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.
Assuming that prediction holds up, it will mean that the 10 warmest years on record all fell within the past 15 years, a measure of how much the planet has warmed. Nobody who is 28 has lived through a month of global temperatures that fell below the 20th-century average, because the last such month was February 1985.
Last year’s weather in the United States began with an unusually warm winter, with relatively little snow across much of the country, followed by a March that was so hot that trees burst into bloom and swimming pools opened early. The soil dried out in the March heat, helping to set the stage for a drought that peaked during the warmest July on record.
The drought engulfed 61 percent of the nation, killed corn and soybean crops and sent prices spiraling.