Photograph by Steven Senne, AP

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Kids play in a water hydrant in Providence, Rhode Island, during a heat wave in June 2012.

Photograph by Steven Senne, AP

2012: Hottest Year on Record for Continental U.S.

Last year was the warmest year on record for the continental U.S. by a wide margin, scientists say.

Temperatures across the continental United States soared in 2012 to an all-time high, making last year the warmest year on record for the country by a wide margin, scientists say. (Related: "July Hottest Month on Record in U.S.—Warming and Drought to Blame?")

"2012 marks the warmest year on record for the contiguous U.S., with the year consisting of a record warm spring, the second warmest summer, the fourth warmest winter, and a warmer than average autumn," Jake Crouch, a climate scientist at the National Climatic Data Center at the U.S. National Oceanic and Atmospheric Administration (NOAA), said in a press conference Tuesday.

According to a new NOAA report, the average temperature for the lower 48 states in 2012 was 55.3 degrees Fahrenheit (13 degrees Celsius), which is higher than the previous 1998 record by one degree Fahrenheit (0.6 degree Celsius).

A single degree difference might not seem like much, but it is an unusually large margin, scientists say. Annual temperature records typically differ by just tenths of a degree Fahrenheit.

"That is quite a bit for a whole year averaged over the whole country," said Anthony Barnston, chief forecaster at Columbia University's International Research Institute for Climate and Society (IRI), who was not involved in the study.

2012: An Odd Year

To put that difference in perspective, said NOAA's Crouch, consider that the entire range of temperature increase between the coldest year on record, which occurred in 1917, and the previous hottest year in 1998 was just 4.2 degrees Fahrenheit (2.3 degrees Celsius).

"2012 is now more than one degree above the top of that. So we're talking about well above the pack in terms of all the years we have data for the U.S.," he added.

2012 was also the 15th driest year on record for the nation: The average precipitation total for the contiguous U.S. was 26.57 inches (67.5 centimeters), 2.57 inches (6.5 centimeters) below average.

Moreover, every single one of the lower 48 states had above average temperatures. Nineteen states had their warmest year on record and an additional 26 states experienced one of their top ten warmest years on record.

2012 was unusual in another way for the nation, according to the NOAA report. Last year was the second most extreme year on record for the U.S., with 11 natural disasters such as Hurricane Sandy and a widespread drought that each cost at least a billion dollars in losses. (See pictures of the U.S. drought.)

Global Warming at Play?

The country's record year can't be explained by natural climate variability alone, noted Kevin Trenberth, a senior scientist at the Boulder, Colorado-based National Center for Atmospheric Research.

"It is abundantly clear that we are seeing [human-caused] climate change in action," Trenberth, who also did not participate in the NOAA report, said in an email. "These records do not occur like this in an unchanging climate." (Test your global warming knowledge.)

Just how much of a role climate change played is still unclear, however. "That's kind of hard to state at this point," NOAA's Crouch said.

"Climate change has had a role in this ... but it's hard for us to say at this time what amount of the 2012 temperature was dependent on climate change and what part was dependent on local variability."

For example, Columbia University's Barnston pointed out, an atmospheric weather pattern known as the North Atlantic Oscillation (NAO) was in the positive phase for much of the winter of 2012, which lead to warmer winters in the eastern U.S.

Warming Trend May Continue

There's no guarantee that the weather pattern will continue in 2013. "It could be in the negative phase, which would make it more like it was a few years ago when we had very snowy winters in the eastern part of the country," Barnston said.

The NAO is an example of "a factor that makes the U.S. annual mean temperature kind of jog up and down from year to year. It won't just gradually go straight up with global warming. It can take big dips and have big jumps."

But if climate change continues unchecked, heat records will become more common, NOAA's Crouch said.

"If the warming trend continues, we will expect to see more warmer than average years."