The hot zone: U.S. warms most in its corners

The United States is warming fastest at two of its corners, in the Northeast and the Southwest, an analysis of temperature records shows.

Northeastern states — led by Maine and Vermont — have gotten the hottest in the last 30 years in annual temperature, gaining 2.5 degrees on average. But Southwestern states have heated up the most in the hottest months: The average New Mexico summer is 3.4 degrees warmer now than in 1984; in Texas, the dog days are 2.8 degrees hotter.

Minnesota's average year-round temperature has climbed 1.26 degrees since 1984, tying it for the 24th-largest increase among states. The Twin Cities' average temperature is up 3.18 degrees.

The contiguous United States' annual average temperature has warmed by 1.2 degrees since 1984, with summers getting 1.6 degrees hotter. But that doesn't really tell you how hot it's gotten for most Americans. While man-made greenhouse gases warm the world as a whole, weather is supremely local. Some areas have gotten hotter than others because of atmospheric factors and randomness.

"In the United States, it isn't warming equally," said Kelly Redmond, climatologist at the Western Regional Climate Center in Reno, Nev. "Be careful about extrapolating from your own back yard to the globe."

For example, while people in the East and Midwest were complaining about a cold winter this year, Nevada and neighboring California were having some of their warmest winter months ever.

To determine what parts of the country have warmed the most, the Associated Press analyzed National Climatic Data Center temperature trends in the Lower 48 states, 192 cities and 344 smaller regions within the states. Climate scientists suggested 1984 as a starting date because 30 years is a commonly used time period and 1984, which had an average temperature, is not a cherry-picked year.

Since 1984, 92 percent of the more than 500 cities and smaller regions within states have warmed and nearly two-thirds of them have warmed by at least a degree. The regions that have warmed the most have been New York's St. Lawrence Valley, northern Maine and the northeastern plains of New Mexico, all of which have warmed by more than 2.5 degrees.

Cities — where data is a tad more suspect because they are based on a single weather station and readings can be affected by urban heating and development — see the greatest variation. Carson City, Nev., and Boise, Idaho, are the cities that have seen the most warming — both year-round and in summer — since 1984. Both cities' average annual temperatures have jumped more than 4 degrees in just 30 years, while Dickinson, N.D., has dropped the most, about 2 degrees.

Southwest warming, especially in the summer, seems to be driven by dryness, because when there is little water the air the ground warms faster, said Katharine Hayhoe, a climate scientist at Texas Tech University in Lubbock.

"Heat and drought are a vicious cycle that has been hitting the Southwest hard in recent years," Hayhoe said.

And in the Northeast, the temperatures are pushed up by milder winters and warm water in the North Atlantic, said Kevin Trenberth, climate analysis chief at the National Center for Atmospheric Research. And less snow on the ground over the winter often means warmer temperatures, said Alan Betts, a climate scientist at Atmospheric Research in Pittsford, Vt.

In the Southeast and mid-Atlantic, industrial sulfur particle pollutants from coal burning may be reflecting sunlight, thus countering heating caused by coal's carbon dioxide emissions, said Pennsylvania State University professor Michael Mann.