The Trump administration has made it abundantly clear that fighting climate change is, at best, a low priority.

President Trump announced in June that the United States will withdraw from the Paris climate agreement. Energy Secretary Rick Perry broached a plan to subsidize coal. Environmental Protection Agency Administrator Scott Pruitt said the science behind climate change is lacking. And White House budget director Mick Mulvaney told reporters that trying to mitigate climate change is “a waste of your money.”

But surprisingly, there’s an international environmental treaty that’s already done more to fight global warming than any other, and the administration actually wants to make it stronger.

It’s a treaty meant to solve a different problem: the hole in the ozone layer.

The Montreal Protocol on Substances that Deplete the Ozone Layer was adopted in 1987 and has more than 197 signatories. More nations are involved in the ozone treaty than are in the United Nations itself. As Brad Plumer explained for Vox last year, the treaty is working. The hole in the ozone layer is healing and is on track to close by 2050.

Ozone fact:

Thanks to #MontrealProtocol, over 99% of chemicals that deplete the ozone layer have been phased out globally.

@NASA #MOP29 pic.twitter.com/ESPa1aJMk5 — UN Environment (@UNEP) November 23, 2017

But this success is not guaranteed. To hit this 2050 goal, the world will need to cut back even more on ozone-depleting chemicals. Activists were concerned that the White House would sideline the Montreal Protocol, especially in light of the United States’ awkward showing at climate change talks earlier this month in Bonn, Germany. There, US officials were forced to defend President Trump’s statements on climate change and tried to make the case for more coal power to environmental campaigners.

Nonetheless, the Trump administration declared its support for the Montreal Protocol last week when delegates from the 197 countries reconvened in the namesake city celebrate the agreement’s 30th anniversary and to discuss how to ramp it up.

The administration even backed a new set of revisions to the treaty that would further ratchet down emissions of harmful gases that deplete ozone and warm up the planet.

From Montreal to Paris: how closing the ozone hole helped reduce global warming

The Montreal treaty formed after scientists detected a growing hole in the ozone layer over Antarctica in the early 1980s and thinning levels of ozone over the rest of the planet. Atmospheric ozone acts as a sunscreen for the planet, blocking cancer-causing ultraviolet light from the sun.

It turned out that certain chemicals that saw wide use in refrigerants and aerosol propellants, namely chlorofluorocarbons (CFCs), ate away at the ozone layer. Startled nations quickly chalked out a plan to phase out the use of CFCs.

One reason the White House may be continuing support for the Montreal Protocol is that it tackles an immediate health concern. The parts of the world that experienced the most thinning of the ozone layer experienced a surge in melanoma and cataracts.

The World Health Organization estimates that a 10 percent decrease in ozone levels leads to an additional 300,000 non-melanoma and 4,500 melanoma skin cancer cases.

“The United States views the Montreal Protocol as of one of the world’s most successful multilateral environmental agreements,” Judith Garber, principal deputy assistant secretary for the Bureau of Oceans and International Environmental and Scientific Affairs at the State Department, told delegates at the 29th meeting of parties to the Montreal Protocol last week. “Truly, few institutions in history can boast such a large positive impact.”

She added that she was grateful for the “heroic work” of her forebears on the Montreal Protocol.

In the US alone, Garber noted, the treaty would avert 280 million cases of skin cancer, about 1.6 million skin cancer deaths, and more than 45 million cases of cataracts over the life of the treaty.

However, she conspicuously avoided mentioning one of its biggest environmental benefits.

The Montreal treaty doesn’t just ban CFCs; it also strictly regulates the CFC replacements that contribute to climate change. (Many CFC replacements are thousands of times more potent than carbon dioxide when it comes to warming the planet.)

The protocol requires countries to prevent these gases from leaking, giving them the flexibility to switch to alternative gases, undertake more aggressive pollution control measures, or deploy more efficient refrigerators and air conditioners that need less ozone-depleting gases in the first place.

The net result has been a vast reduction of emissions of potent greenhouse gases, avoiding a huge amount of warming. In all, the Montreal Protocol (and its subsequent revisions) is the single most effective policy humanity has ever pursued to fight rising global temperatures, as you can see in this chart from the Economist:

The Institute for Governance & Sustainable Development projects that the Montreal Protocol will avert between 100 billion and 200 billion metric tons of carbon dioxide equivalents by 2050, thwarting up to 0.5°C of warming by 2100.

The Montreal Protocol is the most successful climate agreement ever. And it’s growing stronger.

Unlike the 2015 Paris climate agreement, Montreal was ratified as a treaty and is therefore binding. It carries the force of law inside countries and between nations, and it can route funding and assistance to bring noncompliant members in line.

That’s why activists, researchers, and diplomats hail the Montreal Protocol as the most successful climate agreement ever.

And the treaty is still getting stronger. In 2016, parties to the Montreal Protocol met in Kigali, Rwanda, and hammered out the Kigali Amendment, which ratchets down emissions of hydrofluorocarbons (HFCs), another class of refrigerants that warm the planet.

The United Nations Environment Programme declared that the amendment is “the single largest real contribution the world has made so far towards keeping the global temperature rise ‘well below’ 2 degrees Celsius.”

Earlier this month, the amendment received the necessary 20 votes of support it needs from countries to go into effect, an occasion Rwanda’s environment minister described as “historic”:

We did it! With #Sweden ratifying today, the #KigaliAmendment has reached the 20 country threshold needed to enter into force. Thank you to everyone who made this historic moment possible! — Vincent Biruta (@Vbiruta) November 17, 2017

The US backs the Kigali Amendment since it “represents a pragmatic and balanced approach to phasing down the production and consumption of HFCs,” according to Garber, but the revisions need approval from the Senate, and thus the United States was not one of the 20 votes that put the amendment over the top.

The Montreal Protocol provides a convenient out for climate fence sitters

So why the support for Montreal and not Paris from the White House?

For one thing, the Montreal Protocol isn’t overtly about fighting climate change, providing some rhetorical cover to backers (and allowing Garber to praise the Kigali Amendment without talking about its global warming benefits).

Another factor is that the treaty has strong conservative roots, with none other than Margaret Thatcher, a former chemist, and Ronald Reagan, a skin cancer survivor, as key framers of the accord. Reagan called the treaty “a monumental achievement.”

The Montreal Protocol was also unanimously ratified in the Senate, so undoing it would require a huge political lift.

And some of the players most affected by the regulations stemming from Montreal, like the chemicals manufacturer DuPont, have stepped up to provide alternatives to CFCs and HFCs, while heating and cooling hardware manufacturers such as Honeywell are embracing the treaty’s push for better refrigerants as a competitive edge against foreign companies.

But there are still some mixed signals from the White House. Activists noted that the Trump administration’s budget proposal slashed the United States’ contribution to the Montreal Protocol’s Multilateral Fund, a mechanism that channels money from rich countries to poor ones to help meet obligations under the agreement.

Congress approved $41 million for this fund across the State Department and the Environmental Protection Agency in the 2017 fiscal year continuing resolution. But the White House’s budget request for fiscal 2018 zeroes out the EPA’s contribution from $9 million. The State Department’s budget request shows a literal blank line under “Montreal Protocol Multilateral Fund” for 2018.

Ultimately, whether the United States contributes to the Multilateral Fund falls to Congress, and the Senate still has to vote on the Kigali Amendment. While there may be inertia behind the Montreal Protocol, there may not be any oxygen left on Capitol Hill to consider the finer points of an international treaty anytime soon, as lawmakers face a looming debt ceiling deadline and a bruising fight over tax reform.