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.