Global warming: a few skeptics still ask why it's happening

Not all remaining skeptics fit neatly into one pigeonhole. They do agree that the climate has warmed and that humans have pumped more heat-trapping gases into the atmosphere. But some hold that the climate is too complex to reliably forecast its future trends. Other suggest that natural fluctuations in climate remain the main drivers of warming. Still others say that, on balance, warming will be good for humanity.

Within those broad groups, there's overlap and even ambivalence.

"There are days when I am involved in research and I would look to anyone like: This guy's completely on board," says Robert Balling Jr., an atmospheric scientist at Arizona State University who is often is identified as a a skeptic. "Other days, I might be involved in a project that could be seen on the skeptical side. I don't understand how someone working in this field for 15 years can publish nothing" but work supporting the consensus view "and not be a little skeptical."

Some who look at the climate issue through the lens of geological time hold that warming's impact on society pales in comparison with the sudden, natural swings in climate that can occur. The triggers are unknown, and society is woefully unprepared for them, says Australian researcher Robert Carter, one of the testifiers before the Senate Environment and Public Works Committee on Wednesday. The global-warming debate, he argued, is a distraction that keeps people from focusing on what he sees as this greater threat.

The University of Oklahoma's David Deming went further, arguing for a form of geo-engineering to forestall the next ice age. In a phone interview, Dr. Deming said too little is known about how the climate system works to overhaul economies in an effort to affect it. He cites the mechanisms that cause ice ages as an example. And he points to work by Richard Muller, a physicist at the University of California at Berkeley who has suggested an unusual cosmic source for cooling cycles that occur roughly every 100,000 years.

But in an interview, Dr. Muller chuckles and notes that measurements he hoped would bolster his case for periodic swings through a patch of cosmic dust as the culprit so far failed to turn up evidence of dust.

He does have misgivings about computer modeling as a forecast tool and about uncertainties in climate-change science. But given the current state of the science, "we can't rule out that a substantial portion of the warming is due to human influence," he says. "And we have a plausible mechanism that can account for the changes. If we extrapolate those forward, the effects would be bad for the US," even if Canada and Russia might like a warmer climate.

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