Southern Ocean acidification could double by the end of the century | Polarjournal
Like all the world’s oceans and seas, the Southern Ocean is becoming more acidic. The cause is CO2 emissions linked to human activities. Acidification could, however, reach dramatic proportions by the end of the century. Image: Heiner Kubny

By 2100, the acidity of Antarctic coastal waters could double. Mainly due to CO2 emissions, this increase in acidity is a threat to ecosystems and animal life in the Southern Ocean.

A team of researchers from the University of Colorado Boulder has been studying changes in the pH of the Southern Ocean, and their findings are hardly encouraging. The scientists looked at five existing (Ross Sea and South Orkney Islands) and proposed (Weddell Sea, East Antarctica and Antarctic Peninsula) marine protected areas in the coastal waters of the Southern Ocean. And their findings are unequivocal: without a drastic reduction in CO2 emissions as quickly as possible, the acidity of the Southern Ocean could increase by up to 100% by 2100 in the upper 200 meters of the ocean, with serious repercussions for marine fauna and ecosystems.

However, acidification may not be confined to the upper layers. According to the authors of this study, published on January 4, the entire water column of the coastal Southern Ocean, including the seabed, could undergo severe acidification.

The reason for this increase in ocean acidity lies in the role the oceans play in absorbing CO2. Almost 30% of CO2 is absorbed by the world’s oceans. As cold water absorbs more CO2 than warm or temperate water, the Southern Ocean absorbs even more CO2.

From krill and Antarctic toothfish to bivalves and sea urchins, many organisms are directly threatened by the acidification of the Southern Ocean’s coastal waters. The colored map shows marine protected areas. Map: Nissen et al.

By absorbing CO2, ocean water becomes more acidic as the carbon dioxide dissolves. The more CO2 released by human activities, the more the oceans will absorb, maintaining and increasing the acidification process. Hence, for the authors of the study, the urgency of drastically and rapidly reducing our greenhouse gas emissions.

But how could acidification of the Southern Ocean threaten ecosystems and animals?

As water becomes more acidic, its pH decreases. Marine organisms and animals are adapted to live in an environment with, among other things, a slightly alkaline pH. In fact, before the industrial era, the average pH of the oceans was around 8.29. Today, it stands at 8.1. According to the authors of the study, the pH for the Southern Ocean could fall by 0.36 by 2100 in the upper layers (200 meters).

At the base of the food chain. Phytoplankton is made up of cyanobacteria and microalgae. It also plays an essential role in producing the oxygen needed for life in water, while supplying 2/3 of the oxygen found in air. Image: NOAA MESA Project

Phytoplankton grows more slowly and tends to disappear with acidification. More acidic water also has a negative impact on shellfish, dissolving the calcium carbonate that forms their shells.

Phytoplankton forms the basis of the marine food chain, and shellfish such as sea snails and sea urchins are a crucial food reserve for many animal species. Their rarefaction or disappearance could seriously disrupt the top of the chain, where species such as whales and penguins are found.

Using a computer model simulating how seawater in the Southern Ocean would change over the 21st century, the team found that it would become more acidic by 2100.

To avoid this scenario, the authors believe that the only way is to drastically and rapidly reduce CO2 emissions and increase the number of marine protected areas. “Given the cumulative threat to marine ecosystems by environmental change and activities such as fishing, our findings call for strong emission-mitigation efforts and further management strategies to reduce pressures on ecosystems, such as the continuation and expansion of Antarctic MPAs.”, say the authors. The highest greenhouse gas reduction scenario (or preferably the lowest emissions scenario) will be preferred. If not, and if these measures come too late or are too small, the Southern Ocean will become seriously acidified.

Mirjana Binggeli, PolarJournal

Link to the study: Nissen, C., Lovenduski, N.S., Brooks, C.M. et al. Severe 21st-century ocean acidification in Antarctic Marine Protected Areas. Nat Commun 15, 259 (2024).

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