The United States is one of the largest emitters of planet-warming carbon dioxide in the world. But on a per capita basis, our carbon output fell over the last two decades by 30% as  the U.S. population grew and overall emissions dropped, according to a new analysis released this week by the federal Energy Information Administration.

The drop in emissions has a lot to do with changes in how we generate electricity.

Back in 2005, we burned a lot of coal to keep the lights on in the U.S. But a few years later, our energy mix started to change, according to Kevin Nakolan at the Energy Information Administration.

“We had a big boom in natural gas-powered electricity generation,” he said.

This was when fracking became a big thing. Natural gas is still a fossil fuel, but it produces less carbon dioxide than coal. And in the late 2000s, using more natural gas led companies to shut down a lot of coal plants, which lowered carbon emissions.

Then, in the past decade or so, “we saw a lot of that being growth in renewable sources like wind or solar,” Nakolan said.

But going forward, emissions might not fall at the same pace, noted Kenneth Gillingham at Yale. That’s in part because electricity demand is rising.

To meet that demand, “existing power plants that may have become uneconomic are now becoming more economic than they were previously and are more likely to stay on longer,” he said — including coal plants.