William Romaniuk stopped his tractor at the edge of his field and sniffed. A breeze came off the North Saskatchewan River, rippling across rows of ripe wheat, but a caustic scent flooded his nose. An oil well between his crops and the river was leaking, and enough gas was flowing out of a pipe below the rusty pump jack that the farmer could smell gas and see it swirling in the air.

The site had been drilled in 1950 but hadn’t run since 2015. It was now spewing a blend of toxic, egg-smelling hydrogen sulfide and, quite likely, a much larger dose of odorless, flammable methane.

“They said, ‘don’t get too close to it,’” Mr. Romaniuk said, referring to the owners of the well. He put the tractor in gear and skirted the edge of the site, hoping he wouldn’t spark an explosion as he hurried to harvest.

Canada has committed to reducing its planet-warming carbon emissions and has singled out the oil and gas industry as the source of almost half of the country’s annual emissions of methane, a greenhouse gas that can have 80 times the heat-trapping power of carbon dioxide over 20 years. Alberta, the heart of Canadian hydrocarbon extraction, has set a goal of a 45 percent drop in the industry’s methane footprint from active infrastructure by 2025. But the inactive wells — the ones no longer producing oil or natural gas but many still lingering in suspension like zombies — may be as big a threat to the planet.

After decades of booms and busts, an enormous backlog of these inactive wells has built up, and it grows about 6 percent each year. There are now 97,920 wells, like the one on Mr. Romaniuk’s land, that are licensed as temporarily suspended, compared to the province’s 160,000 active wells. The inactive wells are unlikely to be switched on ever again but have not yet been decommissioned. No one knows how many are leaking methane and other pollutants.

“This is a problem that’s been festering for decades,” said Sonya Savage, Alberta’s energy minister.

In Alberta, the amount of methane and other potent greenhouse gases seeping from suspended wells has not been comprehensively studied. But research in British Columbia, another province with a long history of oil and gas development, revealed that more than a quarter of inactive wells in that province were leaking.