Arctic waters are becoming increasingly acidic

Imagine for a moment that you are standing on a pier by the sea holding a bowling ball for some reason. Suddenly you lose your grip and the ball falls into the waves below you. Now imagine the bowling ball is made of carbon dioxide (CO₂): broadly speaking, this would be your share of man-made emissions that are absorbed by the sea every dayto which must then be added those of the other 8 billion people living on the planet.

The reason why so much CO₂ ends up in the oceans is that carbon dioxide is a molecule extremely hydrophilic: that is, it loves to react with water, much more than other atmospheric gases, such as oxygen for example. The first product of this reaction is a compound called carbonic acid, which gives up its hydrogen ion in a short time. In a solution, increasing hydrogen ions also corresponds to greater acidity, and this is why with the growth of the level of CO₂ in the earth’s atmosphere, the planet’s waters have also become more acidic. By the end of the century, models predict that the oceans will reach a level of acidity never recorded for millions of years. Previous periods of acidification and warming have been linked to the extermination of some aquatic species and caused the extinction of others. Scientists believe the current cycle of acidification is happening much faster.

Worrying signs

This change it is hitting more northerly waters more strongly and rapidly of the planetwhere the effects of acidification are already acute, explains Nina Bednaršek, researcher at the National Institute of Biology of Slovenia. Bednaršek studies pteropods, tiny marine molluscs also known as “sea butterflies” for their translucent, glittering shells. Carefully analyzing some specimens from Arctic waters, their exoskeletons reveals an alarming aspect. In the water that has become more corrosive the shells of the once immaculate pteropods, they flake and show cracks, signs that portend an early death. These creatures are like “the canary in the coal mine”Bednaršek points out: they are a key part of the food chain that supports larger fish, crabs and mammals, and a signal of imminent suffering for other speciesas the oceans become more acidic.

The frigid waters of the Arctic are a special case for several reasons, he says Wei Jun Cai, oceanographer at the University of Delaware. The first is that the ice is melting. Typically the ice sheets act as a lid on the underlying water, preventing the exchange of gases between the atmosphere and the ocean. If the ice is gone, water absorbs the extra carbon dioxide in the air. Also, melt water dilutes compounds that might neutralize the acid, then build up without mixing with the deeper water below. The result is a very acidic pool of water that stays close to the surface.

Surprising acceleration

In a studio recently published in the magazine Science, Cai’s team looked at data from maritime missions to the Arctic between 1994 and 2020 and concluded that acidification was occurring three to four times faster than other ocean basins. “We knew the acidification would be quick. But we didn’t know how much”Cai says. The culprit, the scientists speculate, is the rapid decline in summer ice extent over the time span under study. Between 1979 and 2021, the ice at the end of the summer is reduced by an average of 13 per cent every decade (the scenarios projected to 2050 are even more worrying, ed.).

It is difficult, however, to give specific numbers on acidification rates across the entire Arctic seascape. In some places, the water is shallow and mixes heavily with melt water and fresh water from the surrounding continents. In other places it is deeper and is currently blocked by ice all year round. Ideally, researchers prefer to have the complete picture: continuous data from year to year, covering a large territory and several seasons, and recording the sometimes decadal movement of ocean currents. The time factor is also very important in the short term, since local conditions can change drastically from week to week depending on factors such as phytoplankton activity, which may flower briefly in a certain area during the summer period and suddenly suck up some of the extra carbon dioxide. But it’s hard to get in this area of ​​the world. Scientists who study acidification, like Cai, have a limited observatory, which in his case is based on summer missions in a relatively small portion of the sea, mostly still blocked by ice.