Map of above-average temperatures from 2008 to 2012. Image: NASA/Goddard Institute for Space Studies.

The year 2012 was the ninth warmest globally since record keeping began in 1880, said climate scientists today from NASA. NOAA, crunching the numbers slightly differently, said 2012 was the tenth warmest year, and both agencies said a warming pattern has continued since the middle of the 20th century.

NASA had already said last week that for the contiguous United States, 2012 was the warmest year ever recorded. The hottest years on record for the planet, it said, were 2005 and 2010.

"One more year of numbers isn't in itself significant," said Gavin Schmidt, a climatologist at NASA's Goddard Institute for Space Studies in a statement. "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 changing climate has been one of the most contentious political issues of recent decades, with most climate scientists arguing that gases from cars and industry help trap the sun's heat in the atmosphere. But skeptics complain that policymakers could cripple the modern economy if they try to curb industrial emissions based on incomplete science.

President Obama argued for a bill in 2009 to control global warming gases, but it stalled in Congress and he has not tried again. In November, after he won reelection, Obama said he would sit down with "scientists, engineers and elected officials to find out what more we can do to make short-term progress in reducing carbons."

NOAA and NASA scientists said the global temperature in 2012 was moderated by relatively cool temperatures in Alaska, Canada and parts of Asia, and there was also a La Nina - a giant patch of cool water that periodically replaces warmer water in the equatorial Pacific. But NASA also pointed out that 2012 was a year of extremes, with drought and unusual summer heat, for example, afflicting much of the U.S.

"Including 2012, all 12 years to date in the 21st century rank among the 14 warmest in the 133-year period of record," NOAA said. "Only one year in the 20th century - 1998 - was warmer than 2012."