Steven Davis, a University of California at Irvine earth system science professor and the study's lead author, said he and his colleagues were seeking a "way to describe the magnitude of the challenge" in tackling climate change.
Chris Field, who directs the Carnegie Institution for Science's global ecology department at Stanford University, noted that although it is impossible to say whether the world will be "safe" if it limits the temperature rise to 2 degrees Celsius, he compared it to "the number of flat tires I can tolerate on a road trip in my car."
"With even one flat, there is a risk of a serious accident. But because I am a careful driver, and I have a spare, one is probably ok," he wrote in an email. "With two flats on the trip, I know I don't have a spare for the second one, and I understand that the risk of a serious accident is increased. . . . For more than two flats, things get really messy."
Several activists who track international climate talks identified the next three years as critical, saying negotiators need to forge a new pact by 2015 in order 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."
In the United States, a combination of high temperatures and dry conditions last year took a serious toll on the nation's agricultural sector. NOAA's Karl noted that the Midwest had been relatively wet for several years, which had curbed the impact of warmer temperatures.
In 2012, he 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."