While the energy output of the sun does fluctuate, those fluctuations can’t explain recent increases in global temperature, a leading solar physicist says.

Judith Lean, research physicist in the Space Science Division of the U.S. Naval Research Laboratory in Washington, D.C., was responding to a recent Bellingham lecture by Don Easterbrook, a retired geologist. Easterbrook argues that humancaused increases in carbon dioxide and other gases have next to nothing to do with recent climate changes. A report on Easterbrook’s lecture, published in The Bellingham Herald, was linked or posted to a number of Web sites maintained by those who deny that global warming is being caused by burning fossil fuels like gasoline and coal.

Lean’s area of expertise is the sun’s effect on environment and climate. Lean said that, thanks to satellites, scientists now have access to more than 25 years of very precise data about changes in temperature and in the gases that make up the atmosphere. That data has convinced Lean and many other scientists that increased carbon dioxide from human fuel-burning has been warming the Earth.

“The data is so good that you can identify the causes of climate change and you can quantify them,” Lean said.

Among those causes are changes in the sun’s output, volcanic dust from major eruptions, and the El Niño-Southern Oscillation or ENSO, the shift in Pacific Ocean temperatures that affect weather from year to year.

But those three factors taken together don’t come close to explaining the warming that has occurred since 1980. In a published research paper, Lean and a colleague presented evidence that the nonhuman causes could explain only 6 percent of the climate change. Lean said it seems clear to her that carbon dioxide and other gases generated by human activity are pushing global temperatures slowly higher over time, year-to-year fluctuations notwithstanding.

At this point, she added, the cause-and-effect of human activity and global warming is no longer just theory.

“That’s a very straightforward, empirical look at the data,” Lean said.

Easterbrook and other greenhouse gas skeptics point to recent climate data indicating that temperatures have actually cooled a bit since 1998’s temperature peak, even though carbon dioxide levels in the atmosphere have continued to rise.

Thomas Ackerman, director of the Joint Institute for the Study of the Atmosphere and Ocean at the University of Washington, said the 1998 peak was to be expected, given the combined effects of carbon dioxide and the El Niño effect of that year. This year’s much colder temperatures, in Whatcom County and elsewhere, are due to changed Pacific conditions that would be expected to generate colder weather.

Just as it would be wrong to argue that global warming caused 1998’s heat waves, it would be wrong to argue that the chill of 2007-08 means global warming is a hoax, Ackerman said.

“The trend is likely to go up,” Ackerman said. “It doesn’t have any choice but to go up, with all the carbon dioxide that we’re putting into the atmosphere. … It’s like adding insulation to your attic. … That doesn’t mean that every year is going to be hotter than the year before.”

Ackerman said those who discount the effects of carbon dioxide, and offer solar fluctuations as an alternative, can’t back up their theories with scientific data.

“We have a perfectly good physical model that explains what’s going on,” Ackerman said. “We have a bunch of people who say, ‘I don’t like your model.’ ”

He suggested that people resist the science behind global warming because they don’t want to change their lifestyles to combat it.

“There were a lot of people who spent many years denying that smoking was harming their health,” Ackerman said. “They didn’t want to change their lifestyles. They liked smoking.”

Andy Bunn, assistant professor of environmental sciences at WWU’s Huxley College of the Environment, said scientists have long been aware that the atmosphere acts as a blanket to hold in the sun’s heat, and carbon dioxide has more of an insulating effect than other gases in the mix.

“The basic physics and chemistry have been understood for over 100 years,” Bunn said. “You take a heat-trapping gas and put it in the atmosphere, and it’s going to trap heat.”

Bunn said it would be “silly” to argue that this year’s weather disproves global warming.

“The long-term averages show that the planet is about one degree warmer centigrade compared to the long-term mean,” Bunn said.

He scoffed at the idea that atmospheric scientists are part of a conspiracy that is trying to hide evidence that contradicts the impact of carbon dioxide on global temperatures.

“Scientists are way too vain and egotistical to get it together to maintain a conspiracy,” Bunn said.

If he could disprove the current broad scientific consensus on carbon dioxide-caused warming, Bunn said he would do it.

“I would be the most famous scientist since Einstein,” Bunn said. “It would be the biggest scientific discovery of the century.”

In his lecture, Easterbrook said the climate data in the next few years would indicate whether the winter of 2007-08 is, as he suspects, the first in a series of colder winters that will be triggered by solar fluctuation. And he promised to come back to the WWU podium and “eat my words” if that doesn’t happen.