Oilsands giant Imperial Oil continued to call the link between fossil fuels and global temperature rise an “unproven hypothesis” decades after its own research confirmed the industry’s role in global warming, newly released documents show.

That decision made Imperial Oil, which is majority-owned by Exxon, an early supporter of an oil industry campaign of climate denial that continues to slow progress in combating the greatest existential challenge of our time.

Brendan DeMelle, executive director of the research group DeSmog, said the documents show that as early as the 1970s Imperial Oil had confirmed the link between fossil fuels and global warming. DeSmog and the Climate Investigations Center last week published thousands of pages of official Imperial Oil documents found in an archive in Calgary.

DeMelle said that with pressure building in the late 1990s for Canadian climate change solutions that might reduce fossil fuel consumption and hurt the company’s business model, “they start talking about scientific uncertainties and doubt.”