Temperatures in the contiguous United States last year were the hottest in more than a century of record-keeping, shattering the mark set in 1998 by a wide margin, the federal government announced Tuesday.

The average temperature in 2012 was 55.3 degrees, one degree above the previous record and 3.2 degrees higher than the 20th-century average, scientists at the National Oceanic and Atmospheric Administration said. They described the data as part of a longer-term trend of hotter, drier and potentially more extreme weather.

The higher temperatures are due in part to cyclical weather patterns, according to the scientists. But the researchers also said the data provided further compelling evidence that human activity — especially the burning of fossil fuels, which produces greenhouse gases — is contributing to changes in the U.S. climate.

The new report has broad ramifications for policy — and everyday life. Americans who might have thought climate change was a problem for the distant future are experiencing warmer temperatures in their own lifetimes — “something we haven’t seen before,” said Thomas R. Karl, who directs NOAA’s National Climatic Data Center. “That doesn’t mean every season and every year is going to be breaking all-time records, but you’re going to see this with increasing frequency.”

Temperatures were above normal for every month from June 2011 to September 2012, a 16-month stretch that had not occurred since the government began keeping records in 1895. Alaska and the Pacific Northwest did not have record-setting heat last year; a cool-weather pattern over the Pacific Ocean kept temperatures lower.

Tuesday’s report did not address global temperatures. Still, the NOAA analysis has triggered an intense debate over whether global temperatures will reach dangerous levels by the century’s end. In 2009, the world’s leaders pledged to keep global temperatures from rising above pre-industrial levels by two degrees Celsius, or 3.6 degrees Fahrenheit. Now many experts say that goal may be out of reach.

“A hundred years from now, they’re not going to be talking about health care or the fiscal cliff,” said Vanderbilt Law School professor Michael Vandenbergh. “But they will ask, ‘What did you do when we knew we were going to have serious climate change?’ ”

Not all scientists see the situation as urgent. John R. Christy, who directs the Earth System Science Center at the University of Alabama at Huntsville, said some researchers are exaggerating the threat of climate change. He added that the right climate target is “in the mind of the beholder,” given that rising energy demand is a sign that many poor people are struggling “to be lifted out of their current condition.”

“No one in Washington can stop that,” he said. “And, right now, carbon is the most accessible and affordable way to supply that energy — so CO2 emissions will continue to rise because of the undeniable benefit carbon energy brings to human life.”

Judith A. Curry, an atmospheric scientist at the Georgia Institute of Technology, said in an e-mail that it is premature to blame droughts or hurricanes on human-caused warming. “Natural variability continues to dominate the occurrence of extreme weather events,” she said.