Updated December 21 at 2:45 p.m. ET

Not every crucial climate-change story begins with a protest, or a heroic scientist, or a melting ice cap. Some, alas, begin with a brief historical introduction to U.S. natural resources law.

In the autumn of 1948, President Harry Truman made an important declaration about who controlled the huge seams of oil and gas hidden beneath the seafloor around the coast of the United States.

The federal government—“aware of the long-range world-wide need for new sources of petroleum and other minerals”—would try to encourage the development of those resources, he said. It would do so by itself controlling and administering the seabed which would bring forth that black gold. The Supreme Court had set the stage for Truman’s proclamation a year earlier when it ruled the federal government, and not coastal states, owned the seabed.

Congress ratified Truman’s proclamation. Five years later, in 1953, it sought to clarify its terms and restore some rights to the states. It adopted the Continental Outer Shelf Lands Act.

Under that law, the president can sell leases for the right to mine federal oil reserves in the “outer continental shelf,” a legally defined area that begins three miles from shore and extends to the 200-mile international-waters boundary. Under a twin statute passed in the same year, the Submerged Lands Act, individual states could offer oil leases for land within three miles of their coastlines.

Now, there is a new “long-range world-wide need”—and it is not for new sources of petroleum. If the world hopes to stave off catastrophic climate change, it must begin to roll back its usage of fossil fuels like petroleum. It must leave huge seams of oil and natural gas in the ground, permanently untapped.

President Obama has acted on this new reality, and now there is going to be a big fight about that 1953 law.

On Tuesday, President Obama unilaterally blocked oil drilling in some parts of the Atlantic Ocean and almost the entire American-controlled region of the Arctic Ocean. The White House described the ban as “indefinite.”

The Obama administration made the announcement in tandem with the government of Canada, where Prime Minister Justin Trudeau said he would also prevent drilling throughout Canadian-controlled Arctic waters.

Obama took the action under the Continental Outer Shelf Lands Act, that 1953 law. It appears to grant the president two different powers to manage the seabed.

In the 63 years since it was passed, almost all presidents have focused on using the first: formulating “five-year plans” that dictate how new leases will be offered. This March, Obama announced a new draft of his 2017 to 2022 plan, which temporarily blocked new drilling in Virginia and North Carolina. He is expected to finalize that plan before his term ends.

The second provision has been less well known. It reads as follows:

The President of the United States may, from time to time, withdraw from disposition any of the unleased lands of the outer Continental Shelf.

To most water-law experts, it seems like Congress has given any president the ability to permanently retire undersea lands from its continental-shelf portfolio.

This text has never been tested in court, and it has only been invoked by presidents a handful of times. In the late 1950s, several years after the law was adopted, President Eisenhower used this provision to permanently block drilling in a 75-square-mile area of seabed off the Florida Keys. Eisenhower’s order still stands—a testament to this authority—but it has also never been challenged in court. (An unnamed White House official cites this same precedent in an unusually skeptical New York Times article about the ban.)

More recently, Presidents George H. W. Bush and Clinton have used the same provision to temporarily pause all new federal offshore drilling leases in certain areas for 10 years.

Bureau of Ocean Management handout

Most experts seem to think that President Obama’s order, given the unilateral power delegated by this particular area of law, could only be overturned by an act of Congress.

“I think it was quite a realistic thing that Obama did, and it should be upheld—but who knows,” said John Leshy, the former solicitor general to the Department of the Interior during the Clinton administration. Leshy is a professor emeritus of real property law at the University of California Hastings.

“I don’t think those designations by President Obama are going to be that easily undone,” said Robin Kundis Craig, a professor of law at the University of Utah and a leading expert in modern water law. “President Trump couldn’t just come in and unilaterally undo it, because it’s a delegated authority from Congress to deal with federal property.”

Craig compared the continental-shelf provision to a similar one in the Antiquities Act of 1906, which lets the president permanently set aside federally owned lands as a conservation-protected “federal monument.” Obama has used this law to preserve tens of thousands of acres of forest in Maine. He has also invoked it to significantly expand a marine monument off the coast of Hawaii—one originally created, using the same powers, by President George W. Bush.

In both the Antiquities and the Continental Shelf acts, Congress delegates to the president some of its own constitutionally granted power to manage public land. Once delegated, Congress must act again to reclaim that power: “The U.S. Supreme Court has upheld the power of the president across the board” in cases involving the Antiquities Act, Craig said.

“I think the strongest interpretation is that, since the authority to create the reservation came from Congress, it would take an act of Congress to undo it,” she added.

Notably, both of these laws only work in one direction. The president can giveth land to conservation, but he or she cannot taketh it away. In other words, presidents can unilaterally conserve land (or “withdraw [seabed] from disposition”), but he or she cannot unilaterally un-conserve it or re-offer a withdrawn lease. Only Congress can undo it.

Even if the court overturns the ban, experts thought that the steep economics of operating in the Arctic Circle may effectively halt oil exploration there. Last year, Royal Dutch Shell stopped drilling in the Chukchi Sea after a second oil-exploration project there found too little crude to warrant further investment.

Bureau of Ocean Management handout

“They’ve spent a couple billion dollars on a dry hole,” Leshy told me. “There’s no market up there. It’s incredibly costly. It’s perfectly clear to everyone, including the oil industry, that they’re going to have to leave a lot of stuff in the ground, and that’s the first stuff you’d leave in the ground.”

That didn’t mean the ban would go over well with some Arctic communities.

“The industry’s going to scream bloody murder, and then Alaska’s going to scream bloody murder, because Alaska regards itself as totally dependent on the oil industry,” Leshy said. “But Alaska can still drill in state waters, three miles out.”

Experts seemed split on whether the industry would first fight the ban in the courts or try to undo the work through Congress. Trump has made fossil-fuel extraction core to his domestic economic plan, stocking his cabinet with former oil executives and officials from oil and gas-rich states, so he might be looking for a court fight.

And while a Congressional act undoing the ban would almost certainly be successful, it would require getting a law through the Senate, where Republicans do not hold enough seats to overturn a filibuster (at least as the filibuster stands today).

While American environmental advocates rejoiced at the Obama announcement, their Canadian counterparts seemed to scoff. There are no ongoing oil or gas drilling projects in the Canadian Arctic, and Canadians had not anticipated seeing one anytime soon.

“There’s no activity taking place in the Canadian Arctic right now,” Michael Byers, a professor of law at the University of British Columbia, told CTV News. “Saying ‘no’ doesn’t require telling anyone to stop.”