FOR MORE THAN HALF A CENTURY the eminent physicist Freeman Dyson has quietly resided in Princeton, N.J., on the wooded former farmland that is home to his employer, the Institute for Advanced Study, this country’s most rarefied community of scholars.

Lately, however, since coming “out of the closet as far as global warming is concerned,” as Dyson sometimes puts it, there has been noise all around him.

Chat rooms, Web threads, editors’ letter boxes and Dyson’s own e-mail queue resonate with a thermal current of invective in which Dyson has discovered himself variously described as “a pompous twit,” “a blowhard,” “a cesspool of misinformation,” “an old coot riding into the sunset” and, perhaps inevitably, “a mad scientist.”

Dyson had proposed that whatever inflammations the climate was experiencing might be a good thing because carbon dioxide helps plants of all kinds grow. Then he added the caveat that if CO2 levels soared too high, they could be soothed by the mass cultivation of specially bred “carbon-eating trees,” whereupon the University of Chicago law professor Eric Posner looked through the thick grove of honorary degrees Dyson has been awarded — there are 21 from universities like Georgetown, Princeton and Oxford — and suggested that “perhaps trees can also be designed so that they can give directions to lost hikers.”

Dyson’s son, George, a technology historian, said his father’s views have cooled friendships, while many others have concluded that time has cost Dyson something else. There is the suspicion that, at age 85, a great scientist of the 20th century is no longer just far out, he is far gone — out of his beautiful mind.

But in the considered opinion of the neurologist Oliver Sacks, Dyson’s friend and fellow English expatriate, this is far from the case. “His mind is still so open and flexible,” Sacks said. Which makes Dyson something far more formidable than just the latest peevish right-wing climate-change denier.

‘Infinitely smart’

Dyson is a scientist whose intelligence is revered by other scientists — William Press, former deputy director of the Los Alamos National Laboratory and now a professor of computer science at the University of Texas, calls him “infinitely smart.”

Dyson — a mathematics prodigy who came to this country at 23 and right away contributed seminal work to physics by unifying quantum and electrodynamic theory — not only did path-breaking science of his own; he also witnessed the development of modern physics, thinking alongside most of the luminous figures of the age, including Einstein, Richard Feynman, Niels Bohr, Enrico Fermi, Hans Bethe, Edward Teller, J. Robert Oppenheimer and Edward Witten, the “high priest of string theory” whose office at the institute is just across the hall from Dyson’s. Yet instead of hewing to that fundamental field, Dyson chose to pursue broader and more unusual pursuits than most physicists — and has lived a more original life.

Among Dyson’s gifts is interpretive clarity, a penetrating ability to grasp the method and significance of what many kinds of scientists do. His thoughts about how science works appear in a series of lucid, elegant books for nonspecialists that have made him a trusted arbiter of ideas ranging far beyond physics.

Dyson has written more than a dozen books, including “Origins of Life” (1999), which synthesizes recent discoveries by biologists and geologists into an evaluation of the double-origin hypothesis, the possibility that life began twice; “Disturbing the Universe” (1979) tries among other things to reconcile science and humanity. “Weapons and Hope” (1984) is his meditation on the meaning and danger of nuclear weapons that won a National Book Critics Circle Award. Dyson’s books display such masterly control of complex matters that smart young people read him and want to be scientists; older citizens finish his books and feel smart.

Yet even while probing and sifting, Dyson is always whimsically gazing into the beyond. As a boy he sketched plans for English rocket ships that could explore the stars, and then, in midlife, he helped design an American spacecraft to be powered by exploding atomic bombs — a secret Air Force project known as Orion. Dyson remains an armchair astronaut who speculates with glee about the coming of cheap space travel, when families can leave an overcrowded earth to homestead on asteroids and comets, swooping around the universe via solar sail craft.

These ideas attract derision similar to Dyson’s essays on climate change, but he is an undeterred octogenarian futurist.

“I don’t think of myself predicting things,” he says. “I’m expressing possibilities. Things that could happen. To a large extent it’s a question of how badly people want them to. The purpose of thinking about the future is not to predict it but to raise people’s hopes.”

Formed in a heretical and broad-thinking tradition of British public intellectuals, Dyson left behind a brooding England still stricken by two bloody world wars to become an optimistic American immigrant with tremendous faith in the creative imagination’s ability to invent technologies that would overcome any predicament. And according to the physicist and former Caltech president Marvin Goldberger, Dyson is himself the living embodiment of that kind of ingenuity.

“You point Freeman at a problem and he’ll solve it,” Goldberger says. “He’s extraordinarily powerful.” Dyson seems to see the world as an interdisciplinary set of problems out there for him to evaluate. Climate change is the big scientific issue of our time, so naturally he finds it irresistible. But to Dyson this is really only one more charged conundrum attracting his interest just as nuclear weapons and rural poverty have. That is to say, he is a great problem-solver who is not convinced that climate change is a great problem.

Dyson is well aware that “most consider me wrong about global warming.” That educated Americans tend to agree with the conclusion about global warming reached last month at the International Scientific Conference on Climate Change in Copenhagen (“inaction is inexcusable”) only increases Dyson’s resistance. Dyson may be an Obama-loving, Bush-loathing liberal who has spent his life opposing American wars and fighting for the protection of natural resources, but he brooks no ideology and has a withering aversion to scientific consensus.

The Nobel physics laureate Steven Weinberg admires Dyson’s physics — he says he thinks the Nobel committee fleeced him by not awarding his work on quantum electrodynamics with the prize — but Weinberg parts ways with his sensibility: “I have the sense that when consensus is forming like ice hardening on a lake, Dyson will do his best to chip at the ice.”

Dyson said he doesn’t want his legacy to be defined by climate change, but his dissension from the orthodoxy of global warming is significant because of his stature and his devotion to the integrity of science. Dyson has said he believes that the truths of science are so profoundly concealed that the only thing we can really be sure of is that much of what we expect to happen won’t come to pass. In his book “Infinite in All Directions,” he writes that nature’s laws “make the universe as interesting as possible.” This also happens to be a fine description of Dyson’s own relationship to science. In the words of Avishai Margalit, a philosopher at the Institute for Advanced Study, “He’s a consistent reminder of another possibility.”

When Dyson joins the public conversation about climate change by expressing concern about the “enormous gaps in our knowledge, the sparseness of our observations and the superficiality of our theories,” these reservations come from a place of experience. Whatever else he is, Dyson is the good scientist; he asks the hard questions. He could also be a lonely prophet. Or, as he acknowledges, he could be dead wrong.

Second thoughts

It was four years ago that Dyson began publicly stating his doubts about climate change. Speaking at the Frederick S. Pardee Center for the Study of the Longer-Range Future at Boston University, Dyson announced that “all the fuss about global warming is grossly exaggerated.” Since then he has only heated up his misgivings, declaring in a 2007 interview with Salon.com that “the fact that the climate is getting warmer doesn’t scare me at all” and writing in an essay for The New York Review of Books, the left-leaning publication that is to gravitas what the Beagle was to Darwin, that climate change has become an “obsession” — the primary article of faith for “a worldwide secular religion” known as environmentalism.

Among those he considers true believers, Dyson has been particularly dismissive of Al Gore, whom Dyson calls climate change’s “chief propagandist,” and James Hansen, the head of the NASA Goddard Institute for Space Studies in New York and an adviser to Gore’s film, “An Inconvenient Truth.” Dyson accuses them of relying too heavily on computer-generated climate models that foresee a Grand Guignol of imminent world devastation as icecaps melt, oceans rise and storms and plagues sweep the earth, and he blames the pair’s “lousy science” for “distracting public attention” from “more serious and more immediate dangers to the planet.”

A particularly distressed member of that public was Dyson’s own wife, Imme, who, after seeing the film in a local theater with Dyson when it was released in 2006, looked at her husband out on the sidewalk and, with visions of drowning polar bears still in her eyes, reproached him: “Everything you told me is wrong!” she cried.

“The polar bears will be fine,” he assured her.

Not long ago Dyson sat in his institute office, a chamber so neat it reminds Dyson’s friend, the writer John McPhee, of a Japanese living room. On shelves beside Dyson were books about stellar evolution, viruses, thermodynamics and terrorism.

“The climate-studies people who work with models always tend to overestimate their models,” Dyson was saying. “They come to believe models are real and forget they are only models.” Dyson speaks in calm, clear tones that carry simultaneous evidence of his English childhood, the move to the United States after completing his university studies at Cambridge and more than 50 years of marriage to the German-born Imme, but his opinions can be barbed, especially when a conversation turns to climate change. Climate models, he says, take into account atmospheric motion and water levels but have no feeling for the chemistry and biology of sky, soil and trees. “The biologists have essentially been pushed aside,” he continues. “Al Gore’s just an opportunist. The person who is really responsible for this overestimate of global warming is Jim Hansen. He consistently exaggerates all the dangers.”

Dyson agrees with the prevailing view that there are rapidly rising carbon-dioxide levels in the atmosphere caused by human activity. To the planet, he suggests, the rising carbon may well be a MacGuffin, a striking yet ultimately benign occurrence in what Dyson says is still “a relatively cool period in the earth’s history.”

The warming, he says, is not global but local, “making cold places warmer rather than making hot places hotter.” Far from expecting any drastic harmful consequences from these increased temperatures, he says the carbon may well be salubrious — a sign that “the climate is actually improving rather than getting worse,” because carbon acts as an ideal fertilizer promoting forest growth and crop yields. “Most of the evolution of life occurred on a planet substantially warmer than it is now,” he contends, “and substantially richer in carbon dioxide.”

Dyson calls ocean acidification, which many scientists say is destroying the saltwater food chain, a genuine but probably exaggerated problem. Sea levels, he says, are rising steadily, but why this is and what dangers it might portend “cannot be predicted until we know much more about its causes.”

For Hansen, the dark agent of the looming environmental apocalypse is carbon dioxide contained in coal smoke. Coal, he has written, “is the single greatest threat to civilization and all life on our planet.” Hansen has referred to railroad cars transporting coal as “death trains.” Dyson, on the other hand, told me in conversations and e-mail messages that “Jim Hansen’s crusade against coal overstates the harm carbon dioxide can do.” Dyson well remembers the lethal black London coal fog of his youth when, after a day of visiting the city, he would return to his hometown of Winchester with his white shirt collar turned black.

Coal, Dyson says, contains “real pollutants” like soot, sulphur and nitrogen oxides, “really nasty stuff that makes people sick and looks ugly.” These are “rightly considered a moral evil,” he says, but they “can be reduced to low levels by scrubbers at an affordable cost.” He says Hansen “exploits” the toxic elements of burning coal as a way of condemning the carbon dioxide it releases, “which cannot be reduced at an affordable cost, but does not do any substantial harm.”

Science is not a matter of opinion; it is a question of data. Climate change is an issue for which Dyson is asking for more evidence, and leading climate scientists are replying by saying if we wait for sufficient proof to satisfy you, it may be too late.

That is the position of a more moderate expert on climate change, William Chameides, dean of the Nicholas School of the Environment and Earth Sciences at Duke University, who says, “I don’t think it’s time to panic,” but contends that, because of global warming, “more sea-level rise is inevitable and will displace millions; melting high-altitude glaciers will threaten the food supplies for perhaps a billion or more; and ocean acidification could undermine the food supply of another billion or so.”

Dyson strongly disagrees with each of these points, and there follows, as you move back and forth between the two positions, claims and counterclaims, a dense thicket of mitigating scientific indicators that all have the timbre of truth and the ring of potential plausibility.

One of Dyson’s more significant surmises is that a warming climate could be forestalling a new ice age. Is he wrong? No one can say for sure. Beyond the specific points of factual dispute, Dyson has said that it all boils down to “a deeper disagreement about values” between those who think “nature knows best” and that “any gross human disruption of the natural environment is evil,” and “humanists,” like himself, who contend that protecting the existing biosphere is not as important as fighting more repugnant evils like war, poverty and unemployment.

The outsider-insider

The words colleagues commonly use to describe Dyson include “unassuming” and “modest,” and he seems the very embodiment of Newton’s belief that a man should strive for simplicity and avoid confusion in life. Dyson has been in residence at the institute since 1953, a time when Albert Einstein shared his habit of walking to work there, which Dyson still does seven days a week, to write on a computer and solve any problems that come across his desk with paper and pencil. (In his prime, legend held that he never used the eraser.) He and Imme have spent 51 happy years together in the same house, a white clapboard just over the garden fence from the stucco affair once inhabited by their former neighbors, the Oppenheimers.

On some Sundays the Dysons pile into a car still decorated with an Obama bumper sticker and drive to running races, at which Dyson can be found at the finish line loudly cheering for the 72-year-old Imme, a master marathon champion. On many other weekends, they visit some of their 16 grandchildren. During the holiday season the Dysons routinely attend five parties a week, cocktail-soiree sprints at which guests tend to find him open-minded and shy: when friends’ wives give him a hug, he blushes.

One of Dyson’s daughters, the Internet vizier Esther Dyson, says her father raised her without a television so she would read more, and has always been “just as interested in talking to” the latest graduate student to make the pilgrimage to Princeton “as he is the famous person at the next table.” Oliver Sacks says that Dyson has “a genius for friendship.”

But the truth is that Dyson is an elusive particle. When I began spending time with Dyson and asked who his close friends are, the only name he mentioned was John McPhee’s, which surprised McPhee since he said he doesn’t often speak with Dyson even though McPhee teaches nearby at Princeton University. All six of Dyson’s children describe him as a loving, intensely devoted father and yet also suggest that this is a parent with, in the words of his son George, core parts of him that have always seemed “remote.”

William Press said he finds Dyson to be both a “deep” and “magnificently laudable person” and also mysterious and inscrutable, a man with contrarian opinions that Press suspects may be motivated by “a darker side he’s determined the world isn’t going to see.” When I asked Sacks what he thought about all this, he said that “a favorite word of Freeman’s about doing science and being creative is the word ‘subversive.’ He feels it’s rather important not only to be not orthodox, but to be subversive, and he’s done that all his life.”

What may trouble Dyson most about climate change are the experts. Experts are, he thinks, too often crippled by the conventional wisdom they create, leading to the belief that “they know it all.” The men he most admires tend to be what he calls “amateurs,” inventive spirits of uncredentialed brilliance like Bernhard Schmidt, an eccentric one-armed alcoholic telescope-lens designer; Milton Humason, a janitor at Mount Wilson Observatory in California whose native scientific aptitude was such that he was promoted to staff astronomer; and especially Darwin, who, Dyson says, “was really an amateur and beat the professionals at their own game.”

It’s a point of pride with Dyson that in 1951 he became a member of the physics faculty at Cornell and then, two years later, moved on to the Institute for Advanced Study, where he became an influential man, a pragmatist providing solutions to the military and Congress, and also the 2000 winner of the $1 million Templeton Prize for broadening the understanding of science and religion, an award previously given to Mother Teresa and Aleksandr Solzhenitsyn — all without ever earning a Ph.D. Dyson may, in fact, be the ultimate outsider-insider, “the world’s most civil heretic,” as the classical composer Paul Moravec, the artistic consultant at the institute, says of him.

Judgment vs. science

Climate-change specialists often speak of global warming as a matter of moral conscience. Dyson says he thinks they sound presumptuous. As he warned that day four years ago at Boston University, the history of science is filled with those “who make confident predictions about the future and end up believing their predictions,” and he cites examples of things people anticipated to the point of terrified certainty that never actually occurred, ranging from hellfire, to Hitler’s atomic bomb, to the Y2K millennium bug. “It’s always possible Hansen could turn out to be right,” he says of the climate scientist. “If what he says were obviously wrong, he wouldn’t have achieved what he has. But Hansen has turned his science into ideology. He’s a very persuasive fellow and has the air of knowing everything. He has all the credentials. I have none. I don’t have a Ph.D. He’s published hundreds of papers on climate. I haven’t. By the public standard he’s qualified to talk and I’m not. But I do because I think I’m right. I think I have a broad view of the subject, which Hansen does not. I think it’s true my career doesn’t depend on it, whereas his does. I never claim to be an expert on climate. I think it’s more a matter of judgement than knowledge.”

Lately Dyson has been lamenting that he and Imme “don’t see so much of each other. We’re always rushing around.” But one evening last month they sat down in a living room filled with Imme’s running trophies and photographs of their children to watch “An Inconvenient Truth” again. There was a print of Einstein above the TV.

And then there was Al Gore below him, telling of the late Roger Revelle, a Harvard scientist who first alerted the undergraduate Gore to how severe the climate’s problems would become. Gore warned of the melting snows of Kilimanjaro, the vanishing glaciers of Peru and “off the charts” carbon levels in the air. “The so-called skeptics” say this “seems perfectly O.K.,” Gore said, and Imme looked at her husband. She is even slighter than he is, a pretty wood sprite in running shoes. “How far do you allow the oceans to rise before you say, ‘This is no good?’” she asked Dyson.

“When I see clear evidence of harm,” he said.

“Then it’s too late,” she replied. “Shouldn’t we not add to what nature’s doing?”

“The costs of what Gore tells us to do would be extremely large,” Dyson said. “By restricting CO2 you make life more expensive and hurt the poor. I’m concerned about the Chinese.”

“They’re the biggest polluters,” Imme replied.

“They’re also changing their standard of living the most, going from poor to middle class. To me that’s very precious.”

The film continued with Gore predicting violent hurricanes, typhoons and tornados. “How in God’s name could that happen here?” Gore said, talking about Hurricane Katrina. “Nature’s been going crazy.”

“That is of course just nonsense,” Dyson said calmly. “With Katrina, all the damage was due to the fact that nobody had taken the trouble to build adequate dikes. To point to Katrina and make any clear connection to global warming is very misleading.”

Now came Arctic scenes, with Gore telling of disappearing ice, drunken trees and drowning polar bears. “Most of the time in history the Arctic has been free of ice,” Dyson said. “A year ago when we went to Greenland where warming is the strongest, the people loved it.”

“They were so proud,” Imme agreed. “They could grow their own cabbage.”

The film ended. “I think Gore does a brilliant job,” Dyson said. “For most people I’d think this would be quite effective. But I knew Roger Revelle. He was definitely a skeptic. He’s not alive to defend himself.”

“All my friends say how smart and farsighted Al Gore is,” she said.

“He certainly is a good preacher,” Dyson replied. “Forty years ago it was fashionable to worry about the coming ice age. Better to attack the real problems like the extinction of species and overfishing. There are so many practical measures we could take.”

“I’m still perfectly happy if you buy me a Prius!” Imme said.

“It’s toys for the rich,” her husband smiled, and then they were arguing about windmills.