The Earth has exceeded the seventh of nine "planetary boundaries", so defined because they are capable of threatening its overall equilibrium and, consequently, its habitability. Following climate change, biosphere integrity, land exploitation, alteration of biochemical cycles (nitrogen and phosphorus), the use (and pollution) of freshwater and the dispersal of new chemical pollutants, it is now the turn of ocean acidification.

Acidity up 30–40 per cent at ocean surfaces

The finding was revealed on 24 September by the new Planetary Health Check report, published by the Potsdam Institute, which monitors the status of the nine planetary boundaries defined in 2009 by researchers at the Stockholm Resilience Centre, in Sweden. The document explains that the pH of the ocean surface has decreased by approximately 0.1 units compared to the pre-industrial period, roughly the mid-nineteenth century. This equates to an increase in acidity of 30–40 per cent.

This is a damaging phenomenon for a whole range of ecosystems, starting with the most vulnerable, such as coral reefs, whose loss would entail the disappearance of an enormous amount of biodiversity they harbour. But under threat are also the flora and fauna found, for example, in the Arctic region.

Pteropods, the "sentinels" of ocean acidification

Some animal species are already showing signs of ocean acidification: this is the case of pteropods, molluscs equipped with a shell, which play a particularly important role in the carbon dioxide cycle at sea. It is precisely their shell that displays the characteristic signs of deterioration, brought about by the change in pH. Pteropods represent a fundamental food source for numerous species: consequently, their decline could disrupt the entire food chain, with impacts on fisheries and on human beings as well.

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World's oceans fail key health check as acidity crosses critical threshold for marine life.

Scientists call for renewed global effort to curb fossil fuels as seven of nine planetary boundaries now transgressed.
theguardian.com/environment/20…

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But there is more: ocean acidification will also trigger a vicious cycle with regard to climate warming. It is the world's oceans, after all, that absorb a large share of the excess CO2 released into the Earth's atmosphere as a result of human activities. And the more acidic the waters become, the less capable they are of performing this function. "For billions of years, it has been the oceans that have guaranteed the Earth's climate stability. This is why it represents a red alert for our Planet," explained oceanographer Sylvia Earle.

"The system that stabilises the Earth is under ever greater pressure"

"The dynamic we are observing is really moving in the wrong direction. Acidification, falling oxygen levels and the multiplication of marine heatwaves are increasing the pressure on a system that is fundamental to stabilising the Earth's conditions," echoed Levke Caesar, scientist at the Potsdam Institute and one of the report's lead authors. This is why inaction on the part of governments around the world amounts to a deliberate choice: responsibility for the future of the Planet depends on the decisions made today.