Their divide mirrors a much larger political split, as the US Senate begins to debate a climate bill written in large part by Massachusetts Senator John F. Kerry. All parties to the debate have the same evidence to draw on; their conclusions are another matter. Lindzen and Emanuel’s collision spotlights the ultimate sticking point: What steps should we take, and at what cost? That is: How much insurance against the possibility of catastrophe should a prudent planet buy?

The story of the scientists’ relationship is much more than a curiosity. The fact that these serious-minded colleagues and longtime friends disagree so vehemently highlights the immense difficulty of finding common ground on human-caused global warming. That’s because their disagreements are not just about interpretations of scientific data, but about how they assess the risks, amid the uncertainty over global warming’s future impact.

Lindzen has implied that Emanuel is hyping the evidence and making a play for fame and funding in the age of Obama and Gore. In a letter savaging an opinion piece by Emanuel in the Globe, he branded the reasoning “more advocacy than assessment.’’

Emanuel thinks Lindzen’s key theories don’t hold up, and just two weeks ago went public with his criticism, penning a tart letter to the editor rebutting Lindzen’s Journal piece — “irresponsible and misleading,’’ he called it, “advancing spurious hypotheses.’’

“I don’t see how a climate scientist can look at the evidence and not see risk,’’ he said recently.

Emanuel, an equally respected researcher, emerged as a preeminent voice on climate change’s potential dangers after he published a paper three weeks before Katrina that suggested global warming might be making hurricanes more powerful. Named one of the most influential people in the world by Time magazine, Emanuel, 55, says he has been persuaded by the evolving science that man-made climate change is a real threat.

Lindzen, a leading specialist on atmospheric physics, has emerged as one of the most prominent climate change skeptics in the world. At age 70, he speaks at home and overseas, arguing that there is little to worry about from emissions of heat-trapping gases from power plants, factories, and cars. We should “go back to dealing with real science and real environmental problems such as assuring clean air and water,’’ he wrote in The Wall Street Journal on Earth Day.

But then the climate changed between them. Friends became intellectual foes, dueling icons in one of the world’s most acrimonious political debates.

Richard Lindzen and Kerry Emanuel are both brilliant and convivial. Both study the atmosphere and climate at the Massachusetts Institute of Technology, where their offices overlooking the Charles River are one floor apart. In an academic world often dominated by liberals, both have strong conservative streaks and once agreed that the evidence for catastrophic man-made global warming just wasn’t there.

It is no surprise they grew to be friends.

Emanuel would borrow Lindzen’s slides for his talks and the duo would have lunch and the occasional dinner together. They weren’t best friends, but far closer than mere colleagues. In 1990, Emanuel’s future wife, Susan, who was living in France at the time, attended a scientific talk he gave in England. Lindzen was there and asked a question of Emanuel that Susan saw as hostile. She marched up afterward to give him a piece of her mind — and ran headlong into his charm.

Back then, Emanuel agreed there wasn’t yet enough evidence. Computer models that tried to project future warming were woefully inadequate. Temperature data showing recent warming didn’t demonstrate a clear trend.

He reviewed the evidence and came away a skeptic about the projections of future catastrophe. He came to see opportunism in some of those loudly sounding the global warming alarm — especially as they raced to obtain a piece of the growing pot of federal research funding on the topic. The professor who once cast his presidential vote for Democrat Michael Dukakis became a Republican.

“That’s when I thought, wow, things have gotten really out of hand,’’ Lindzen said recently.

In 1988, he began questioning an emerging environmental issue: Man-made climate change. An economist had written him, saying he had been interrupted by then-Senator Al Gore at a Washington lunch for daring to suggest that there was uncertainty about the case for global warming.

Lindzen delights in questioning many assumptions that most people accept as truths. He smokes Marlboro Lights and doesn’t worry much about dying from them. He doubts that acid rain was ever much of a problem.

It was classic wry Lindzen and Emanuel found himself drawn to this colleague with the bushy beard and piercing eyes, this entertaining conversationalist with wide-ranging interests, from food to photography. Emanuel, who is more buttoned-down and cautious — he dislikes attending large sporting events because the crowd mentality unsettles him — saw a little of himself in Lindzen’s aversion to the status quo.

Emanuel, who had recently voted for Ronald Reagan, was espousing his views. Lindzen, at that time a registered Democrat, looked up and said, as Emanuel recalls: “You’re to the right of Attila the Hun.’’

Emanuel had to laugh. He and Lindzen were at lunch in a university dining hall in the early 1980s, shortly after both arrived at MIT in what is now the program in Atmospheres, Oceans and Climate.

“If these two guys can’t agree on the basic conclusions of the social significance of [climate change science], how can we expect 6.5 billion people to?’’ said Roger Pielke Jr., a University of Colorado at Boulder professor who writes a climate blog.

Still, immersion in the controversy seemed to help Emanuel find a public voice. He wrote a 2007 article in a national magazine, the Boston Review, explaining climate change science, and it was made into a book. He penned an opinion piece in The Boston Globe this past February. His message is measured: Climate change may not be as bad as some computer models predict, but the odds are just as good it could be worse.

So-called hurricane wars broke out among climate scientists. Colleagues, including Lindzen in private, questioned Emanuel’s findings, and Emanuel himself came to temper his conclusions. Earlier this year, hurricane specialists — including Emanuel — agreed that it is not clear warming oceans have already made hurricanes fiercer, but that it is likely they eventually will.

He came to be viewed, Pielke said, “not as scientist but a symbol in the debate.’’

According to his friends, the mild-mannered and deliberative professor was unprepared for the limelight.

Later that month, Katrina slammed into New Orleans, catapulting Emanuel to fame. The link between hurricanes and global warming instantly gained a much wider audience: Promotions for the Al Gore global warming movie, “An Inconvenient Truth,’’ featured smokestack emissions fueling a hurricane.

In August 2005, Emanuel published a paper in the journal Nature that linked rising North Atlantic and North Pacific sea surface temperatures, possibly from global warming, to fiercer hurricanes in the previous 30 years. Hurricanes are fed by warm ocean water, and he projected they would become more powerful if climate change continued heating oceans.

“It’s like a trial,’’ Emanuel said in an interview. “None of the evidence is perfect, but it all points in one direction.’’

As the decade progressed, Emanuel said it became hard for him to ignore the growing evidence that man-made climate change could indeed cause problems. Computer models of the climate, while still imperfect, improved, and there were 10 more years of temperature data. He was swayed.

Intrigued, Emanuel tried to verify Lindzen’s analysis with his own calculations. He — and other scientists — couldn’t, and Emanuel began to think that his friend may have gotten it wrong.

To the contrary, he argues, water vapor and clouds could actually have a cooling effect.

In an early 1990s paper, Lindzen challenged a widely-accepted assumption about global warming — that the amount of water vapor will increase in the atmosphere as the earth heats up, amplifying warming.

It wasn’t long before she was friends with Lindzen and his wife, Nadine, a native of France. Emanuel and Susan stayed at the Lindzens’ apartment in Paris. The Lindzens visited a home the Emanuels bought in Burgundy. Back home, the couples began dining three or four times a year — get-togethers that would continue even as the husbands’ scientific views diverged.

Lindzen watched his colleague become a media star with growing unease.

He says that man-made emissions are a small factor in climate change and doesn’t agree that global warming poses a worrisome threat; and his water vapor theory is not the only reason. “The evidence as I see it says that the risk is so small,’’ he said in an e-mail. “We are proposing trillion dollar solutions to a problem that is much less serious.’’

He began to see questionable motivations in Emanuel’s transformation into a scientist outspoken about the possible dangers from global warming.

Emanuel “would tell me that he really felt that it would be a mistake not to take advantage of the issue . . . there is funding . . . it could benefit the department,’’ Lindzen said in an interview. “I always took a more moralistic view. There has to be a foundation.’’

What had been an academic dispute was about to become personal.

Debate gets hotter Emanuel, after casting an uncharacteristic vote for Obama in part because of his promise to combat climate change, threw a small inauguration party with his wife in their Lexington home. Emanuel, after casting an uncharacteristic vote for Obama in part because of his promise to combat climate change, threw a small inauguration party with his wife in their Lexington home.

Lindzen wasn’t happy about Obama’s win, of course, but he and his wife good-naturedly went to be with friends. Champagne flowed.

But Obama’s inauguration proved to be a turning point, both for climate policy and for Lindzen and Emanuel. The new president’s pledge to sharply reduce greenhouse gas emissions, and international climate treaty talks, catapulted global warming into the headlines like never before. With the stakes raised, the climate debate became more polarized and venomous, and scientists including Lindzen and Emanuel were tugged into the fray.

In March 2009, Lindzen took a jab at Emanuel and another colleague in a speech for the Heartland Institute, a conservative think tank, that assailed the political nature of climate science and funding. Lindzen said many scientists endorse man-made global warming because it “just makes their lives easier.

“For example, my colleague, Kerry Emanuel, received relatively little recognition until he suggested that hurricanes might become stronger in a warmer world,’’ the speech reads. “He then was inundated with professional recognition.’’

The comments, while literally true, suggested that Emanuel’s science was tainted by other motivations. Emanuel won’t talk about the incident, other than to say that he never spoke to Lindzen about the speech and that he’s over it. Lindzen said his words were interpreted incorrectly by some people. Among several MIT colleagues, however, the comments became infamous.

In July, Emanuel confronted his friend after he saw that Lindzen had signed on to a strongly worded letter to Congress by climate skeptics. “We are flooded with claims that the evidence is clear, that the debate is closed, that we must act immediately, etc, but in fact there is no such evidence; it doesn’t exist,’’ the letter said.

Emanuel fired off an e-mail asking Lindzen to explain himself. Of course there was evidence, Emanuel argued, and Lindzen knew it.

“I saw it as untruthful,’’ Emanuel said in an interview. “And it was to Congress.’’

Lindzen agreed the letter could have been written better, but felt it needed to get to Congress as soon as possible.

Months later, Emanuel and Lindzen participated in MIT’s The Great Climategate Debate, about the significance of a batch of leaked e-mails among top climate scientists, which had sparked questions about their integrity and data.

Emanuel sat next to Lindzen at the event last December. At one point — as the two sparred about funding sources for climate skeptics and scientists — Emanuel stopped in midsentence.

He touched Lindzen on the shoulder, and said: “By the way, I want to make something very clear. He’s not part of the machine. Dick Lindzen is his own machine.’’

Lindzen laughed. But the pleasantry could not mask a tough year for their relationship. Lindzen attended Emanuel’s annual department Christmas party in his home, and the two saw each other at an autumn wedding, but they have not picked up the phone to socialize with each other in the past year.

Then late last month, Emanuel publicly scolded Lindzen for his Earth Day piece.

“Mr. Lindzen clings to his agenda of denial,’’ Emanuel wrote.

When asked whether his relationship with Emanuel has changed because of their views on global warming, Lindzen said opaquely in an e-mail, “That is a complicated question.’’

Emanuel is also loath to talk about it. He acknowledges, however, that their relationship has become “strained.’’

Beth Daley can be reached at bdaley@globe.com.

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