Cenovus Energy Inc.’s Christina Lake oil-sands site vented the most natural gas in more than four years into the atmosphere when a wildfire forced an emergency shutdown of the facility in May.

Christina Lake released the equivalent of 151,511 cubic feet a day of climate-warming gas in May, up from less than 5,000 cubic feet a day in April, Alberta Energy Regulator data show. The amount vented was the most for any month since January 2021. The monthly increase in venting was the most for any facility in Alberta and only eight other sites, all of which primarily produce gas rather than oil, released more in total.

Cenovus along with other oil-sands producers including Canadian Natural Resources Ltd. and MEG Energy Corp. curtailed the equivalent of about 350,000 barrels a day of production in late May and early June as a wildfire threatened well sites, prompting the companies to evacuate workers. The Christina Lake site produces about 230,000 barrels a day.

“Most of the venting in May was directly related to the safe shutdown of the Christina Lake facility at the end of the month due to the wildfire in the area,” the company said in an email. “In an emergency shutdown, we can’t burn or use that gas for operations, which leaves venting as the only option.”

The sudden methane release caused by the shutdown of one of Canada’s largest oil-producing sites illustrates just one negative impact of the wildfires that erupt each summer in Canada, which themselves emit massive amounts of carbon dioxide into the atmosphere. Extreme forest fires in 2023 emitted about 640 million metric tons of carbon, according to National Aeronautics and Space Administration, equal to the annual fossil fuel emissions of a large industrialized nation. Methane concentration in the atmosphere has more than doubled over 200 years and is estimated to account for 20% to 30% of global warming since the Industrial Revolution.