Hours after being sworn in as the new U.S. Secretary of Transportation, Sean Duffy took aim at the main way the federal government regulates miles per gallon for cars and pickup trucks — also a principal way that it regulates air pollution and addresses climate change.

Here are five reasons why the Trump administration’s action matters. Duffy ordered his chief of the National Highway Traffic Safety Administration to "propose the recission or replacement of any fuel economy standards" necessary to bring the rules into line with Trump's priority of promoting oil and biofuel.