John Christy, a professor of atmospheric science at the University of Alabama in Huntsville, says he remembers the morning he spotted a well-known colleague at a gathering of climate experts.

“I walked over and held out my hand to greet him,” Dr. Christy recalled. “He looked me in the eye, and he said, ‘No.’ I said, ‘Come on, shake hands with me.’ And he said, ‘No.’ “

Dr. Christy is an outlier on what the vast majority of his colleagues consider to be a matter of consensus: that global warming is both settled science and a dire threat. He regards it as neither. Not that the earth is not heating up. It is, he says, and carbon dioxide spewed from power plants, automobiles and other sources is at least partly responsible.

But in speeches, congressional testimony and peer-reviewed articles in scientific journals, he argues that predictions of future warming have been greatly overstated and that humans have weathered warmer stretches without perishing. Dr. Christy’s willingness to publicize his views, often strongly, has also hurt his standing among scientists who tend to be suspicious of those with high profiles. His frequent appearances on Capitol Hill have almost always been at the request of Republican legislators opposed to addressing climate change.

“I detest words like ‘contrarian’ and ‘denier,’ ” he said. “I’m a data-driven climate scientist. Every time I hear that phrase, ‘The science is settled,’ I say I can easily demonstrate that that is false, because this is the climate — right here. The science is not settled.”

Dr. Christy was pointing to a chart comparing seven computer projections of global atmospheric temperatures based on measurements taken by satellites and weather balloons. The projections traced a sharp upward slope; the actual measurements, however, ticked up only slightly.

Such charts — there are others, sometimes less dramatic but more or less accepted by the large majority of climate scientists — are the essence of the divide between that group on one side and Dr. Christy and a handful of other respected scientists on the other.

“Almost anyone would say the temperature rise seen over the last 35 years is less than the latest round of models suggests should have happened,” said Carl Mears, the senior research scientist at Remote Sensing Systems, a California firm that analyzes satellite climate readings.

“Where the disagreement comes is that Dr. Christy says the climate models are worthless and that there must be something wrong with the basic model, whereas there are actually a lot of other possibilities,” Dr. Mears said. Among them, he said, are natural variations in the climate and rising trade winds that have helped funnel atmospheric heat into the ocean.

Dr. Christy has drawn the scorn of his colleagues partly because they believe that so much is at stake and that he is providing legitimacy to those who refuse to acknowledge that. If the models are imprecise, they argue, the science behind them is compelling, and it is very likely that the world has only a few decades to stave off potentially catastrophic warming.

And if he is wrong, there is no redo.

“It’s kind of like telling a little girl who’s trying to run across a busy street to catch a school bus to go for it, knowing there’s a substantial chance that she’ll be killed,” said Kerry Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology. “She might make it. But it’s a big gamble to take.”

By contrast, Dr. Christy argues that reining in carbon emissions is both futile and unnecessary, and that money is better spent adapting to what he says will be moderately higher temperatures. Among other initiatives, he said, the authorities could limit development in coastal and hurricane-prone areas, expand flood plains, make manufactured housing more resistant to tornadoes and high winds, and make farms in arid regions less dependent on imported water — or move production to rainier places.

Dr. Christy’s scenario is not completely out of the realm of possibility, his critics say, but it is highly unlikely.

In interviews, prominent scientists, while disagreeing with Dr. Christy, took pains to acknowledge his credentials. They are substantial: Dr. Christy, 63, has researched climate issues for 27 years and was a lead author — in essence, an editor — of a section of the 2001 report of the United Nations Intergovernmental Panel on Climate Change, the definitive assessment of the state of global warming. With a colleague at the University of Alabama in Huntsville, Dr. Roy Spencer, he received NASA’s medal for exceptional scientific achievement in 1991 for building a global temperature database.

That model, which concluded that a layer of the atmosphere was unexpectedly cooling, was revised to show slight warming after other scientists documented flaws in its methodology. It has become something of a scientific tit for tat. Dr. Christy and Dr. Spencer’s own recalculations scaled back the amount of warming, leading to further assaults on their methodology.

Dr. Christy’s response sits on his bookshelf: a thick stack of yellowed paper with the daily weather data he began recording in Fresno, Calif., in the 1960s. It was his first data set, he said, the foundation of a conviction that “you have to know what’s happening before you know why it’s happening, and that comes back to data.”

Dr. Christy says he became fascinated with weather as a fifth grader when a snowstorm hit Fresno in 1961. By his high school junior year, he had taught himself Fortran, the first widely used programming language, and had programmed a school computer to make weather predictions. After earning a degree in mathematics at California State University, Fresno, he became an evangelical Christian missionary in Kenya, married and returned as pastor of a mission church in South Dakota.

There, as a part-time college math teacher, he found his true calling. He left the pastoral position, earned a doctorate in atmospheric sciences at the University of Illinois and moved to Alabama.

And while his work has been widely published, he has often been vilified by his peers. Dr. Christy is mentioned, usually critically, in dozens of the so-called Climategate emails that were hacked from the computers of the University of East Anglia’s Climatic Research Center, the British keeper of global temperature records, in 2009.

“John Christy has made a scientific career out of being wrong,” one prominent climate scientist, Benjamin D. Santer of the Lawrence Livermore National Laboratory, wrote in one 2008 email. “He’s not even a third-rate scientist.”

Another email included a photographic collage showing Dr. Christy and other scientists who question the extent of global warming, some stranded on a tiny ice floe labeled “North Pole” and others buoyed in the sea by a life jacket and a yellow rubber ducky. A cartoon balloon depicts three of them saying, “Global warming is a hoax.”

Some, including those who disagree with Dr. Christy, are dismayed by the treatment.

“Show me two scientists who agree on everything,” said Peter Thorne, a senior researcher at Norway’s Nansen Environmental and Remote Sensing Center who wrote a 2005 research article on climate change with Dr. Christy. “We may disagree over what we are finding, but we should be playing the ball and not the man.”

Dr. Christy has been dismissed in environmental circles as a pawn of the fossil-fuel industry who distorts science to fit his own ideology. (“I don’t take money from industries,” he said.)

He says he worries that his climate stances are affecting his chances of publishing future research and winning grants. The largest of them, a four-year Department of Energy stipend to investigate discrepancies between climate models and real-world data, expires in September.

“There’s a climate establishment,” Dr. Christy said. “And I’m not in it.”