On Friday, a federal judge ruled that President Trump could not re-authorize drilling in Arctic waters after President Obama removed those waters from drilling in 2016.
If the Alaskan judge's ruling withstands appeal, it would mean that the Trump Administration would have to seek approval from Congress to re-open federal waters north of Alaska to oil and gas drilling. Congress, currently divided with a Republican majority in the Senate and a Democratic majority in the House, would be unlikely to agree to such a request.
Why are these waters in question?
In December 2016, the outgoing Obama Administration invoked a 1953 law called the "Outer Continental Shelf Lands Act" (OCSLA) to remove about 125 million acres of Arctic waters north of Alaska from potential leasing to oil drilling operations. (This excluded roughly 3.2 million acres of lease-able waters adjacent to the coast, approximately 200,000 of which are currently under active leases to oil and gas companies.)