Global-warming skeptics: Might warming be 'normal'?

In a small college town like Corvallis, Ore., it's not unusual that George Taylor would ride a bike to his job on the Oregon State University campus. He commutes this way for the exercise, he says, but also because it's good for the planet.

Mr. Taylor manages the Oregon Climate Service, and much of his work has to do with global warming. "I'm certainly in favor of doing prudent things to reduce the human impact," he says.

But unlike most climate scientists, he does not believe that anthropogenic (human-caused) greenhouse gases – mainly from coal-fired power plants and motor vehicles spewing carbon dioxide – are the main culprits. In fact, he says, "It's my belief that in the last 100 years or so natural variations have played a bigger role."

Among the forces of nature he cites are changes in solar radiation, "very significant influences" of the tropical Pacific (El Niño and La Niña events in decades-long cycles), as well as changes in Earth's tilt and orbit over cycles lasting thousands of years.

Above all, says Mr. Taylor, who is past president of the American Association of State Climatologists, "The climate system is very, very complex, and the more we learn, the more we see that we really don't understand it."

Taylor may be in the minority among climate experts, but he is not alone.

Other planets in our solar system have expanding and contracting ice caps, too, other skeptics point out, and those worlds have no people as far as we know – certainly no gas-guzzling muscle cars and trucks. Antarctica and Greenland at times have been warm and green before humankind in­­vented machines, indicating to these skeptics that this is just a natural cycle.

In Phoenix, where it's been very hot indeed this summer, Warren Meyer has written "A Skeptical Layman's Guide to Anthropogenic Global Warming." He is not a professionally trained climate scientist, but he studied physics and engineering at Princeton University, then earned an MBA at Harvard University before entering the business world.

Like Taylor, Mr. Meyer cites other possible factors – ocean oscillations and currents, sunspot cycles, and recovery from the "Little Ice Age" (which ran for roughly three to four centuries, up to the mid-19th century) – to argue that "we are a long way from attributing all or much of current warming to man-made carbon dioxide."

He says he's carefully studied the official reports and assertions about global warming and come to the conclusion that "it's a funny sort of anthropomorphic hubris to say that we know what 'normal' is or even know what the cycles are.

"Look, there's a lot going on here that we've observed for a very short time," Mr. Meyer says. "We have all these complicated cycles happening, and many of them last for thousands or millions of years. And we've observed them carefully for – what? – 30 years?"

Is climate 'feedback' positive or not?

Meyer's engineering background is in feedback and control theory, so he especially takes issue with the belief among many climate scientists (as well as activists such as former Vice President Al Gore) that what had been a long-term, stable climate system is now dominated by "positive feedback ." Positive feedback means that as temperatures rise in extraordinary fashion there will be a tendency for global warming to speed up. One example is when light-colored sea ice melts to reveal darker ocean water, which in turn absorbs more heat, which melts more ice.

Meyer contends that in physics (and in nature) the tendency is just the opposite: a "negative feedback" will occur as CO2 levels rise – in other words, cooling mechanisms will set in. In the case of carbon dioxide and global temperature, "future CO2 has less impact on temperature than past CO2," he says.

One bit of recent research may give some weight to Meyer's argument.

Researchers at the University of Alabama's Earth System Science Center in Huntsville studied heat-trapping tropical clouds thought to result from global warming. They found an apparent decrease in such clouds as the atmosphere warms, allowing more infrared heat to escape from the atmosphere. The cloud decrease appears to be "negative feedback," meaning that as warming continues it sets off another process that counters its effects.

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