For 25 years, Exxon Mobil published advertorials in the New York Times denouncing or denying the effects of climate change. But according to a Harvard study, the oil giant and gas giant was well aware of how dangerous climate change really was.

A new academic study published Tuesday in the journal Environmental Research Letters provides further evidence of Exxon’s attempt to systematically sow the seeds of doubt in Americas’ minds on climate change — despite having conducted their own research acknowledging both climate change and humans’ role in causing it.

Authors Geoffrey Supran and Naomi Oreskes, both historians of science at Harvard who study how industry impacts scientific knowledge, had been interested in Exxon for a while. After a 2015 investigative report from InsideClimate News and the LA Times revealed that Exxon had systematically misled the public on the effects of climate change, the company accused journalists of “cherry-picking” the evidence and asked the public to read all the documents themselves. So that’s exactly what Supran and Oreskes did.

They systematically matched 187 internal reports and peer-reviewed research papers with public advertising from Exxon, including “all the documents that Exxon Mobil provided plus some additional ones that InsideClimate News in the Los Angeles Times had found, plus this whole set of advertorial advertisements that were formatted to look like editorials,” Oreskes said in an interview with VICE News. “We read those as well in order to get a complete picture.”

And the complete picture, according to Oreskes, “is very, very clear.” Although the company contributed to the scientific conversation and accepted the scientific consensus on climate change in private, the message it relayed to the public was one of doubt and disinformation. More than 80 percent of internal documents acknowledged that climate change was happening, whereas over 80 percent of the messages relayed to the public expressed doubt about the science.

Those messages included advertorials placed every Thursday in The New York Times between 1972 and 2001 — many of which focused on climate change — at the current-day equivalent of $30,000 per ad. In all, the authors estimate, Exxon spent somewhere close to $15 million promoting disinformation on the accuracy of climate change science.

The authors refer to this sort of corporate-sponsored purposeful spread of disinformation as the “tobacco strategy,” wherein a corporation deliberately obfuscates negative scientific results, much like the tobacco industry did in suppressing research into the detrimental effects of tobacco smoke on human health. Oreskes says she wasn’t surprised Exxon adopted the tactic, because many people who worked for the tobacco industry now work for or with the fossil fuel industry.

Oreskes and Supran were specifically concerned by the timing of the advertorials, which ramped up after the U.N. Framework Convention on Climate Change in Rio in 1992, and after the Kyoto protocol in 1997. “The science of climate change is too uncertain to mandate a plan of action that could plunge economies into turmoil,” read one of Exxon’s Times ads, published shortly before the Kyoto protocol was signed.

And Exxon wasn’t just trying to confuse the public, Oreskes says. The study also notes obfuscation of Exxon’s own research that showed that some of Exxon’s oil and gas reserves would have become unusable if world leaders had agreed on a carbon budget — something that would have greatly affected the future of the company.

“We certainly think it’s pertinent to shareholders who might be uncertain about this shareholder resolutions and uncertain about, you know, this question of whether or not Exxon Mobil has been fully forthcoming,” Oreskes said. “So we think it’s very relevant to shareholders, [and] we think it’s relevant to consumers.”