The U.S. is burning less and less coal each year, thanks to cheap natural gas and new pollution rules. From a climate perspective, that’s a huge deal — less coal means less carbon. But here’s the catch: if the U.S. just exports its unused coal abroad, the end result could actually be more carbon.

Aerial view of Powder River Basin in Wyoming. (Alex Brandon)

If that scenario actually transpired (and, admittedly, it’s a bet that natural gas prices will remain low for a long while), it would make a huge difference for America’s global-warming contributions. Deutsche Bank estimates that carbon pollution from the electricity sector would drop 44 percent below 2005 levels — that’s about a 16 percent cut in all U.S. emissions. And that’s without Congress even passing a climate bill.

Except that the story doesn’t end there. The United States still has plenty of coal sitting underground, especially in Wyoming’s vast Powder River Basin. And other countries around the world would love to get their hands on that coal to burn for electricity. That’s why, over the past decade, even as domestic coal use has dropped, exports have surged (mostly to Europe). In 2010, exports accounted for 7.5 percent of all coal production, up from 4.4 percent in 2005. As Steven Hayward of the American Enterprise Institute observes, that boom in overseas demand has helped coal production hold steady in the past decade.

So here’s one possible future: If we’re not going to burn our coal, someone else will. One Tokyo shipping company, Daiichi Chuo Kisen Kaisha, says that U.S. coal exports could double in the next three or four years. In Washington state, coal companies are proposing two large export terminals that would help ship tens of millions of tons of coal from the Powder River Basin to countries like China. That, in turn, could make coal even cheaper in places like China — which might spur the country to build even more coal power plants than its current, already hectic pace. And, since carbon-dioxide heats up the planet no matter where it’s burned, this outcome could cancel out many of the global-warming benefits of the U.S. coal decline.

Now, this outcome is hardly set in stone. Environmental groups like the Sierra Club are trying to block the coal-export terminals in Washington. (It’s quite possible that this could become another Keystone XL pipeline fight, especially since, as Eric De Place points out, the climate impact is potentially much larger.) Green groups are also trying to mount legal challenges to new coal leases in the Powder Basin River area — my colleague Juliet Eilperin wrote an excellent story on that ongoing battle. Alternatively, China has recently started exploiting its own large shale-gas reserves, which might crimp demand for coal overseas.

Still, it’s another reminder that climate change remains a global issue — and one that will take far more than a smattering of U.S. regulations and a boom in cheap natural gas to solve.