It's official: 2012 was the warmest year ever recorded in the contiguous United States, the National Oceanic and Atmospheric Administration said yesterday.

The average temperature in the lower 48 states reached 55.3 degrees Fahrenheit, shattering the previous record set in 1998 by a full degree. The government's temperature records for the contiguous United States go back to 1895.

What's remarkable, said NOAA climate scientist Jake Crouch, is that the other 117 years in that temperature record fell within a 4.2-degree range, or envelope.

No other year has approached the heat 2012 brought to the lower 48 states, he said. "We are well above the pack for all the years we have data for the United States," Crouch explained.

"Climate change has had a role in this," he told reporters, cautioning that it is still hard for scientists to tease out how much of this year's searing heat was caused by natural variability and how much was sparked by man-made climate change.

The contiguous United States endured a record-warm spring, its second-warmest summer and fourth-warmest winter, and a warmer-than-average autumn. Every state there recorded unusually warm temperatures last year.

There was one notable national exception, however: far-flung Alaska, which experienced its 11th-coolest year since records there began in 1918.

Warmer years to come
But overall, scientists said, it is the unusually warm years that will become more common as man-made climate change intensifies.

"Going into the future, we would expect warmer years, or years with temperatures much above the 20th century average, to become more frequent," Crouch said. "2012 was an outlier in terms of looking at the past record for the contiguous United States, but as we move forward and the warming trend continues, we would expect to see more warmer-than-average years."

Globally, 2012 appears to be the eighth-warmest year in a record that goes back to 1880, he said. NOAA will release its global temperature analysis for 2012 next week.

But in the United States, it is clear that last year was not only warm but extreme in other ways. The Northeast suffered a devastating blow from Superstorm Sandy, while a smoldering drought gripped a wide swath of the country's midsection beginning in early summer.

Record warmth and notable dryness helped sustain the drought, which peaked in July with 61 percent of the country affected. It remains a force in a wide area of the central United States even now. Water levels on the Mississippi River are so low that barge operators expect the federal government to halt barge traffic later this month, cutting off a major shipping artery through the middle of the country.

All told, 2012 stands as the nation's second-most extreme weather year on record, according to NOAA scientists who maintain the U.S. Climate Extremes Index.

The index tracks extreme weather activity, monitoring extremes of temperature and precipitation and tropical cyclones that make landfall. (That means Sandy, which transformed from a tropical storm to a supercharged nor'easter before it hit land, wasn't included in the tally, researchers said.)

"Big heat and big rain are the types of extreme events that we are seeing most often, and those, not coincidentally, are the extremes where climate science has made the most confident connections on how they are going to evolve in a warming world," said Deke Arndt, chief of the climate monitoring branch at NOAA's National Climatic Data Center.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500