In a surprising bit of good news, CO2 emissions are dropping in the United States, and much of the credit goes to the use of natural gas.

In a surprising turnaround, the amount of carbon dioxide being released into the atmosphere in the U.S. has fallen dramatically to its lowest level in 20 years, and government officials say the biggest reason is that cheap and plentiful natural gas has led many power plant operators to switch from dirtier-burning coal.

Many of the world’s leading climate scientists didn’t see the drop coming, in large part because it happened as a result of market forces rather than direct government action against carbon dioxide, a greenhouse gas that traps heat in the atmosphere.

Natural gas has been controversial due to fracking, but the carbon emission benefits can be substantial if gas replaces coal. It’s also adding to job growth and other economic activity as well.