Greenhouse gases fueled 2006 U.S. heat

By Deborah Zabarenko, Environment Correspondent

WASHINGTON (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 September 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.