The 1987 Montreal Protocol, which phased out the production and use of chemicals that were depleting the ozone layer, has long been considered one of the most successful environmental treaties in history. New research finds that the global pact achieved another unforeseen benefit: delaying the melting of Arctic sea ice.

In a study published Monday in the Proceedings of the National Academy of Sciences, researchers from the University of Exeter and Columbia University found that the implementation of the Montreal Protocol is delaying the first ice-free Arctic summer by up to 15 years. That’s because the chemicals banned under the agreement are also potent greenhouse gases.

“Our results show that the climate benefits from the Montreal Protocol are not in some faraway future: the protocol is delaying the melting of Arctic sea ice at this very moment,” Lorenzo Polvani, one of the study’s authors, said in a press release. 

The study authors ran a series of climate models based on two different scenarios: one that included levels of ozone-depleting substances that would be expected if the Montreal Protocol never existed, and another accounting for the global treaty. The researchers concluded that the protocol is postponing the first ice-free Arctic summer by a decade or more, and entirely due to the phasedown of ozone-depleting chemicals. 

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The Montreal Protocol was created to address a hole in the stratospheric ozone layer over the Antarctic. The ozone layer protects the Earth from harmful ultraviolet radiation that causes skin cancer and cataracts in humans. The treaty phased out almost 100 chemicals — including aerosols used in hair spray and other products, refrigerants, and solvents — that were found to be responsible for destroying stratospheric ozone.

Those banned chemicals, collectively called ozone-depleting substances, or ODS, are also potent greenhouse gases, with up to tens of thousands times the global warming potential of carbon dioxide. The report authors estimate that 1 metric ton of avoided ODS emissions leads to 7,000 square meters (more than 75,000 square feet) of avoided Arctic sea loss. By way of comparison, 1 metric ton of carbon dioxide emissions results in about 3 square meters (about 32 square feet) of sea ice loss. 

Given the potency of ODSs as a greenhouse gas, the authors are not surprised at this outsize impact on Arctic sea ice levels. “Nonetheless, such a large mitigating impact of the Montreal Protocol on Arctic sea ice loss is remarkable if one keeps in mind that the protocol was aimed at preventing ozone depletion in the Antarctic stratosphere, and little was known of its effect on Arctic sea ice when the protocol was signed,” the authors noted.

According to their projections, the Montreal Protocol has already prevented more than half a million square kilometers (about 193,000 square miles) of sea ice loss. By 2030, that amount will rise to more than 1 million square kilometers, and to 2 million square kilometers of prevented Arctic sea ice loss by 2040.

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The Montreal Protocol was the first treaty to achieve universal ratification by all countries. It has also enjoyed bipartisan support in the U.S. for decades, including when the Senate approved an amendment to the treaty in 2022. In a 2022 report, a U.N. panel found that the treaty has eliminated 99 percent of ozone-depleting chemicals so far. And since peaking in size in 2006, the hole in the ozone layer has continued to decrease. 

The federal Environmental Protection Agency estimates that the implementation of the Montreal Protocol will prevent more than 443 million cases of skin cancer and 63 million cases of cataracts for Americans born between 1890 and 2100. 

Study authors noted that how long the Montreal Protocol can help postpone the first ice-free Arctic summer depends on how much greenhouse gas emissions, largely produced from fossil fuels, are released in the future.

“The first ice-free Arctic summer — with the Arctic Ocean practically free of sea ice — will be a major milestone in the process of climate change,” Polvani said.

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