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Cold winter doesn't mean global warming is over

All of the snow and chilly temps this winter almost convinced us that global warming isn't happening, after all.  Reality check: recent climate data shows that it's still hotter than it used to be.

The planet's average temperature was 57.9 degrees Fahrenheit (14.4 degrees Celsius) last year, making it the eighth warmest year on record since 1880, according to the National Climatic Data Center (NCDC). But your goose bumps weren’t deceiving you: 2008 was also the coldest year since the beginning of the decade, according to data from NASA's Goddard Institute for Space Studies. (Hat tip to Ars Technica for alerting us to this info in their analysis of climate trends yesterday.)

Global warming is responsible for the overall upward temperature trend, and any snow outside our window shouldn’t convince us that Earth has stopped heating up, says Richard Heim, an NCDC meteorologist. "Most of the top 10 warmest years have happened in the last decade and a half," Heim tells ScientificAmerican.com. "Global warming does not mean every year will be warmer than the previous year. Global warming means there's an increasing frequency of warmer temperatures and a decreasing frequency of cooler temperatures, and that’s definitely what we're seeing."

So how to explain the relatively colder winters we've had in the latter half of this decade? Goddard attributes them to La Niña, a cyclical pattern of cold sea-surface temperatures in the Pacific Ocean. La Niña in the first half of 2008,  followed by a neutral period in the latter half, likely had something to do with it, Heim says, but adds that global warming is about long-term, rising temperature trends over time. "It's kind of like a drunk driver," Heim says. "The car is weaving back and forth, but it's still progressing forward."

View towards Back Bay, Boston, January 2009 by essygie via Flickr

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