False-color image showing ozone concentrations above Antarctica on Oct. 2, 2015. Image: NASA/Goddard Space Flight Center

In 1989, amidst mounting scientific evidence, dozens of nations joined forces to sign a treaty aimed at halting the expansion of a massive hole in Earth’s ozone layer. Nearly thirty years later, the Montreal Protocol has done just that. But it has also done something its architects never intended. It has become one of America’s most effective tools in the fight against climate change.

That’s the surprising conclusion of a study published this week in Geophysical Research Letters, which takes a fresh look at the consequences of the nearly 30-year-old treaty that phased out chlorofluorocarbons (CFCs) and later, hydrochlorofluorocarbons (HCFCs), after scientists determined that these compounds were destroying ozone in Earth’s stratosphere. Since CFCs and HCFCs are also potent greenhouse gases, phasing them out has had a major impact on US climate pollution: Between 2008 and 2014, the Montreal Protocol led to greenhouse gas reductions with roughly half the benefit of all other climate regulations enacted by the EPA.

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“This is something that’s been talked about for a while, this dual benefit of the Montreal Protocol limiting damage to the ozone layer, also curtailing climate change,” Rachel Cleetus, climate policy manager and lead economist with the Climate and Energy program at the Union of Concerned scientists, told Gizmodo. “It’s because all these ozone depleting substances are also very potent global warming gases.”

Indeed, the CFCs and HCFCs used as refrigerator coolants, aerosol propellants, flame retardants, and more have heat-trapping potentials hundreds to thousands of times greater than carbon dioxide on a timescale of centuries. While prior research has highlighted the climate benefits of the Protocol on a global scale, the new study looked specifically at what its implementation meant for climate pollution in the US.

Under the Montreal Protocol—which was enforced by the EPA’s Clean Air Act—the US saw a a near-complete phaseout of CFCs beginning in 1996, and a 95 percent decline in HCFCs since 1998. Pulling data from the National Oceanic Atmospheric Administration’s atmospheric monitoring network, Lei Hu from the University of Colorado Boulder and her colleagues demonstrated that from 2008 to 2014, the elimination of these substances had the equivalent climate impact of reducing CO2 emissions by 170 million tons per year. Projecting forward, the researchers found that the continued implementation of the Montreal Protocol and its amendments could shave some 500 million tons of CO2 off our carbon footprint annually by 2025, compared with 2005 emissions levels.

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For context, 500 million tons of CO2 is roughly a quarter of what we need to cut to meet our Paris Climate Agreement target, of reducing emissions 26 to 28 percent by 2025. It’s also close to the annual US emissions from the entire agriculture sector.

There is a catch: Today, the climate benefits of slashing CFCs and HCFCs are being offset, in part, by the use of hydrofluorocarbons (HFCs), which don’t destroy ozone like their nastier cousins, but are still hundreds to thousands of times more effective at trapping heat than CO2. The good news is, last year, the US and other countries adhering to the Montreal Protocol agreed to limit future production and consumption of HFCs, too, with the adoption of the Kigali amendment.

The legally-binding amendment, which mandates rich countries stop production and use of HFCs by 2018, was hailed by then-Secretary of State John Kerry as the “single most important step” in the fight against climate change. This, less than a week after 170 nations ratified the Paris Climate Agreement.

Of course, as positive as this is, we still need to bring down carbon emissions if we’re going to halt global warming. Unfortunately, Trump and EPA head Scott Pruitt are currently working to do the exact opposite, withdrawing the US from the Paris Agreement and unwinding Obama-era climate regulations. Perhaps the most important thing the Montreal Protocol has to offer today—aside from the immediate environmental benefits—is a salient reminder that meaningful action is possible.

“As a policy framework, [the Montreal Protocol] has been an incredible success,” Cleetus, who was not involved with the new study, said. “It’s pretty striking how quickly countries moved together. What’s also striking is how the protocol has matured and been updated over time.”

Kevin Trenberth, an atmospheric scientist at the National Center for Atmospheric Research, agreed. “This is a wonderful advertisement of the success of the Clean Air Act just at a time when it is being undermined by the Trump administration,” he told Gizmodo. “If one takes appropriate actions it has consequences and can indeed work.”

[Geophysical Research Letters]

Correction: An earlier version of this post stated that “the continued CFC/HCFC phaseout could shave some 500 million tons of CO2 off our carbon footprint annually by 2025.” In fact, to reach 500 million tons CO2-equivalent emissions reductions, the US will also need to implement a phaseout of HFCs as stipulated in the Kigali amendment. The text has been updated to reflect this.