By Environment Correspondent Deborah Zabarenko
WASHINGTON (Reuters) - Last year was among the top 10 warmest in the modern global record, two U.S. climate-watching agencies reported on Tuesday, less than a week after 2012 was declared the hottest ever in the contiguous United States.
The U.S. space agency NASA and the National Oceanic and Atmospheric Administration jointly issued two reports on 2012 world temperatures. NASA ranked last year the ninth-warmest since record-keeping began in 1880, while NOAA found last year was the tenth-warmest.
The difference in the two rankings may be due to NASA's extrapolation of temperatures in areas with no weather stations, particularly near the poles, according to James Hansen, director of NASA's Goddard Institute for Space Studies in New York.
The 2012 global surface temperature, including land and water, was 1 degree F (.56 degree C) warmer than the 1951-1980 average. That was enough to increase extreme high temperatures last year, Hansen reported.
Last year was also the 36th consecutive year with a global temperature hotter than the 20th century average, scientists from the two agencies told a media briefing.
Senator Barbara Boxer, a California Democrat who chairs the Senate environment committee, said the reports "make clear that the Earth is warming, and the trend is going in the wrong direction. We cannot afford to ignore these warnings and must make plans to address this serious threat. The health and well-being of our communities and families depends on it."
And while the moving five-year mean temperature for the globe has been flat for a decade, that doesn't mean global warming has stalled, according to Hansen, a longtime advocate for action to curb greenhouse gas emissions that contribute to climate change.
"APPARENT STANDSTILL" IN WARMING?
Noting that each of the last three decades has been warmer than the one that preceded it, he said an "apparent standstill" in global warming could be due to weak El Nino patterns, which would normally heat up things in many places, and strong La Nina patterns, which have a corresponding cooling effect.
Hansen said it also might be due to fast-developing countries like China and India, where increased particulate air pollution from fossil-fueled vehicles and industries can reflect sunlight and keep temperatures lower.
Despite evidence that human activities that emit carbon dioxide contribute to climate change, some skeptics maintain that the rise in global temperatures is due to natural variability or other non-human factors. Others question whether temperatures are in fact rising.
NOAA said in its report that most parts of the world were hotter than average over the course of 2012, including most of North and South America, most of Europe and Africa and western, southern and far northeastern Asia.
Most of Alaska, far western Canada, Central Asia, parts of the eastern and equatorial Pacific, southern Atlantic and parts of the Southern Ocean around Antarctica were cooler than average, according to NOAA's National Climatic Data Center.
The two agencies also issued their report on global snow and ice cover, finding that the Northern Hemisphere had its 14th largest winter snow cover in 47 years of record-keeping. By spring, though, Northern Hemisphere snow cover shrank to the sixth-smallest size on record, NOAA said.
Arctic sea ice - an important global weather-maker - shrank to its smallest size ever in 2012, 49 percent below the average and 293,400 square miles (760,000 square km) below the previous record smallest, set in 2007.
By contrast, Antarctic sea ice was above average for most of 2012.
(Reporting By Deborah Zabarenko; editing by Marilyn Thompson and Christopher Wilson)