For nearly 40 years ExxonMobil publicly raised doubt about the dangers of climate change even as scientists and execs inside the oil giant acknowledged the growing threat internally, according to a Harvard University study.

"We conclude that ExxonMobil misled the public," the researchers wrote in the peer-reviewed study that was published on Wednesday.

The Harvard study could add to the controversy and legal scrutiny surrounding Exxon's (XOM) handling of climate change.

Exxon dismissed the Harvard study as "inaccurate and preposterous," saying in a statement that the research was "paid for, written and published by activists."

The Harvard researchers examined 187 public and private communications from Exxon about climate change between 1977 and 2014, ranging from internal documents and peer-reviewed studies to company pamphlets and editorial-style advertisements in The New York Times known as "advertorials."

The study found that the more public-facing the Exxon communication, the more doubt it expressed about climate change.

Exxon's advertorials "overwhelmingly emphasized only the uncertainties, promoting a narrative inconsistent with the views of most climate scientists, including ExxonMobil's own," the Harvard study concluded.

Exxon's internal communications broadly acknowledged that global warming is "real, human-caused, serious and solvable," the research found.

About 80% of Exxon's internal documents that were examined acknowledged that climate change is both real and human-caused, compared with just 12% of advertorials published in the op-ed pages of the Times. Doubt was expressed by 81% of Exxon's advertorials.

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For instance, the Harvard study cited an internal Exxon document in 1979 that soberly stated: "The most widely held theory is that...the increase [in atmospheric carbon-dioxide] is due to fossil fuel combustion."

That document warned of "warming of the earth's surface" and "dramatic environmental effects before the year 2050" caused by fossil fuel consumption.

Contrast that with what Exxon told the world two decades later. In a 1997 Times op-ed, Exxon cautioned that the "science of climate change is too uncertain" and "we still don't know what role man-made greenhouse gases might play in warming the planet."

"Let's not rush to a decision at Kyoto," Exxon said of the the international global warming pact being negotiated at the time. It warned that action on the protocol could "plunge economies into turmoil."

The Harvard study found that Exxon did not "suppress" climate science as some have alleged. Rather, the researchers said that Exxon "misled non-scientific audiences about climate science."

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Exxon rejected the findings, noting that in 2000, it published two op-eds that said climate change may pose a "legitimate long-term risk."

"Our statements have been consistent with our understanding of climate science," Exxon said, adding that the "risk of climate change is clear and warrants action."

Exxon is being investigated by New York Attorney General Eric Schneiderman over allegations it concealed the risk of climate change. Schneiderman has accused Secretary of State Rex Tillerson of using the pseudonym "Wayne Tracker" to send emails related to climate change while he served as Exxon's CEO.

In response to the Harvard study, a spokeswoman for Scheniderman said his investigation has "uncovered significant evidence indicating Exxon may have misled" investors and the public about climate change risks.

Climate activists similarly pounced on the Harvard findings. Greenpeace put out a statement urging state authorities to "act now to hold polluters accountable for the biggest crisis facing humanity."

Exxon lost a key climate change battle in May when shareholders approved a proposal urging the company to do more to disclose the risks it faces from a global crackdown on carbon emissions.