By Deborah Zabarenko, Environment Correspondent

WASHINGTON, Aug 28 (Reuters) - Greenhouse gas emissions -- not El Nino or other natural phenomena -- pushed U.S. temperatures for 2006 close to a record high, government climate scientists reported on Tuesday.

The annual average U.S. temperature in 2006 was 2.1 degrees F (1.16C) above the 20th century average and the ninth consecutive year of above-normal U.S. temperatures, researchers at the National Oceanic and Atmospheric Administration wrote.

This is in line with a global warming trend over the past century that most climate scientists attribute to human-made greenhouse gas emissions. Greenhouse gases, notably carbon dioxide from petroleum-fueled vehicles and coal-fired power plants, build up in the atmosphere and hold in the sun's heat like the glass walls of a greenhouse.

But other factors also play a role, and when figures for 2006 indicated a near-record-heat year for the contiguous 48 states -- the area for which there are the best statistics -- U.S. climate scientists wondered if this warmth was due to climate change or to the naturally occurring El Nino.

El Nino seemed a logical culprit, since there were active El Nino patterns of warm water in the Pacific in 2006 and in the hottest U.S. year of 1998, said Martin Hoerling of the U.S. climate administration.

IF NOT EL NINO, THEN WHAT?

Hoerling and his co-authors, writing in the Sept. 5 edition of Geophysical Research letters, looked back through history and found that El Nino does not generally cause a rise in U.S. average annual temperatures. But if not El Nino, what was it?

To find out, they used computer simulations of the impact of greenhouse gas emissions on temperature that were used by the U.N. Intergovernmental Panel on Climate Change. The panel reported, with 90 percent probability, that human activities contribute to global warming.

They compared the 18 different simulations for 2006, which included projections of greenhouse gas emissions, with the actual average temperature for the United States, and found a correlation, Hoerling said.

"What we found was a very strong footprint of the observed warming, consistent with the greenhouse gas effect," Hoerling said in a telephone interview.

Preliminary data suggested that 2006 was a record warm year for the contiguous 48 U.S. states but updated numbers showed last year was 0.08 degrees F (.04C) cooler than 1998.

For most states, 2006 ranked among the 10 hottest years since 1895. Globally, 2005 was the warmest, edging out 1998, with 2006 ranked about sixth for the world, Hoerling said.

Hoerling said the difference in U.S. average temperatures between 2006, 1998 and 1934 was minuscule.

"Those three years are so close to one another ... that's not really a relevant concern," he said.

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