In January 2013, the National Climatic Data Center in Asheville, N.C., announced that 2012 was the hottest year ever in the contiguous United States since records began in 1895. The year 2012 brought a blistering March heat wave, a severe drought in the Corn Belt and a massive storm that caused broad devastation in mid-Atlantic states.
The temperature differences between years are usually measured in fractions of a degree, but 2012 blew away the previous record, set in 1998, by a full degree Fahrenheit.
If that does not sound so impressive, consider that 34,008 new daily high records were set at weather stations across the country, compared with only 6,664 new record lows, according to a count maintained by the 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 in 2012.
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 new record would have been set without the backdrop of global warming caused by the human release of greenhouse gases.