

Scientists said that natural variability almost certainly played a role in last year's extreme heat and drought. But many of them expressed doubt that such a striking record would have been set without the backdrop of global warming caused by the human release of greenhouse gases. And they warned that 2012 was likely a foretaste of things to come, as continuing warming makes heat extremes more likely.

Even so, the last year's U.S. record is not expected to translate into a global temperature record when figures are released in coming weeks. The year featured a La Nina weather pattern, which tends to cool the global climate overall, and scientists expect it to be the world's eighth- or ninth-warmest year on record.

In addition to being the warmest year, 2012 turned out to be the second-worst on a measure called the Climate Extremes Index, surpassed only by 1998.

Experts are still counting, but so far 11 disasters in 2012 have exceeded a threshold of $1 billion in damages.

Among the cities that set temperature records in 2012 were Nashville; Athens, Ga.; and Cairo, Ill., all of which hit 109 degrees June 29; Greenville, S.C., which hit 107 degrees July 1; and Lamar, Colo., which hit 112 degrees June 27.

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