In 2009, the Environmental Protection Agency declared that the rising concentrations of greenhouse gases in the atmosphere threatened public health and welfare . This “endangerment finding,” as it’s known in legal jargon, may have sounded self-evident to those who had been following climate science for decades, but its consequences for U.S. policy were tremendous: It allowed the EPA to issue rules limiting emissions from U.S. vehicles, power plants, and other industrial sources. While those rules have not always survived court challenges and changing presidential administrations, the regulatory authority underpinning them has proven remarkably stable.

On Tuesday, President Donald Trump’s EPA took a major step toward changing that. At a truck dealership in Indianapolis, EPA Administrat

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