Long-term forecast: Hotter and hotter

Global warming will mean more years like North Carolina's warmer-than-average 2006, an environmental advocacy group said Thursday.

Environment North Carolina examined temperature patterns and found that Raleigh, Greensboro and Asheville all experienced maximum temperatures in 2006 of 2 degrees or more above average.

"Throw out the record books, because global warming is raising temperatures in North Carolina and across the country," said Margaret Hartzell, field organizer for Environment North Carolina. The group examined data from 2000 to 2006 and compared them with average temperatures over a 30-year period from 1971 to 2000.

The findings add to recent conclusions that global warming is affecting natural weather variation, but scientists caution it's too early to blame global warming on this year's string of blazing hot August days in North Carolina.

In February, an international panel of scientists concluded that the evidence of global warming is "unequivocal" and that much of the temperature change in the past 50 years has likely been caused by human activities such as burning coal and oil. Those activities raised the levels of heat-trapping carbon dioxide gases in the atmosphere, creating the greenhouse effect.

An accelerating trend

The average global temperature increased about 1 degree between 1900 and 2000, and the rate of warming has accelerated over the past 30 years.

This week, four scientists at the National Oceanic & Atmospheric Administration's research lab in Boulder, Colo., said that greenhouse gases likely accounted for more than half of the widespread warmth across the continental United States last year. Natural variability likely accounted for the rest, they said.

The average temperature in 2006 for the continental United States was 2.1 degrees above the 20th century average and marked the ninth consecutive year of higher-than-normal temperatures. It ranked as the second warmest year on record after 1998, according to NOAA.

"It's likely that about half of the warming that occurred in 2006 resulted from human influences, via the change in the atmosphere," Martin Hoerling, a meteorologist at NOAA's Earth System Research Laboratory in Boulder, said in an interview.

Hoerling was the lead author of the peer-reviewed study that will appear Sept. 5 in a publication of the American Geophysical Union.

The scientists set out to explore whether it was coincidence that 2006 and 1998, the two warmest years on record, coincided with El Nino events -- the warming of the surface of the tropical Pacific Ocean that affects weather patterns in the United States. When temperatures broke U.S. records in 1998, many scientists attributed the unusual warmth to the influence of the El Nino.

Using temperature data from the past 10 El Nino events dating to 1965, Hoerling and his fellow scientists found a slight cooling trend across the country in El Nino years.

From that, the scientists concluded that factors other than El Nino were responsible for the unusually warm weather in 1998 and 2006.

The computer climate models the researchers analyzed showed greenhouse gases produced warmth over the nation much like the warming pattern last year.

The scientists also predicted that there is a 16 percent chance that 2007 will bring record-breaking warmth.

"Technically, we won't know until the end of the year," Hoerling said. "The fact that it is warm is entirely consistent with the statistics and physics of what was working in 2006 -- namely greenhouse gas. It's very likely a cause this year because the influence we diagnosed last year -- the gases -- are still there."