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The drought was the worst since the 1950s and slightly behind the Dust Bowl of the 1930s, meteorologists said. During a drought, the ground is so dry that there’s not enough moisture in the soil to evaporate into the atmosphere to cause rainfall, which leads to hotter, drier air. This was fed in the U.S. by La Nina, which is linked to drought.

Scientists say even with global warming, natural and local weather changes mean that temperatures will go up and down over the years. But overall, temperatures are climbing. In the United States, the temperature trend has gone up 1.3 degrees over the last century, according to NOAA data. The last year the U.S. was cooler than the 20th-century average was 1997.

The last time the country had a record cold month was December 1983.

What has scientists so stunned is how far above other hot years 2012 was. Nearly all of the previous 117 years of temperature records were bunched between 51 and 54 degrees, while 2012 was well above 55.

“A picture is emerging of a world with more extreme heat,” said Andrew Dessler, a Texas A&M University climate scientist. “Not every year will be hot, but when heat waves do occur, the heat will be more extreme. People need to begin to prepare for that future.”

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Online:

National Climatic Data Center summary of US weather in 2012, http://1.usa.gov/UHhwpx

NCDC chart showing 2012 versus other years, http://1.usa.gov/13eHTrJ

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Seth Borenstein can be followed at http://twitter.com/borenbears