The oil and gas industry has known for decades that its wells were releasing far more planet-warming methane than companies acknowledged, according to industry documents identified by an environmental group.

In a report on the findings released on Monday, the Center for Climate Integrity cited, among other examples, a 1979 report commissioned by the research arm of the American Gas Association, an industry group. That report said that major gas fields in the United States could be releasing far more planet-warming gases than was being officially reported.

Methane, a main component in natural gas, is a particularly potent greenhouse gas when released into the atmosphere. It has “always been unclear what the industry directly knew” about its methane releases, said Steven Hamburg, chief scientist at the Environmental Defense Fund, who has researched the question for years and wasn’t involved in the center’s report.

The Center for Climate Integrity is a nonprofit that has called for the oil industry to be held financially responsible for the effects of climate change, which has been caused by the burning of fossil fuels. The group said it examined decades’ worth of industry documents, available in university libraries and other public archives, to draw its conclusions.

Despite industry findings like these, the group said in its report, the industry publicly underplayed the concerns over methane. And as the natural gas industry faced increasing competition from electric stoves and heaters, industry groups worked on an advertising campaign that highlighted gas as a clean fuel, according to documents cited by the Center for Climate Integrity.

The American Gas Association, whose research arm commissioned the 1979 review, did not respond to requests for comment.

Since 1990, natural gas production has surged in the United States, transforming the nation from a net importer of the fuel to a leading exporter. By the 2010s, the United States had overtaken Russia as the world’s largest producer of natural gas.

The American Petroleum Institute, the largest U.S. trade group for the oil and natural gas industry, called the report “the latest in a coordinated campaign” against an industry it said was vital to the American economy. It also criticized a recent effort by state and local governments to hold the fossil fuel industry accountable for its role in accelerating climate change.

“We continue to believe that energy policy belongs in Congress, not a patchwork of courtrooms,” said Andrea Woods, an A.P.I. spokeswoman.

In recent years, as concerns have mounted over the oil and gas industry’s role in warming the planet, companies have invested in technology to more aggressively detect and fix leaks from gas fields, pipelines and other sources. They’ve also moved to eliminate routine flaring, reversing a longstanding industry practice designed to burn off excess natural gas produced at oil and gas fields.

The environmental group’s report casts a harsh light on how the oil and gas industry has previously communicated the role of natural gas in global warming.

Carbon dioxide, which is building up in vast quantities in the atmosphere, remains the main driver of global warming. But methane is roughly 80 times more potent at trapping the sun’s heat over a 20-year period than carbon dioxide, making it a particularly intense short-term driver of climate change. The production of oil and gas is now recognized to be a major source of methane emissions, along with agriculture, landfills and natural sources like wetlands and swamps.

Fossil fuel groups have long highlighted how burning natural gas is cleaner than burning coal, the dirtiest fossil fuel. And switching away from coal-burning power plants to gas-burning ones has indeed helped the United States rein in its carbon dioxide emissions.

Still, scientific evidence has grown that drilling for gas releases large amounts of methane into the atmosphere. A landmark study in the journal Science, published in 2018, found that the U.S. oil and gas supply chain was leaking roughly 60 percent more methane than the Environmental Protection Agency had estimated. Gas also seeps from underground city pipelines, loose valves, and gas stoves and other appliances, research has found.

The documents released Monday suggest the oil and gas industry has long been aware of these concerns.

Notes from a 1966 Shell study group indicated that natural gas was released in “embarrassingly large quantities” at oil fields, often through flaring or venting. In 1968, a report commissioned by the American Petroleum Institute found that at least some of the methane found in the atmosphere was likely coming from the world’s gas fields.

Research into the health effects of gas was also progressing. By the early 1960s, physicians were identifying gas appliances like gas stoves as sources of indoor pollution. Internal polling conducted by gas groups had found that the public viewed natural gas as a pollutant.

The American Gas Association turned to public relations firms, including Hill & Knowlton, to craft a response, the documents show. Hill & Knowlton, now part of the Burson agency, urged the industry to mount a “proactive” campaign to quiet fears before critics demanded regulation. In 1972, the group launched its “Gas, clean energy for today and tomorrow” campaign designed to rebrand natural gas as an environmentally friendly fuel.

The group also cited a 1990 statement by Patrick Gillam, then a managing director at the oil giant BP. He told industry leaders at the time that while gas was cleaner than coal, leaks during the production and distribution of natural gas likely made its environmental benefits “more limited than some enthusiasts expect.”

A spokeswoman for Shell, Krista Edwards, said the report mischaracterized Shell’s actions and intent. Shell has been a leader in reducing methane emissions, she said, and was working toward a goal of near-zero methane emissions by 2030.

Paul Takahashi, a spokesman for BP, said methane emissions were “an important issue” for the company and that it was “focused on measuring and reducing them” across its operations. Burson did not respond to a request for comment.

The 1990 statement tracked research in the public sphere. In 1976, a study led by scientists at the National Aeronautics and Space Administration said the climate effects of gases like methane had been “overlooked or underestimated.”

And in 1985, scientists at the federally funded National Center for Atmospheric Research estimated that global warming stemming from increasing levels of trace gases in the atmosphere, including methane, could potentially be as large as the warming from projected increases in carbon dioxide.

“We as scientists put out what we knew at the time, and I think by and large the results of our study were accepted everywhere,” said Hanwant B. Singh, a retired research director at NASA and a co-author of the 1985 study. “Nobody said, ‘This is blatantly wrong.’ Even the gas industry accepted it.”

Wei-Chyung Wang, a co-author of the 1976 study and now professor emeritus at the State University of New York at Albany, said the industry’s actions over the years were to be expected. After all, he said, chlorofluorocarbons, which were used widely until the 1980s in refrigerants and aerosols, weren’t banned until it became indisputable that the chemicals were destroying the ozone layer.

For oil and gas companies to take action, he said, they felt that stronger evidence was likely needed. “I am not really surprised by the industry’s behavior.”

The growing concerns were missing from a major joint study the gas industry published with the Environmental Protection Agency in 1996. The study concluded that methane emissions were negligible and that climate concerns were small.

Documents identified by the Center for Climate Integrity show that the E.P.A. waived its technical review of the study’s findings, because the agency lacked the expertise to conduct it. In the years since, the conclusions have been criticized for relying on small samples of self-reported industry data.

Joe von Fischer, a professor at Colorado State University and co-author of a 2020 paper examining the E.P.A.’s use of industry-reported data, said it meant the agency had severely undercounted methane emissions. With natural gas leaks, he said, “the more you look, the more you find.”

A spokeswoman for the E.P.A., Brigid Hirsch, described the Center for Climate Integrity as “full of left-wing climate cultists” and said “the Trump E.P.A. understands we can both protect human health and the environment and grow the economy at the same time.”