Scientists long have issued the warnings: The modern world's appetite for cars, air conditioning and cheap, fossil-fuel energy spews billions of tons of carbon dioxide into the atmosphere, unnaturally warming the world.

Yet, it took the dramatic images of a hurricane overtaking New Orleans and searing heat last summer to finally trigger widespread public concern on the issue of global warming.

Climate scientists might be expected to bask in the spotlight after their decades of toil. The general public now cares about greenhouse gases, and with a new Democratic-led Congress, federal action on climate change may be at hand.

Problem is, global warming may not have caused Hurricane Katrina, and last summer's heat waves were equaled and, in many cases, surpassed by heat in the 1930s.

In their efforts to capture the public's attention, then, have climate scientists oversold global warming? It's probably not a majority view, but a few climate scientists are beginning to question whether some dire predictions push the science too far.

"Some of us are wondering if we have created a monster," says Kevin Vranes, a climate scientist at the University of Colorado.

Vranes, who is not considered a global warming skeptic by his peers, came to this conclusion after attending an American Geophysical Union meeting last month. Vranes says he detected "tension" among scientists, notably because projections of the future climate carry uncertainties — a point that hasn't been fully communicated to the public.

The science of climate change often is expressed publicly in unambiguous terms.

For example, last summer, Ralph Cicerone, president of the National Academy of Sciences, told the U.S. House Committee on Energy and Commerce: "I think we understand the mechanisms of CO2 and climate better than we do of what causes lung cancer. ... In fact, it is fair to say that global warming may be the most carefully and fully studied scientific topic in human history."

Vranes says, "When I hear things like that, I go crazy."

Nearly all climate scientists believe the Earth is warming and that human activity, by increasing the level of greenhouse gases such as carbon dioxide, has contributed significantly to the warming.

But within the broad consensus are myriad questions about the details. How much of the recent warming has been caused by humans? Is the upswing in Atlantic hurricane activity due to global warming or natural variability? Are Antarctica's ice sheets at risk for melting in the near future?

To the public and policymakers, these details matter. It's one thing to worry about summer temperatures becoming a few degrees warmer.

It's quite another if ice melting from Greenland and Antarctica raises the sea level by 3 feet in the next century, enough to cover much of Galveston Island at high tide.

Models aren't infallible

Scientists have substantial evidence to support the view that humans are warming the planet — as carbon dioxide levels rise, glaciers melt and global temperatures rise. Yet, for predicting the future climate, scientists must rely upon sophisticated — but not perfect — computer models.

"The public generally underappreciates that climate models are not meant for reducing our uncertainty about future climate, which they really cannot, but rather they are for increasing our confidence that we understand the climate system in general," says Michael Bauer, a climate modeler at NASA's Goddard Institute for Space Studies, in New York.

Gerald North, professor of atmospheric sciences at Texas A&M University, dismisses the notion of widespread tension among climate scientists on the course of the public debate. But he acknowledges that considerable uncertainty exists with key events such as the melting of Antarctica, which contains enough ice to raise sea levels by 200 feet.

"We honestly don't know that much about the big ice sheets," North says. "We don't have great equations that cover glacial movements. But let's say there's just a 10 percent chance of significant melting in the next century. That would be catastrophic, and it's worth protecting ourselves from that risk."

Much of the public debate, however, has dealt in absolutes. The poster for Al Gore's global warming movie, An Inconvenient Truth, depicts a hurricane blowing out of a smokestack. Katrina's devastation is a major theme in the film.

Judith Curry, an atmospheric scientist at the Georgia Institute of Technology, has published several research papers arguing that a link between a warmer climate and hurricane activity exists, but she admits uncertainty remains.

Like North, Curry says she doubts there is undue tension among climate scientists but says Vranes could be sensing a scientific community reaction to some of the more alarmist claims in the public debate.

For years, Curry says, the public debate on climate change has been dominated by skeptics, such as Richard Lindzen of the Massachusetts Institute of Technology, and strong advocates such as NASA's James Hansen, who calls global warming a ticking "time bomb" and talks about the potential inundation of all global coastlines within a few centuries.

That may be changing, Curry says. As the public has become more aware of global warming, more scientists have been brought into the debate. These scientists are closer to Hansen's side, she says, but reflect a more moderate view.

"I think the rank-and-file are becoming more outspoken, and you're hearing a broader spectrum of ideas," Curry says.

Young and old tension

Other climate scientists, however, say there may be some tension as described by Vranes. One of them, Jeffrey Shaman, an assistant professor of atmospheric sciences at Oregon State University, says that unease exists primarily between younger researchers and older, more established scientists.

Shaman says some junior scientists may feel uncomfortable when they see older scientists making claims about the future climate, but he's not sure how widespread that sentiment may be. This kind of tension always has existed in academia, he adds, a system in which senior scientists hold some sway over the grants and research interests of graduate students and junior faculty members.

The question, he says, is whether it's any worse in climate science.

And if it is worse? Would junior scientists feel compelled to mute their findings, out of concern for their careers, if the research contradicts the climate change consensus?

"I can understand how a scientist without tenure can feel the community pressures," says environmental scientist Roger Pielke Jr., a colleague of Vranes' at the University of Colorado.

Pielke says he has felt pressure from his peers: A prominent scientist angrily accused him of being a skeptic, and a scientific journal editor asked him to "dampen" the message of a peer-reviewed paper to derail skeptics and business interests.

"The case for action on climate science, both for energy policy and adaptation, is overwhelming," Pielke says. "But if we oversell the science, our credibility is at stake."

eric.berger@chron.com