President Trump announced on Monday that he planned to relax limits on pollution from cars, saying that the move wouldn’t “mean a damn bit of difference to the environment.”

But decades of science show that the pollution from automobile tailpipes has harmed the environment and public health, from the days when leaded gasoline sent neurotoxins into the air and soil to the carbon dioxide emissions that are heating the planet right now.

Transportation is the largest source of greenhouse gas emissions in the United States.

It accounts for about a third of all U.S. emissions contributing to climate change, which is leading to more frequent and intense extreme weather like deadly heat waves, droughts, floods, and storms.

The Biden administration tightened limits on pollution from tailpipes as a way of pressuring automakers to sell more electric vehicles, which do not emit carbon dioxide. It required automakers to achieve fuel economy standards of an average of 65 miles per gallon for all the car models they sell by 2031.

Mr. Trump said he intended to go back to the fuel economy standards in place in 2020 during his first administration, when automakers were required to achieve a fleetwide average of about 40 miles a gallon.

“It doesn’t mean a damn bit of difference, either, for the environment,” Mr. Trump said on Monday in the Oval Office. “It doesn’t matter.”