In an effort to curb rising carbon emissions, the U.S. Environmental Protection Agency is proposing new fuel efficiency standards for medium- and heavy-duty trucks. Currently, these large vehicles account for 20 percent of all greenhouse gas emissions in the U.S. — but only account for 5 percent of the cars on American roads.

The EPA plans to completely retool the standards for all semis, large pickup trucks, buses, and work trucks, along with the National Highway Traffic Safety Administration. The new standards may cut vehicle CO2 emissions by 1 billion metric tons — and fuel costs by $170 billion. The changes will begin to be phased in between 2021 and 2027.

Heavy duty trucks account for 20 percent of all greenhouse gas emissions in the U.S.

The EPA aims to reduce the impacts of climate change while saving manufacturers money. "Once upon a time, to be pro-environment you had to be anti-big-vehicles. This rule will change that," Anthony Foxx, the U.S. transportation secretary, said in a statement. "In fact, these efficiency standards are good for the environment – and the economy. When trucks use less fuel, shipping costs go down. It’s good news all around, especially for anyone with an online shopping habit."

Carbon dioxide levels are at the highest they’ve been in 650,000 years, according to the National Oceanic and Atmospheric Administration. The amount of this heat-trapping gas has steadily increased in our atmosphere over the years, reaching an all-time average high this year of 400 parts per million.