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.”