Last year was the ninth warmest since 1880, say NASA scientists, and the warmest recorded by far for the continental US.
The average temperature in 2012 was about 58.3 degrees Fahrenheit - 1.0 F (0.6 C) warmer than the mid-20th century baseline. The average global temperature has risen about 1.4 degrees F since 1880, says the team.
"One more year of numbers isn't in itself significant," says climatologist Gavin Schmidt. "What matters is this decade is warmer than the last decade, and that decade was warmer than the decade before. The planet is warming. The reason it's warming is because we are pumping increasing amounts of carbon dioxide into the atmosphere."
The carbon dioxide level in the atmosphere was about 285 parts per million in 1880, the first year in the Goddard Institute for Space Studies (GISS) temperature record. By 1960, it was about 315 parts per million and is now over 390 parts per million.
While it was a hot year for the world in general, the US saw its warmest twelve months ever.
"The US temperatures in the summer of 2012 are an example of a new trend of outlying seasonal extremes that are warmer than the hottest seasonal temperatures of the mid-20th century," says GISS director James E Hansen.
"The climate dice are now loaded. Some seasons still will be cooler than the long-term average, but the perceptive person should notice that the frequency of unusually warm extremes is increasing. It is the extremes that have the most impact on people and other life on the planet."
With the exception of 1998, the nine warmest years in the record have all occurred since 2000, with 2010 and 2005 the hottest on record. The last year that experienced cooler temperatures than the 1951 to 1980 average was 1976.