This story was originally published by Grist and is reproduced here as part of the Climate Desk collaboration.

In 2007, the Supreme Court ruled that the Environmental Protection Agency has the authority to regulate greenhouse gases, because they meet the Clean Air Act’s definition of air pollutants. It was a pivotal moment for climate regulation in the United States. That ruling led the EPA to find that six key greenhouse gases, including carbon dioxide, endanger public health and welfare.

The agency then utilized this so-called endangerment finding to issue rules limiting tailpipe emissions from vehicles during the Obama and Biden administrations — a key tool for reducing the nearly 30 percent of US emissions attributable to transportation. Over the years, the EPA has depended on its endangerment finding to regulate climate-warming gases from coal plants, aircraft, and other industrial sources.

The finding, which underpins several major EPA rules, is now at risk.

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