SAN FRANCISCO —  Three independent research groups have concluded that 2008 was a comparatively cool year on planet Earth — a feverish chill on our warming world.

The year's average global temperature was the 9th or 10th warmest since reliable record-keeping began in 1850, and the coldest since the turn of the 21st century, according to separate surveys by the United Nations' World Meteorological Organization, NASA's Goddard Institute of Space Studies, and the U.S. National Climatic Data Center.

Each used slightly different methods to rank 2008 based on world-wide land and sea-surface temperatures through November.

For the time being, no one knows whether this temperature drop heralds a lasting retreat from global warming or a temporary dip.

Last summer was relatively cool world-wide, for example, while global land temperatures in October were the warmest for that month in more than a century, government weather records show.

Taken together, the result was a year that ran slightly less than one degree warmer than the 20th century mean.

In matters of climate, the unusual is becoming routine, as higher temperatures make weather patterns more unstable.

"As a result of climate change," says Peter Stott, head of climate monitoring at the U.K.'s Hadley Center, which helped prepare the U.N. figures, "what would have once been an exceptionally unusual year has now become quite normal."

Despite the ups and downs of annual temperature swings, though, the planet has grown steadily warmer in recent decades, affecting everything from New England winters and the Siberian spring to western droughts and tropical cloud cover.

That's according to eight new government and university climate studies presented last month during a meeting in San Francisco of the American Geophysical Union, an international scientific society of 50,000 researchers who study Earth and its environment.

Moreover, almost all of the warming in North America has taken place since 1970, says a team of government and academic experts at the U.S. Climate Change Science Program.

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