WASHINGTON — Last year was the hottest on record for the continental United States, shattering the previous mark set in 1998 by a wide margin, the National Oceanic and Atmospheric Administration announced Tuesday.
The average temperature was 55.3 degrees, 1 degree above the previous record and 3.2 degrees more than the 20th-century average. Temperatures were above normal in every month between June 2011 and September 2012, a 16-month stretch that hasn't occurred since the government began keeping such records in 1895.
Federal scientists said that the data were compelling evidence that climate change is affecting weather in the United States and suggest that the nation's weather is likely to be hotter, drier and potentially more extreme than it would have been without the warmer temperatures.
Last year's record temperature is "clearly symptomatic of a changing climate," said Thomas R. Karl, who directs NOAA's National Climatic Data Center. Americans can now see the sustained warmth over the course of their own lifetimes — "something we haven't seen before." He added, "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."
Alaska and the Pacific Northwest didn't experience record-setting heat last year; a cool-weather pattern over the Pacific Ocean kept temperatures lower.
Although the new analysis focuses on the United States, it 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 2 degrees Celsius, or 3.6 degrees Fahrenheit. Now many academics and policymakers say that goal may be out of reach.
"We have to begin the conversation about cruising past 2 degrees, because we're on course for that," said John Podesta, who chairs the liberal think tank Center for American Progress. "It's hard to contemplate and scary to contemplate, but it has to be addressed at this point."