A recent Northern voyage on a Norwegian research vessel highlighted how soaring carbon dioxide levels and the resulting acidification of the oceans present an especially grave threat to the Arctic’s cold seas and the rich marine life that they harbor.

From the deck of a Norwegian research ship, the ravages of climate change in the Arctic are readily apparent. In the Fram Strait, the ocean passageway between Norway’s Arctic islands and the east coast of Greenland, seas that should be ice-covered in early September shimmer in the sunlight. Glaciers that muscled across mountains a decade ago are now in rapid retreat, leaving behind walls of glacial till. Rivers of meltwater gush off the Greenland Ice Sheet. But some of the biggest changes taking place in these polar seas are invisible. Under disappearing ice cover, these waters are rapidly growing more acidic as decades of soaking up humanity’s carbon emissions take their toll on ocean chemistry. A soup of brash ice — the wreckage of old floes — surrounds the RV Kronprins Haakon as Colin Stedmon crouches on deck filling bottles with water that has just been hoisted from the ocean depths. The 329-foot icebreaker is steaming 700 nautical miles south of the North Pole on a Norwegian Polar Institute research cruise studying climate change impacts in this gateway to the Arctic Ocean. The samples of dissolved carbon, pH, and other measurements being collected by Stedmon, a chemical oceanographer from the Technical University of Denmark, will reveal how rapidly acidification is intensifying. “Warm, fresh, and sour,” says Stedmon of the changes sweeping Arctic seas, which, along with the Southern Ocean surrounding Antarctica, are acidifying faster than any other marine waters on the planet. He and the rest of the crew of researchers from across Europe are trying to decipher how a warming Arctic is, as Stedmon puts it, “melting ice, freshening seawater, and reducing its ability to resist acidification.”

Cold polar waters, where CO2 is most soluble, have absorbed the lion’s share of CO2 emissions.

The Arctic is a bellwether for acidification, oceanographers say. Since the Industrial Age, the planet’s oceans have stored up to 30 percent of human CO2 output, with cold polar waters, in which the gas is the most soluble, absorbing the lion’s share. Those same cold waters and unique environmental conditions make the Arctic especially susceptible to the rapidly shifting ocean chemistry wrought by that excess carbon. The result: rising acidity, which eats up the minerals vital to shell-building creatures, as well as posing other dangers to Arctic marine life. Acidification is underway throughout the world’s oceans, according to a new Oceans and Cryosphere report by the Intergovernmental Panel on Climate Change. But the various ocean regions will respond differently to the same amount of carbon dioxide, says Alessandro Tagliabue, a University of Liverpool biogeochemist and one of the lead authors on the ocean changes chapter of the report. As our carbon emissions continue to soar, acidic conditions will race across the high latitudes first, according to the report. And Arctic ecosystems and people — already stressed by rising temperatures, vanishing ice, and the myriad other effects of climate change — are particularly threatened by acidification’s impacts. “The polar regions are especially vulnerable because of a systemic vulnerability that is linked to their chemical states today, which makes them very, very close to tipping over the edge into extremes of acidification,” says Tagliabue.

Oceanographer Colin Stedmon prepares water samples for acidification measurements aboard the Kronprins Haakon. Lawrence Hislop / Norwegian Polar Institute

That tipping point involves certain carbonate minerals that are essential to shell-building organisms. Carbonate ions, normally present at high, or “saturated,” concentrations in seawater, help buffer the acid produced when carbon dioxide reacts with water. But as marine carbon levels climb, more and more carbonate ions are being used up, lowering the ocean’s ability to buffer, and causing acidity — measured as a drop in pH — to rise. Those same carbonate ions are needed by creatures like starfish and clams, whose shells or skeletons are made of the calcium carbonate minerals aragonite or calcite. As the carbonate levels in seawater decrease, mollusks and other shell-building creatures find it increasingly difficult to get enough ions to build and maintain their shells. And at a sufficiently low carbonate concentration — called undersaturation — the shells begin to corrode. Models predict that large parts of the Arctic will cross this threshold as early as 2030, and researchers forecast that most Arctic waters will lack adequate aragonite for shell-building organisms by the 2080s. As the corrosive water spreads, it will spill into neighboring regions such as the North Atlantic, where it could impact the ocean food web and threaten important fisheries. Already, high levels of acidification in the cold waters of the North Pacific have caused some oyster die-offs in the U.S. Pacific Northwest. Not only do these cold waters act like a sponge for atmospheric carbon, but many areas are also being diluted with freshwater from melting ice and increasing river flows, which further reduces their buffering capacity. To make matters worse, marine life in the Arctic is particularly vulnerable to the effects of acidification because it is accustomed to consistent pH levels — unlike ecosystems, such as estuaries, where creatures have adapted to variable pH conditions. “So this is getting sprung on them,” Stedmon says, gesturing at the mushy ice rolling with the ocean swell on a blustery September day.

Melting away the ocean’s icy lid exposes yet more water to take up yet more carbon dioxide from the atmosphere.

These effects are coming on top of widespread climate upheaval in the Arctic, including excessive heat and ice-melt. Parts of Greenland saw temperatures hit 40 degrees Fahrenheit above normal in June. After a staggering July heat wave, the Greenland Ice Sheet shed 31 billion metric tons of ice in three days. This year’s runoff from Greenland is projected to add about 329 billion tons of fresh water into the surrounding seas. Meanwhile, last month Arctic summer sea ice tied for the second-lowest extent since satellite measurements began in 1979 — more than 2 million square kilometers less than normal. Melting away the ocean’s icy lid exposes yet more water to take up yet more carbon dioxide from the atmosphere . Chemical oceanographers Agneta Fransson, of the Norwegian Polar Institute, and Melissa Chierici, of Norway’s Institute of Marine Research, were among the first to start tracking the spread of acidification in the polar oceans. In 2005, they began to see low pH, aragonite-deficient water in the Canadian Arctic Archipelago. In the Fram Strait, which they’ve been monitoring since 2011, their latest results show a pH drop of as much as 0.009 points a year in the ocean’s upper 200 meters, along with falling aragonite saturation. That may not sound like much, but as Fransson points out, the pH scale is logarithmic, so even a small change can have a large impact. Depth is a key aspect of acidification, Fransson says. Aragonite naturally tends to be scarcer at lower depths. “Now organisms can escape this low saturation by migrating to the upper surface,” she says. But she and other researchers are concerned about “shoaling” of aragonite saturation, with corrosive water building up closer to the surface in some places. That will shrink the zone where shell-building organisms can find enough free carbonates.

The Kronprins Haakon moored in drift ice off the coast of northeastern Greenland in September. Lawrence Hislop / Norwegian Polar Institute

“We call it the saturation horizon,” Fransson says. “That’s going to be critical in the near future when we have more uptake of CO2, and more freshwater and more organic matter from rivers that can affect pH and CO2.” Widespread ocean acidification poses a severe threat to the planet. Global acidification of the oceans 252 million years ago, caused by massive volcanic eruptions, is believed to have obliterated marine life in Earth’s largest mass extinction. And a new study suggests that the asteroid impact that wiped out dinosaurs 66 million years ago also set off widespread acidification that devastated marine ecosystems for millennia. Although at different rates, all parts of the Arctic are being affected by acidification. Around Alaska, surface waters in the Beaufort Sea likely crossed the aragonite undersaturation threshold for at least part of the year around 2001, according to the 2018 National Climate Assessment. The Chukchi Sea is projected to follow by 2030 and the Bering Sea around 2065. “Alaska does have a really fast rate of ocean acidification,” says Jessica Cross, an oceanographer with the National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory in Seattle and lead author of a recent report on the spread of corrosive water in the Pacific Arctic. “It’s important because it has the potential to impact a lot of commercial and subsistence fisheries in the Arctic and around Alaska in general.”

Studies show that fish and other organisms could suffer neurological and physiological problems from exposure to elevated levels of CO2.

Some of the most vulnerable organisms to ocean acidification are tiny, says Cross. For instance, some species of pteropods — tiny shelled snails also known as “sea butterflies” — “fully dissolve in water that is close to the saturation threshold.” And pteropods are eaten by salmon and other fish, so impacts on their populations could reverberate up the food chain to commercial fisheries, she says. Oyster and clam aquaculture, which is a growing industry in the Gulf of Alaska, are also at risk as acidification spreads. “There’s some evidence that suggests in the future, the growing season for these organisms could be reduced by as much as 40 percent, which is a pretty substantial hit on your bottom line,” says Cross. Crabs are highly susceptible too, especially as juveniles when they start building shells. Contact with acidic water could also potentially damage cells or impair metabolism in some marine animals. “Higher organisms like fish are trying to maintain a pH balance in their cells like we do,” says Tagliabue. “There will be a cost to adapting to those conditions.” And at the levels predicted by the end of this century, studies have shown that fish and other organisms could suffer neurological, behavioral, and physiological problems from exposure to elevated levels of carbon dioxide. The Arctic seas off Siberia’s northern coast are an area of special concern, according to the Arctic Monitoring and Assessment Program. Thawing underwater permafrost is adding extra fuel to the acidification process there by injecting yet more carbon into the system.

Accumulations of floating ice in the Fram Strait off the coast of northeastern Greenland. Cheryl Katz / Yale e360

The Southern Ocean, which surrounds Antarctica, is also growing short on available carbonate, and is projected to be undersaturated by the end of this century. Nikki Lovenduski, an oceanographer at the University of Colorado-Boulder, says the Southern Ocean is particularly vulnerable to acidification because the carbonate ion concentration there is already very low. A recent modeling study she co-authored finds that shelled creatures such as Antarctic pteropods face a “fatal horizon” when corrosive water suddenly shoals and drastically shrinks their habitable zone. Although that’s not expected to happen until century’s end, when it does, it will be abrupt. “Basically, within a period of a year, it’ll go from a depth of 1,000 meters to a depth of 300 meters,” she says. “And that’s a huge, huge change for an organism with a shell.” Not all marine life will be harmed by acidification and carbonate shortage. Research suggests that some organisms, such as Antarctic krill — the bedrock of the Southern Ocean food web and an important commercial fishery used in products like fish oil supplements and animal feed — will be unaffected or able to adapt. But relatively little research has been done on high-latitude creatures, and scientists say much more needs to be learned about how the various species will respond. In the Arctic, where fisheries already provide around a tenth of the commercial catch worldwide, more southerly species like Atlantic cod are moving in as waters warm. But models suggest that as acidification progresses across the high latitudes, fish populations will decline, impacting the global food supply and threatening subsistence fishing by the Arctic’s indigenous people.