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Research released shows that Exxon Mobil Corp. expressed its knowledge of climate change differently to the public than in internal communications.

In a paper published in Environmental Research Letters, two Harvard University researchers conclude that for nearly 40 years, Exxon behaved like two separate companies targeting two separate audiences. It published internal reports and peer-reviewed research saying that climate change was real, serious and man-made, while simultaneously telling citizens that climate science was far from settled.

The report is one of the most thorough accounts of Exxon’s climate change statements to date, and the authors say it is the first peer-reviewed analysis to scrutinize four decades’ worth of the oil giant’s statements about climate change and its climate research.

“What we’ve done is look at the whole cherry tree,” said Geoffrey Supran, who co-authored the paper with Naomi Oreskes. “When you dig down, there was a very small effort going on the science side.”

Since 2015, Exxon has disputed critics’ assertions that it was downplaying climate risks, despite revelations that it had a detailed grasp of human-caused climate change as early as the 1970s.

Exxon accused journalists of “cherry picking” its records to reach that conclusion, and it challenged critics to examine the company’s climate record.

“Read all these documents and make up your own mind,” Ken Cohen, an Exxon spokesman, said in October 2015.

So the Harvard researchers did.

Supran and Oreskes — a professor of science history and co-author of the book “Merchants of Doubt,” which chronicles how industry groups inject confusion about climate change into the public sphere — reviewed 187 Exxon records, public and internal, published between 1977 and 2014.

They identified a “systematic discrepancy between what Exxon said about climate in private and academic circles versus what it said in public,” Supran said.

Eighty-three percent of the peer-reviewed work by Exxon scientists and 80 percent of Exxon’s internal files noted that climate change is real and driven by humans, the report found.

But public statements, campaigns and ads — such as those Exxon ran in The New York Times every Thursday from 1972 to 2001 — tell a different story.

Supran and Oreskes found that 12 percent of Exxon’s “advertorials” intended for the public described climate change as real and man-made, “with 81 percent instead expressing doubt.”

An Exxon spokesman declined to comment on the record.

Exxon has often criticized the work of Oreskes, and of 350.org founder Bill McKibben and other environmental researchers and advocates, dismissing it as biased against the company.

Oreskes and Supran said they are aware of those optics. But they pointed out that while they may begin with an opinion, peer-reviewed reports have to stand up to scrutiny from other experts.

“It’s not just opinion,” Supran said of the report. “We’re providing an independent and empirical report that Exxon has known about the basics of climate science for decades.”

In recent years, Exxon constructed a “red herring” by pointing to its contributions to climate change and arguing that climate advocates are accusing the company of withholding scientific results, according to Oreskes.

“They’ve set up a straw man,” she said by phone. “So they’re rejecting something that no one has ever claimed.”

“The claim was never that they suppressed science,” she added. “The claim was that there was a mismatch or a discrepancy between what their own scientists were doing versus what Exxon Mobil said to the outside world when they communicated publicly.”

The Rockefeller Family Fund, which has been critical of Exxon, supported the research.

Representatives of the fund did not respond to requests for comment.