James Hansen testifies before the Senate Foreign Relations Committee during a hearing about the Keystone XL pipeline project, in 2014. Photograph by Chip Somodevilla/Getty

It has been just a week since negotiators from a hundred and ninety-six nations agreed, for the first time, on the need to cut the world’s greenhouse-gas emissions. The accord is a couple of decades late, and its terms are far too weak. Nonetheless, the agreement is an enormous achievement, and it has been responsible for a rare, and well-earned, moment of optimism among climate-change activists. Finally, we can focus more seriously on finding the best approaches to reducing carbon emissions and then implementing them.

What better time, then, to describe as “climate deniers” some of the people who have done the most to push humanity to save itself and the planet? I am not referring to the many insipid comments made by the people who are seeking the Republican Presidential nomination, although they are indeed stunning. (A tweet from the Donald: “The concept of global warming was created by and for the Chinese in order to make US manufacturing non-competitive.”) At least he didn’t blame it on Syrian refugees—yet.

Any number of people have attacked the truth from the right. But this week, Naomi Oreskes, a professor of the history of science at Harvard University, took a different approach: in an article published on Wednesday in the Guardian, she said that “four climate scientists”—she didn’t name them, but linked to a piece by James Hansen, the former NASA chief scientist; Kerry Emanuel, a professor of meteorology at M.I.T.; Tom Wigley, perhaps the most distinguished climate scientist in Australia; and Ken Caldeira, of the Carnegie Institution, who contributed to the team from the International Panel on Climate Change, which won the 2007 Nobel Peace Prize—had adopted a new form of “denialism.”

It was an interesting choice. There is perhaps nobody who has done more to alert the world to the dangers of climate change than Hansen or Caldeira. But the article that the four men wrote, published in the Guardian as the Paris summit got underway, argued that “To solve the climate problem, policy must be based on facts and not on prejudice. The climate system cares about greenhouse gas emissions—not about whether energy comes from renewable power or abundant nuclear power.”

According to Oreskes, suggesting that nuclear power play some role in limiting carbon emissions and solving global warming is not just wrong but “a strange form of denial that has appeared on the landscape of late, one that says renewable sources can’t meet our energy needs.”

It is one thing to wonder about the value of nuclear energy—I was mostly opposed, too, until I saw Robert Stone’s compelling documentary, “Pandora’s Promise.’’ But to label Hansen (whom my colleague Elizabeth Kolbert has Profiled for this magazine) or Caldeira as denialists is absurd.

You don’t have to be a supporter of nuclear power to realize that words have meanings, and “denialism” is not a synonym for disagreement. (I should note that I’ve written a book and articles in this magazine on the subject.) Rather, it implies willful, and often hateful, ignorance. Denialists are people who inhabit their fictions and run from the truth. They don’t believe there was a Holocaust or a final solution; they insist, despite indisputable molecular evidence, as well as the deaths of tens of millions of people, that AIDS is not caused by H.I.V. They are people who believe that angels are real and that evolution is false.

Oreskes is certain that we won’t need nuclear power to reduce greenhouse-gas emissions. This is a legitimate and essential debate. But it should be possible to have it without denigrating positions held by people who have spent their careers, quite courageously, trying to solve the world’s biggest problem. Other well-known scientists, like Stanford’s Mark Jacobson, agree with Oreskes. Jacobson is a serious scholar, and I have seen him debate this issue with Stewart Brand, the environmental futurist who created the Whole Earth Catalogue. Brand supports nuclear power and Jacobson does not. Notably, though, the two men managed to air their differences in a very public forum without resorting to attacks that would have only weakened their common goals.

None of the men Oreskes attacked have argued that we should lessen our growing commitment to solar or wind power. As far as I can tell, they would all be happy to rely on the sun and the atmosphere to power the planet. But none of them believe that this is entirely possible. As the environmental activist Ben Heard tweeted earlier this week, “This thrash of insult against senior climate scientists for wanting all tools deployed is foul and degrades us all.”

No political group in Western life has been more adept at committing fratricide (and suicide) than the people of the left. Now, after decades mired in the mud, we have a broad and widely accepted framework to address global warming. Is this really how we want to begin the discussion?