President Barack Obama just made millions of acres of the Arctic and Atlantic Oceans off limits to oil and gas drilling. It’s a move that will protect critical habitats for marine life. And it will be challenging for the next administration to undo.

It will be challenging for the next administration to undo

The president used a 1953 law called the Outer Continental Shelf Lands Act to safeguard 3.8 million acres of the Atlantic Ocean off of the East Coast of the United States — from southern Virginia to off of Cape Cod. He also protected 115 million acres in the Arctic Ocean — including all of the Chukchi Sea off the northwest coast of Alaska, and most of the Beaufort Sea off Alaska’s northern edge.

This 1953 act allows the Department of the Interior to govern which companies have the rights to drill into the ocean floor more than three miles off the coast of the US — which is under federal, and not state, control. It also gives the president the right to put large, un-leased sections of ocean off limits for future oil and gas exploration. Companies that already have oil and gas drilling rights in these regions will not lose them under the new rules.

This provision hasn’t been used very often, says Robin Kundis Craig, an environmental law professor at the University of Utah. So the fact that President Obama invoked it, she says, “is a statement that our marine areas have other kinds of values to them, besides offshore oil and gas.”

The new protected area in the Atlantic includes deep canyons that are critical for sustaining a diverse range of marine life. Warming waters are making it increasingly difficult for most fish species in the Atlantic to survive, according to the Department of the Interior. So preserving these canyon refuges will also help to conserve the region’s fish, as well as the livelihoods of the fishers who depend on them.

For the Arctic, the Department of the Interior lists two reasons to protect these waters: for one thing, many species that indigenous peoples rely on for food and incorporate into cultural traditions are vulnerable because of climate change and human activity. Drilling could threaten them still further. The other reason is more pragmatic: it’s hard to make sure Arctic drilling is being done safely. The conditions are harsh, the region is remote, and a spill could be devastating.

President Obama’s freeze on Arctic drilling comes with a matching promise from Canada. Prime Minister Justin Trudeau announced that the country will block new offshore drilling in its own Arctic waters indefinitely — though the policy will be reevaluated in five years. Trudeau also pledged to work with indigenous peoples when creating policy — possibly in response to recent pushback against his decision to allow a crude oil pipeline to carry more oil. Both countries agreed to find the most sustainable shipping routes through the region.

“Our marine areas have other kinds of values to them, besides offshore oil and gas.”

Still, it’s an interesting move, Craig told The Verge. “Because commercially, at least right now, no one has been rushing to develop the Arctic,” she says.

Royal Dutch Shell, for example, began exploring the possibility of drilling in the Chukchi Sea off the northwest coast of Alaska in 2008. But the company gave up in 2015 because there was too little oil and gas to make the expense of drilling in a frozen, remote region worth it. “The price of oil is pretty low right now, so that’s an expensive way to get your oil,” Craig says. “It’s also pretty hazardous.”

President Obama’s withdrawal won’t be simple for the next administration to take back — in fact, although the law isn’t completely clear, it probably would take an act of Congress. “It’s not so easy that the next president can just say, ‘No, I’m rescinding that,’” Craig says. “And how willing Congress is to do those things on any given days is anybody’s guess. We could put Las Vegas odds on that.”