When an expected record-breaking 36 million Americans take their holiday road trips this Fourth of July weekend, they’ll be part of what is quickly becoming our nation’s biggest source of carbon dioxide emissions — transportation.

This winter, for the first time since 1979, carbon dioxide emissions from cars, trucks, and SUVs surpassed the carbon pollution from electric power plants, which have been America’s chief climate polluter for more than 35 years, U.S. Energy Information Administration data released this week show.

“The major significance is it shows that decarbonization is about more than just the power sector,” said Sam Ori, executive director of the Energy Policy Institute at the University of Chicago.