While those across the marine sector recover from the blow delivered this week by the breakdown in negotiations to address ocean plastic pollution, corners of the scientific community suggest that the subject at the top of the world’s agenda right now ought instead to be the worsening threat of ocean acidification.

Once thought to be the concerns of surface level layers of the ocean, headlines over the past week indicate that ocean acidification is now sinking into marine regions as deep as 1,500 metres – posing new threats to the marine life and organisms that dwell there, including sea butterflies, sea snails, and cold-water corals.

It’s long been established that the uptake of carbon dioxide impacts the acidity of the ocean’s surface. Earlier this year, scientists called upon world leaders gathered at COP16 this October to turn greater attention to the growing concerns over ocean acidification by addressing the global carbon emissions crisis and the impacts of climate change upon biodiversity and the environment.

The ocean is, after all, the largest natural sink of carbon dioxide, responsible for the absorption of around a quarter of the world’s annual emissions. As those emissions increase, so too does the impact of ocean acidification.

And one such impact, new studies have revealed, is that it is reaching deeper waters and impacting ecosystems in a way that few have fully understood, until now.

It was the work of Jens Daniel Müller and his colleague, Nicolas Grube at the Federal Institute of Technology Zurich in Switzerland that brought this to world’s attention this week, having developed a 3D reconstruction of how carbon dioxide moves through the ocean, based on global measurements of currents and other circulation patterns. By using this model to estimate how the carbon dioxide the oceans have absorbed since 1800, the team of researchers have been able to illustrate just how greenhouse gas emissions have affected deep-water acidity since the start of the industrial revolution.

What was found was there is a clear indication of acidification down to depths as far as 1,000 metres below the surface across most of the ocean. This went deeper still in some areas, such as the North Atlantic – where the Atlantic Meridional Overturning Current (AMOC) routinely carries carbon from the surface to deeper waters – where acidification has reached depths of 1,500 metres beneath the surface.

In the research paper (published recently in Science Advances under the title ‘Progression of Ocean Interior Acidification over the Industrial Era’) Müller and Grube position that ocean acidification driven by the uptake of human-caused C02 represents “a major threat to ocean ecosystems” while drawing attention to the matter that, even now, “little is known about its progression beneath the surface”.

Ocean acidification comprises multiple changes in the marine carbon dioxide system. At its most simplistic, acidification occurs when a series of chemical reactions drop the pH levels of the ocean, making it more acidic. This poses a serious threat to marine life whose survival depends on the chemical stability of their ecosystems.

“Carbon is an important ingredient to the ocean’s ecosystem; it’s the mineral that organisms use to build their shells; it’s heavily relied upon by all sorts of organisms from corals to crabs and lobsters, to mollusks and oysters,” said director of science at Plymouth Marine Laboratory, Professor Steve Widdicombe – a veteran of ocean acidification research unconnected with the latest study.

“The pH of the ocean is also crucially important, because it can influence the physiology of cells. We have measured the impact of ocean acidification in the laboratory for the past 20 years, and we’ve seen case after case of its negative impact on growing organisms.”

Coral reefs are among those organisms at particular risk. The acidic waters hinder their skeleton-building abilities, resulting in coral bleaching and eventual death. This then creates a domino effect on marine systems dependent on the ecosystems healthy coral reefs provide.

“When it comes down to it, ocean acidification is a way bigger problem than plastics right now,” Widdicombe told Oceanographic Magazine. “Because the changing pH of the ocean is invisible. Couple ocean acidification with the warming temperatures of our seas and the rapid loss of oxygen and you’ve got a way bigger problem than plastics.

“If we could see ocean acidification, we’d be way more scared.”

To offer a clearer picture of the spread of ocean acidification, Müller and Grube developed an ocean model simulating the effects of rising atmospheric CO2 levels based on historical data spanning over two centuries, with CO2 estimates for the years 1800, 1994, 2004, and 2014. 

Starting with a standard ocean model that simulates water movement and chemistry, the pair then added data points on CO2 levels and acidification indicators like protons concentrations, pH levels, and aragonite saturation states, enabling them to map acidification trends.

What they found is that acidification is moving deeper into the ocean, with the average depth impacted by acidification measuring around 1,000 metres by 2014. In regions influences by the Atlantic Meridional Overturning Current, acidification reached depths of up to 1,500 metres.

At these depths and ocean acidification becomes an even greater concern as the ‘ocean interior’ is home to many organisms – such as pteropods (sea slugs and mollusks) or the other highly diverse organisms inhabiting the sea floor, such as cold water corals – much more sensitive to changes in the pH balance of their ecosystems. 

“It’s been speculated that these organisms might be more sensitive to ocean acidification than those living at the surface,” state Müller and Grube.

It’s notable that the most severe worsening of ocean acidification has taken place over the course of the last two decades, accelerating at a pace that both Müller and Grube refer to simply as “remarkable”.

“Relative to the change that had occurred until 1994, the acidification progressed by around 50% between 1994 and 2014. This is primarily due to the near exponential growth of atmospheric carbon dioxide, pushing an exponentially growing amount of carbon into the ocean, when then leads to an exponential growth of the accumulated carbon at depth,” they said.

The publication of Müller and Grube’s research follows calls from the scientific community made earlier this year upon world leaders to address the “critical threat” that ocean acidification poses by investing in research and developing national ocean acidification plans before the ticking clock on a phenomenon “already unraveling marine life, food chains, and entire economies” runs out.

“As our oceans acidify, fisheries are projected to see declines in key species, threatening food security for millions of people globally,” said Professor Widdicombe. “Coastal economies that depend on marine biodiversity will suffer significant losses. These aren’t hypothetical scenarios.”

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