Melting sea ice is acidifying the Arctic Ocean

A warmer climate has caused significant amounts of sea ice in the Arctic Ocean to melt over the past few decades. Without an ice barrier between seawater and air, the ocean can rapidly absorb CO2 from the atmosphere. As WEI-JUN CAI and ZHANGXIAN OUYANG explain, this process lowers the pH of the ocean’s surface water—and it is unfolding three to four times more quickly in the Arctic Ocean than in other oceans, threatening all forms of life.

Once perennially ice-covered, increasingly large areas of the Arctic Ocean are now ice-free for longer periods. With the loss of its “ice cap,” the ocean’s fresh and carbon-deficient surface water is exposed to increasing atmospheric CO2, which it absorbs.

You might think this process could mitigate climate change. It does—but it also reduces both the pH level of the water and the saturation state of aragonite (a carbonate mineral) in the seawater, causing acidification.

Aragonite is produced by tropical and cold-water corals, pteropods and some molluscs. An ocean’s aragonite saturation state is a measure of carbonate ion concentration, so it is commonly used to track ocean acidification. When the saturation state falls below 3, these organisms become stressed. Below 1, shells and other aragonite structures begin to dissolve.

For years, we were not able to quantify how quickly seawater pH and aragonite would decline in the Arctic Ocean because of the scarcity of observations in this remote area. But now we have some clues.

When we first saw the preliminary results derived from a compiled dataset from 47 Arctic research cruises from 1994 to 2020, we were stunned: the data revealed that seawater pH can decrease at the swift rate of about 0.1 unit per decade in the ice-free Central Arctic Ocean—a rate much faster than in other oceans. It took more than 100 years for the pH in the Atlantic, Pacific and Indian oceans to drop by 0.1 unit. How is it possible that this could happen in the Arctic Ocean within just two and a half decades? What caused it? Could we trust our results?

Three connected processes

With rigorous analysis, we validated the rapid rates of decline in seawater pH and aragonite saturation state in the Arctic Ocean. We found that the declines were significantly associated with the decrease in sea ice extent.

Simply speaking, there are three processes behind how melting sea ice controls and accelerates acidification in the Arctic Ocean.

First, after the air-sea barrier disappears, the carbon-deficient water under the sea ice is exposed to the atmosphere and can rapidly take up CO2.

Second, the relatively lighter meltwater forms a stable layer at the top of the ocean that does not easily mix into the deeper waters. As a result, the CO2 absorbed from the atmosphere accumulates and concentrates in the surface water.

And last but not least, the comparatively fresh (less salty) sea ice meltwater changes the chemistry of the seawater by diluting the carbonate ion concentration. A lower carbonate ion concentration indicates a lower buffer capacity for neutralizing CO2 molecules. This is why pH decreases more quickly in sea ice water that has been diluted by meltwater than it does in normal seawater.

A threat to food webs, biodiversity and food security

Given expectations that both temperatures and sea-ice loss will continue to increase, the lower pH and decreased aragonite saturation state in the summer Arctic Ocean may cause more severe acidification. This would be bad news for Arctic sea life, food webs, ecosystems and Indigenous Peoples.

For example, researchers know that sea butterflies [1] are already struggling in acidifying polar oceans. A type of mollusc that is a “canary in the coal mine” for ocean acidification, a sea butterfly has an aragonite shell (which forms from calcium and carbonate ions) that will start to dissolve in corrosive waters (those with low pH and aragonite saturation state under 1).

Because sea butterflies are a key species in Arctic food webs and an important food source for fish and whales, their impairment may cascade throughout marine food webs, impact the diversity of species, sabotage marine ecosystems, restructure relationships between predator and prey in the ocean, and threaten Indigenous People’s food security.

Unfortunately, we are far from having a complete understanding of all the consequences that are likely to be induced by the rapid declines in pH and aragonite saturation state for the Arctic ecosystem. A recent model study predicted that the synergetic effect of earlier seasonal ice melting and amplified atmospheric warming in the summer will accelerate summer ocean acidification by the end of the century, not only in the Central Arctic Ocean but also in the shelf seas, which are key harvesting areas for Arctic fisheries.

Does all life in the Arctic Ocean have time to adapt to such rapid increases in acidic conditions? Sadly, we suspect that many shell-building organisms do not.