"To imply that any scientist who has questions about global warming is somehow part of an orchestrated campaign" by industry or interest groups greatly oversimplifies the spectrum of motivations among those outside the consensus view, says Annie Petsonk, a lawyer with Environmental Defense. "It is much more complicated than that."
History shows that science is a field in which it can be difficult to achieve consensus – even when the question at hand has no public-policy implications. When the question gets tangled up with politics, economics, and lifestyles, the ranks of the unconvinced can thin far more grudgingly.
"Science moves slowly," says John Wallace, an atmospheric scientist at the University of Washington who used to be skeptical about human-induced warming. He cites the controversy over smoking and health as an example of skepticism's durability when public policy is involved. Even after accounting for the influence of the tobacco lobby, "there were scientifically very conservative people who maintained ... doubt until very late in the game – much to the detriment of a lot of smokers," he says. They insisted, he says, on absolute certainty on the link between smoking and health.
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."