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The term ocean acidification was coined in 2003, but in the Beaufort Sea scientists have been tracking its effects since 1997.

Over the past 23 years, the Arctic Ocean has become increasingly acidic. While most of the increase has been driven by rising carbon emissions, a recent paper shows how acidification has been intensified by an influx of fresh water from melting sea ice. In 2006, the Beaufort Sea passed a crucial tipping point: ever since, the water has been corrosive to animals’ shells.

Among the victims of this shift are Arctic sea butterflies—small marine snails with delicate shells. In Beaufort Sea fjords, 70 percent of sea butterflies have weakened shells.

Being a creature with a shell is difficult in the Arctic Ocean. Cold water absorbs more carbon dioxide from the air than warm water, and dissolved carbon dioxide makes the water more acidic. Acidified water ties up the calcium carbonate that animals use to build their shells. When the concentration of calcium carbonate in the water dips, organisms must expend energy to keep their shells intact. That’s been the reality in the Beaufort Sea for the past 14 years.

But according to the new research led by Yuanxin Zhang, an oceanographer from the Tokyo University of Marine Science and Technology, melting sea ice—one side effect of global warming—is further depressing the already-low levels of calcium carbonate in the Beaufort Sea, adding stress to animals with shells.

When seawater freezes, salt and calcium carbonate are pushed out, resulting in sea ice that is basically fresh water. The older the ice, the fresher it is. So when multi-year Arctic sea ice began disappearing rapidly in the late 1990s and early 2000s, it sent a pulse of fresh water into the sea that diluted calcium carbonate and rapidly turned the Beaufort Sea corrosive.

In the future, melting permafrost and rising rivers will push more fresh water into the Arctic Ocean, which could further dilute calcium carbonate—especially in coastal areas.

“Organisms have some ability to flex their body systems,” says Zhang. But like most high-latitude systems, the Beaufort Sea food web is highly dependent on a relatively small number of species. “Once one [species] is gone, the impact will be bigger than in other oceans.”

While Zhang’s research shows worrisome annual trends in the decreasing level of calcium carbonate, University of Alaska Fairbanks marine biologist Amanda Kelley says it’s important to remember that there are huge swings hidden inside these annual averages.

“There’s an intense seasonality in polar regions,” says Kelley. “There’s just so much sun in the summer.” In some nearshore areas, the amount of carbon dioxide consumed by phytoplankton and algae in the summer can swamp broader trends to cause dramatic swings in acidity in just a few hours.

The global effects of increasing acidity and rising temperature layer onto a system that is never static. Whether or not organisms can withstand further changes in calcium carbonate is influenced by the resilience they’ve built to the swings of the past and by their susceptibility to the multiple stressors of the future. It’s unclear how much longer the Arctic’s creatures will be able to adapt.