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A newly released trove of documents from the US oil industry is a walk through a forgotten past — a world before the science of climate change was controversial.

The records, amassed over four years of research by the Center for International Environmental Law (CIEL), outline how much American petroleum companies knew about the dangers of carbon emissions from fossil fuels long before global warming became the hot-button issue it is today. The center began releasing those documents this week, including a 1968 report to the American Petroleum Institute (API) that urged more research into reducing their carbon footprints.

"There are numerous points along this path where the industry was on notice that this was a rising risk," said Carroll Muffett, the president of CIEL. By 1968, "You have two paths in front of you. One is to respond to this rising risk, notwithstanding the continuing uncertainty, or the alternative is to try to discredit the science. And I think history has shown what path they chose."

Oil executives started huddling about pollution controls in 1946 as the public began raising concerns about smog in cities like Los Angeles, the documents lay out. Those meetings eventually resulted in a "Smoke and Fumes Committee" being set up by the API, the industry's top trade association.

By 1957, scientists from ExxonMobil's corporate ancestor, Humble Oil, identified the burning of fossil fuels as a contributor to the rise of carbon dioxide concentrations in the atmosphere. Then in 1968, Elmer Robinson — a meteorologist who led environmental research at the Stanford Research Institute (SRI), which had extensive ties to the industry — warned API that rising carbon dioxide concentrations in the atmosphere "may be the cause of serious world-wide environmental changes."

"This is SRI, their own consulting scientists, delivering a very clear message to industry leaders that climate change poses a potentially significant risk," Muffett said. "It could affect sea-level rise, glacial melt, lead to potentially severe environmental damage worldwide."

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Gretchen Goldman, lead analyst for the Center for Science and Democracy at the Union of Concerned Scientists, said the documents show the oil industry not only knew climate change was happening, "but were at the cutting edge of the science when the science was in its infancy."

"These new studies push the timeline back for when we have evidence that the fossil fuel industry knew about climate change," she said. The Humble study in particular "takes it as a given" that fossil fuels are adding CO2 to the atmosphere, with warming a likely effect.

"It's remarkable to see them have this sophisticated a position this early," she said.

The 1968 Robinson report recognizes the uncertainties in the science at the time — but Robinson "is clearly and quite explicitly concerned" about CO2, Muffett said. And he said the industry's leading players responded by trying to poke holes in the science and delaying action.

"From my perspective, one of the testaments to the effectiveness of the climate-denial movement — the climate skepticism that they sought to sow — is that the clock on climate change keeps getting reset over and over again," he said. "Constantly, the public is persuaded that climate change is a new issue, some new theory that is just being tested, just being proved."

Well before the Robinson report, popular media discussed the likely effects of carbon emissions in terms that will be familiar to today's audience. In 1956, Time magazine noted that "in 50 years or so," a CO2 buildup "may have a violent effect on the earth's climate." A 1958 science film produced by Bell Labs and Hollywood legend Frank Capra talked about possible melting of the polar ice caps if global temperatures rise "even a few degrees."

By 1965, US President Lyndon Johnson's science advisers warned him that "Man is unwittingly conducting a vast geophysical experiment" by burning vast amounts of fossil fuels, risking "measurable and perhaps marked changes in climate."

"The science at this point was many decades old," Muffett said. "That I think is a real testament to the power of the skepticism that has been sown. People keep forgetting that, and the clock keeps getting reset. As a result, we have lost decades — many decades — in the response to climate change."

The API did not respond to requests for comment on the documents. But with industry heavyweight ExxonMobil already under fire over what it knew about climate change and when, the "Smoke and Fumes" papers are now raising similar questions about a wider chunk of the petroleum sector.

"It raises the question of how broadly these things were known beyond ExxonMobil," Goldman said. The records offer a window into how the industry's thinking on climate change evolved, "And I think it's reasonable to think ExxonMobil was not an outlier in having this information."

Similar reports by Inside Climate News and the Los Angeles Times last year have led to investigations of ExxonMobil in 17 states. Prosecutors are looking into whether the company misled investors by making misleading statements about climate change and its potential impact on the company's bottom line.

Muffett said the CIEL will be releasing more documents in the coming months. But he said one of the "real tragedies" in the records is the knowledge of how much time that could have been spent preparing for the effects of warming has been lost.

"There was a moment where we could have responded much, much sooner before a lot of coastal infrastructure was built — before entire city economies or state economies were built on coastlines," he said. "There were moments when we, as a country, could have made very different choices about our future. And at those moments, what people were hearing about climate science and the threat of climate change was very different from what industry appeared to know."