Melting sea ice leads to acidic Arctic Ocean | Polarjournal
The decline in sea ice in the Arctic Ocean is no fiction anymore, despite annual fluctuations. This leads to larger areas of water that are in exchange with the atmosphere. This again puts more CO2 into the water and changes the pH value in the direction of “acidic”. Image: Dr Michael Wenger

Melting sea ice in the Arctic Ocean has serious consequences for for wildlife and algae that live on and under the ice. Higher water temperatures, because the dark surface absorbs more heat, and a change in salt concentrations, because more fresh water is floating in the Arctic Ocean, are two of these consequences. Now an international research group has discovered that there is also a link between dwindling sea ice and the pH of seawater.

The more sea ice melts, the larger the areas of water that are in exchange with the atmosphere and thus more carbon dioxide can be absorbed by the Arctic Ocean. This in turn leads to a faster acidification of the water due to a decreasing pH value. Since 1994, the value has decreased around three to four times faster than in other regions of the world. This is the result of the work of a 20-member international research group after analyzing and modeling data from 26 years of measurements in the western part of the Arctic between the Chukchi Sea and the northeastern Canadian Basin. The study was published recently in the journal Science.

Just like at home, when you pump carbon dioxide into your water, the amount of carbon dioxide in the ocean increases, lowering the pH. The colder the water, the more carbon dioxide you can pump in. Thus, while the oceans have buffered the global increase in carbon dioxide in the atmosphere, they have become more acidic as a result. Image: EvaK via Wikicommons CC BY-SA 2.5

The problem of ocean acidification due to increasing amounts of carbon dioxide from the atmosphere has been known for some time. Think of it like producing sparkling water at home: Carbon dioxide is added to still water, increasing the carbonic acid in the water and thus lowering the pH value. If we now transfer this to the world’s oceans, we get the same picture: increasing amounts of carbonic acid, decreasing pH. Because this is also temperature-dependent (the colder the water, the greater its gas solubility and thus a higher carbon dioxide uptake), polar waters would be predestined for acidification. But the sea ice cover has so far protected the underlying Arctic Ocean from excessive acidification.

However, when the research team examined data from the past 26 years, they discovered a three- to four-fold acceleration in Arctic Ocean acidification and a correlation to Arctic ocean ice cover. The team believes the reasons for the increased acceleration are the larger water surface area, which leads to more carbon dioxide in the water; the simultaneous dilution of the water by the melted ice, which leads to less conversion of carbonic acid to bicarbonate (a natural process that depends on water chemistry); and the reduced mixing of surface water with deeper layers, which would buffer the acidification effect.

The consequences for organisms living in the upper water layers have not yet been fully explored. It is known that a lower pH results in shell-forming organisms such as algae, crabs, mussels and snails being restricted in building their shells on the one hand because the acid dissolves the shell and at the same time less calcium carbonate is available for building up a shell at all. Furthermore, many important physiological processes are pH-dependent, for example for healthy development. Even a small change in the value towards acidification can thus cause large deficits. As organisms are at the base of food webs, higher levels such as fish, seals, and then polar bears and humans are affected. Co-author and acidification expert Wei-Jun Cai of the University of Delaware puts it succinctly: “We believe that by 2050, all the ice in summer will be gone. And if we continue the current trend for another 20 years, summer acidification will be very, very severe. How will that affect the biology there?” This is a question that research teams will now need to get to the bottom of quickly.

Dr Michael Wenger, PolarJournal

Link to the study: Qi et al. (2022) Science 377 1544 – 1550 Climate change drives rapid decadal acidification in the Arctic Ocean from 1994 to 2020; DOI: 10.1126/science.abo038

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