On Tuesday, President Obama announced that he was putting hundreds of millions of acres of federally owned waters in the Arctic and Atlantic Ocean off limits “indefinitely” to future oil and gas drilling.

Environmental groups had been calling for precisely this move ever since the election — as a way of preempting Donald Trump. What’s interesting here is that while Obama can legally put these areas off limits by executive order, it’s not at all clear that Trump can unilaterally reverse this move, at least not without Congress passing a brand-new bill. The dispute hinges on an obscure section of a 1953 law, the Outer Continental Shelf Lands Act, that has never really been tested in courts.

First, let’s look at the lands in question. The White House placed 115 million acres of federal waters north of Alaska, in the Chukchi and Beaufort seas, off limits to future oil and gas exploration. (A smaller section of the Beaufort sea near the coast will continue to be available for future leasing.)

Shell had earlier tried to search for crude in this icy and treacherous region — but finally gave up last September after a lack of early success combined with low oil prices made the whole project much too risky and costly. (Shell had been struggling with the region’s floating ice and harsh weather for years before that.)

Obama also placed 3.8 million acres in the north Atlantic Ocean off limits “indefinitely” to future oil and gas leasing. Earlier in his presidency, his Interior Department had floated the idea of opening up some federal waters further south, in the mid-Atlantic, for leasing. But he later reversed course after coastal communities in places like Virginia and North Carolina began fretting about the potential effects of oil spills on tourism there:

So how did he pull this off? Simple. Section 12(a) of the Outer Continental Shelf Lands Act says, “The President of the United States may, from time to time, withdraw from disposition any of the unleased lands of the outer Continental Shelf.” So that’s what Obama did — he withdrew these areas from disposition. (The Outer Continental Shelf includes the submerged sea floor between the US continent and the deep ocean. Companies have long been drilling in the Gulf of Mexico OCS.)

Now, the striking thing about this 1953 law is that it doesn’t say a future president can undo a permanent withdrawal like this. As a brief from two environmental groups, the Natural Resources Defense Council and EarthJustice, explains, this strongly implies that only Congress can put these areas back into consideration for oil and gas leasing.

So it’s unclear what happens next. No president has ever tried to reverse a permanent withdrawal made under section 12(a). But Trump has vowed to open up new federal lands to oil and gas drilling as president, and the oil industry is already encouraging him to challenge this move by Obama. “There is no such thing as a permanent ban,” said Reid Porter of the American Petroleum Institute in a press release. He notes that George W. Bush issued a memo to reverse a Clinton-era time-limited withdrawal, though the NRDC notes that that was a different situation.

Perhaps Trump will try to overturn this ban and we’ll see what the courts have to say. Or, alternatively, the GOP-controlled Congress could try to pass a new bill that explicitly places these areas back into play for oil and gas drilling (though that would presumably have to get by a Senate filibuster first and isn’t so easy).

It’s also unclear how much oil and gas drilling would actually occur in these areas if they were made available again. Current low oil prices have scared away companies like Shell from engaging in costly and risky oil exploration in the Arctic. (Oil companies can only drill up there in the summer, when the thickest ice has melted, and even then, hurricane-force storms and floating icebergs can pose extreme challenges for drilling crews.) Along the Atlantic, meanwhile, state governments are often opposed to drilling and would likely have a say in the issue.

In a statement, the White House pointed to the need to shift away from fossil fuels in order to address climate change as well as the dangers of an oil spill in an ecologically sensitive Arctic region that would be difficult to clean up (on account of all that ice). Here’s the key White House quote:

These actions ... protect a sensitive and unique ecosystem that is unlike any other region on earth. They reflect the scientific assessment that, even with the high safety standards that both our countries have put in place, the risks of an oil spill in this region are significant and our ability to clean up from a spill in the region’s harsh conditions is limited. By contrast, it would take decades to fully develop the production infrastructure necessary for any large-scale oil and gas leasing production in the region — at a time when we need to continue to move decisively away from fossil fuels. In 2015, just 0.1 percent of U.S. federal offshore crude production came from the Arctic and Department of Interior analysis shows that, at current oil prices, significant production in the Arctic will not occur. That’s why looking forward, we must continue to focus on economic empowerment for Arctic communities beyond this one sector.

This move was done in conjunction with Prime Minister Justin Trudeau’s government in Canada, which placed some of its own waters off limits to fossil-fuel exploration, though that Canadian ban will be reviewed every five years.

Further reading

— Ever since Trump’s election, the Obama administration has been busy trying to finish up rules and hem in its successor. Juliet Eilperin wrote an insightful piece looking at those moves for Outside magazine.

— The American Petroleum Institute is deeply unhappy with this decision; you can read its response here. So is the Arctic Slope Regional Council, which has 11,000 Alaska Native shareholders and is broadly in favor of drilling; its response is here.

— Environmental groups, meanwhile, are thrilled (for now at least); for weeks, they’ve been calling on Obama to restrict oil drilling in these areas.

— Here’s an in-depth earlier look at the many, many challenges of drilling in the Arctic.