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.
But some Democrats, including Rep. Edward J. Markey (Mass.), seized upon the report to press the Obama administration to move aggressively on climate change — including imposing new carbon limits on existing power plants through regulation.
In light of the extreme weather events the United States experienced in 2012, Markey said, “it’s time for the politicians to start catching up.”
White House spokesman Clark Stevens said President Obama has already restricted carbon emissions on vehicles and new power plants, adding: “The president has made clear that his administration will continue to build on this progress, and climate change will be a priority in his second term.”
Key Republicans, including Rep. Ed Whitfield (Ky.), who heads the Energy and Commerce subcommittee on energy and power, have indicated that they will fiercely oppose any new government rules to curb greenhouse-gas emissions.
Last year, the United States experienced several weather events — including extreme storms, a historic drought and wildfires — in addition to record heat.
Deke Arndt, chief of the National Climatic Data Center’s climate-monitoring branch, said the higher temperatures were “consistent with what we would expect in a warming world.” He also said that current science shows the accumulation of carbon in the atmosphere leads to more frequent “big heat” and “big rain” events.
A combination of high temperatures and dry conditions last year took a serious toll on the nation’s agricultural sector.
In 2012, NOAA’s Karl said, “both the day and the nighttime temperatures were breaking their all-time records,” and that, combined with drier conditions, amounted to “a double whammy.”
Despite researchers’ concerns, global carbon emissions continue to rise. The International Energy Agency estimated last month that coal will come close to surpassing oil as the world’s top energy source in 2017 , when an additional 1.2 billion metric tons will be burned annually. In late November, the World Resources Institute reported there are nearly 1,200 proposed coal plants around the globe, three-quarters of which are planned for China and India.
By Jan. 1 of this year, the Kyoto Protocol was supposed to have cut the world’s greenhouse-gas output by 5 percent compared with 1990 levels. But while the signatories as a whole are likely to meet that target — in part because of the shutdown of Eastern European factories during the 1990s — global carbon emissions overall rose 54 percent in that same period, according to the Global Carbon Project.
Many experts are discussing whether they should continue pressing for ambitious carbon cuts in the near term or adjust their goals given the prospect of a much warmer world.
“We have to begin the conversation about cruising past” the 2009 pledge on limiting global temperature increases, said John D. Podesta, who chairs the Center for American Progress, a liberal think tank, and is a member of a United Nations advisory panel addressing climate change and other issues. “It’s hard to contemplate and scary to contemplate, but it has to be addressed at this point.”
In 2004, Princeton University professors Robert Socolow and Stephen Pacala wrote an influential paper outlining how the world could stabilize its greenhouse-gas emissions by mid-
century through a series of “wedges,” using current technology, such as sharply increasing nuclear power worldwide, eliminating deforestation and converting conventional plowing to no-tillage farming.
Socolow has more recently published an article in the Vanderbilt Law Review that he describes as his “let’s get real here” lecture. He says environmentalists will have to accept that fossil fuels will not disappear in the next few decades.
Several activists who track international climate talks insist, however, that the next three years are critical, saying negotiators need to forge a new pact by 2015 to lock in needed carbon cuts. Alden Meyer, who directs strategy and policy for the Union of Concerned Scientists, said major emitters will not agree to meaningful cuts until they view it as in “their national self-interest.”