Damage to coral reefs, loss of habitats, and a threat to survival for shell-building marine creatures are among the impacts already being felt across the ocean from a concerning process called ocean acidification. 

Until now, ocean acidification has not been deemed to have crossed its ‘planetary boundary’, but a major new study led by the UK’s Plymouth Marine Laboratory and the US-based National Oceanic and Atmospheric Administration has found this safety limit was reached five years ago.

The findings, published in ‘Ocean Acidification: Another Planetary Boundary Crossed’ in the journal Global Change Biology earlier this week – which coincides with the United Nations Ocean Conference in Nice, France this week – highlights that the threat to marine ecosystems worldwide is far more extensive than previously understood.

Using the latest physical and chemical measurements in the upper ocean, the research team concluded that by 2020 the average ocean condition worldwide was already very close to – and in some regions beyond – the ‘danger zone’ for ocean acidification.

Exploring deeper into the ocean – down to around 200 metres below the surface – it was discovered that around 60% of these deeper waters had crossed the boundary, compared to 40% of the water at the surface. This increase has major implications for the survival of many sea creatures, especially those that build shells or skeletons from calcium carbonate.

Selected tropical and subtropical coral reefs have lost 43% of their suitable habitats, sea butterflies in polar regions have lost up to 61% of their habitat, and coastal shellfish species have lost 13% of their global coastline habitats in which they can sustain their essential biological processes.

Based on these findings, scientists have recommended the revision of the previous safety limit – establishing that even a 10% change from pre-industrial levels would be harmful to ocean ecosystems. 

The entire ocean surface has already surpassed this stricter limit by around the year 2000.

“Looking across different areas of the world, the polar regions show the biggest changes in ocean acidification at the surface,” said Professor Helen Findlay from Plymouth Marine Laboratory. “Meanwhile, in deeper waters, the largest changes are happening in areas just outside the poles and in the upwelling regions along the west coast of North America and near the equator.”

Most ocean life doesn’t just live at the surface level and the waters below are home to many more different types of plants and animals. Since these waters are changing so much, the impacts of ocean acidification, the paper has stated, could be far worse than we first thought.

“This has huge implications for important underwater ecosystems like tropical and even deep-sea coral reefs that provide essential habitats and nursing refuge for many species, in addition to the impacts being felt on bottom-dwelling creatures like crabs, sea stars, and other shellfish such as mussels and oysters,” said Professor Findlay.

Ocean acidification also reduces the availability of calcium carbonate, a crucial building block that many marine organisms need to form shells and skeletons. As pH levels drop, calcifying species such as corals, oysters, mussels, and tiny sea butterflies struggle to maintain their protective structures, leading to weaker shells, slower growth, reduced reproduction, and decreased survival rates.

“Ocean acidification isn’t just an environmental crisis – it’s a ticking time bomb for marine ecosystems and coastal economies,” said Professor Steve Widdicombe, co-chair of the Global Ocean Acidification Observing Network and the co-focal point for the UN’s Sustainable Development Goal 14 Target 3 – aiming to minimise and address the impacts of ocean acidification.

“As our seas increase in acidity, we’re witnessing the loss of critical habitats that countless marine species depend on and this, in turn, has major societal and economic implications.

“From the coral reefs that support tourism to the shellfish industries that sustain coastal communities, we’re gambling with both biodiversity and billions in economic value every day that action is delayed.”

The study suggests that conservation measures should focus on the regions and species most vulnerable to acidification. The authors also stress the importance of protection or suitable management measures for those areas least compromised to ensure their longevity.

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