The first Arctic summer without ice is projected to occur sometime around the middle of the century. That milestone might have come even earlier were it not for the Montreal Protocol, an international treaty banning ozone-destroying gases.
In 1987, most of the world’s countries convened in Montreal to agree on a plan to phase out the use of chlorofluorocarbons (CFCs) and other aerosols that eat away at the ozone layer. These ozone-depleting substances are also greenhouse gases, some thousands of times more potent than carbon dioxide.
The primary concern then was preserving the ozone layer, but the resulting treaty had unexpected climate benefits too. It has reduced warming, lessened the intensity of tropical cyclones and helped store more carbon in plants.
The effects of the treaty have also delayed – but not prevented – sea ice loss in the Arctic due to climate change, according to an analysis by Mark England at the University of Exeter in the UK and his colleagues. He and his team used climate and ice models to project Arctic sea ice extent with and without the Montreal Protocol, accounting for lowered CFC emissions, and the resulting rebound of the ozone layer. They also considered the effects of the gases used to replace CFCs.
The analysis showed that without the Montreal Protocol, the first Arctic summer without ice would have occurred at some point between 2030 and 2038, depending on the model. With the treaty in place, they find the first summer without ice can be expected between 2037 and 2054, assuming moderate to high future greenhouse gas emissions.
The first ice-free Arctic summer – defined as the first September with less than 1 million square kilometres of ice – would have consequences for ecosystems and communities that depend on the ice, as well as shipping routes, says England. “It is an alarming indicator for how much we’ve changed the climate system,” he says.
England says the best projections suggest the occurrence of ice-free Arctic summers is “approaching inevitability” even with substantial emissions reductions. But how frequently they occur will depend on the amount of greenhouse gases released, he says. Controlling potent but short-lived methane emissions could be especially impactful.
The work of the Montreal Protocol itself is also hardly complete. In April, researchers reported a sharp rise in the atmospheric concentration of CFCs since 2010, despite the ban on their production. And many of the aerosols used to replace CFCs are themselves potent greenhouse gases now set to be phased down after an update to the original treaty.