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Acid test: Are the world’s oceans becoming too ‘acidic’ to support life?

A hogfish in the coral reefs

A hogfish in the coral reefs of Cuba. Image by abrice Dudenhofer / Ocean Image Bank.

  • The world’s oceans absorb about a quarter of humanity’s carbon dioxide emissions, buffering us against higher atmospheric CO2 levels and greater climate change. But that absorption has led to a lowering of seawater pH and the acidification of the oceans.
  • The process of ocean acidification is recognized as a leading threat to ocean life due to its impairment of calcifying organisms and other marine species. The full impacts of acidification are unknown, but at some point reduced pH could be disastrous biologically.
  • Researchers have designated ocean acidification as one of nine planetary boundaries whose limits, if transgressed, could threaten civilization and life as we know it. But there is debate as to whether there is a global boundary for this process, since acidification impacts some regions and species more or less than others, making it hard to quantify.
  • Scientists agree that the primary solution to ocean acidification is the lowering of carbon emissions, though some researchers are investigating other solutions, such as depositing alkaline rock minerals into oceans to lower the pH of seawater.

Hours after being born, oysters are already working to form their protective, chalk-layered shells. Drawing calcium and carbonate from seawater, they combine the two to form hardened shells.

But as humans have pumped voluminous sums of carbon dioxide into the atmosphere, this ancient process has come under threat. It’s estimated that the global ocean absorbs around 30% of human carbon emissions. While this carbon sequestration creates a powerful buffer against climate change by reducing the amount of CO2 flowing into the atmosphere, it changes the chemistry of seawater, decreasing the pH and causing seawater to become more acidic.

Besides affecting oysters, ocean acidification can dissolve the aragonite (a form of calcium carbonate) shells of pteropods, tiny marine snails that swim through the water column, and which whales, seabirds and fish rely on as a food source. Acidification slows corals’ ability to grow their skeletons. Marine animals like sea urchins find it more difficult to reproduce. New research has also shown that ocean acidification can exacerbate other issues, including the impacts of marine heat waves, compounding stress on an already stressed-out ocean.

Ocean acidification is considered to have such wide-ranging global impacts that scientists have designated it as one of nine planetary boundaries responsible for regulating and maintaining Earth’s functionality. Each of these nine boundaries refers to a biophysical subsystem or process that has a clear limit to which it can withstand anthropogenic changes. The theory, which was first introduced in a 2009 paper and updated in a 2015 paper, suggests that Earth can function properly if humanity remains within the “safe operating limits” of these boundaries. But once a certain threshold is crossed for one or more of these boundaries, the concept suggests that Earth will move into a new and dangerous state — one that is far less supportive of biological life.

Oysters draw calcium and carbonate from seawater and combine the two to form hardened shells. But due to the increased CO2 in the atmosphere, this ancient process has come under threat. Image by Nanditha Chandraprakash/Mongabay.

The other boundaries include climate change, biosphere integrity, ozone depletion, atmospheric aerosol pollution, the water cycle, biogeochemical flows of nitrogen and phosphorus, land-system change, and release of novel chemicals.

Pushing oceans to the brink

Katherine Richardson, a professor in biological oceanography at the University of Copenhagen and co-author of the planetary boundary studies, says the ocean has played a key role in the global carbon cycle throughout geological time, and that jeopardizing the abilities of calcifying organisms to build shells and skeletons could potentially have a “tremendous effect on the global climate.” This is because calcium carbonate stores carbon; but once it’s no longer able to do so, the atmospheric concentration of CO2 can increase, she says.

“In principle, if you push ocean acidification far enough, so that you don’t have these calcifying organisms, it could have a feedback on the performance of the Earth system,” Richardson told Mongabay.

While the theory of planetary boundaries asserts that some Earth processes have an overall global threshold beyond which it’s not safe to transgress, many researchers view ocean acidification as possessing numerous thresholds, depending on the marine species, ocean ecosystem, or region involved.

Or as ocean acidification expert Ken Caldeira puts it, ocean acidification isn’t one looming cliff-edge catastrophe, but rather a process of “progressive deterioration.”

A pteropod shell is shown to dissolve over time in seawater with a lower pH. When carbon dioxide is absorbed by the ocean from the atmosphere, the chemistry of the seawater changes. Image by NOAA.

‘A huge experiment with our Earth’

In the 1990s, marine biologist Ulf Riebesell began a research project looking at the effects of rising carbon dioxide on marine microalgae. He hypothesized that increased carbon dioxide would improve the process of photosynthesis and help the microalgae grow. But when it came to calcifying algae, he noticed that CO2 had the opposite effect, impeding the calcification process essential to its survival.

Riebesell wasn’t the only scientist to discover the damaging impacts of excessive CO2 on the marine environment. At a 2004 symposium in Paris, Riebesell found himself in the company of many other scientists who’d also conducted studies on this newly uncovered phenomenon.

“That’s when it all started,” Riebesell of the GEOMAR Helmholtz Centre for Ocean Research Kiel, and coordinator of the German research network on ocean acidification BIOACID, told Mongabay. “That’s when all of the sudden our community became aware [that this pH destabilization is] ongoing and potentially harming the ocean. That’s also when the term ‘ocean acidification’ was born.”

Since then, research on ocean acidification has proliferated. While scientists have generally found that increased CO2 has a negative impact on marine life, they also discovered animals and plants responding to decreasing pH in surprising ways. For instance, marine zoologist John Spicer of Plymouth University co-authored a study that found that ophiuroid brittlestars (Amphiura filiformis) actually grow stronger calcium skeletons when exposed to highly acidic waters — but that seeming benefit came at a substantial cost to the organism in the form of muscle wastage.

A study that found that ophiuroid brittlestars (Amphiura filiformis) grow stronger calcium skeletons when exposed to highly acidic waters — but that came at a cost in the form of muscle wastage. Image by NOAA Ocean Exploration via Flickr (CC BY-SA 2.0).

“What happens to human populations when the cost of living goes up? Everything gets harder,” Spicer told Mongabay. “That’s what will happen to life in the oceans. Because in some ways, the biggest threat … that ocean acidification poses is that more energy has to be put into defending yourself, to maintaining yourself.”

While the majority of studies on the subject have looked at the effects of ocean acidification on individual species, scientists are also trying to understand its potential impacts on entire ecosystems. To do this, many researchers have turned to naturally occurring CO2 venting sites, where large amounts of volcanic CO2 are flushed into the ocean from cracks in the seafloor. While calcifying organisms tend to be scarce in these regions, these ocean-bottom venting sites are not always devoid of life.

“You get a very different community,” Spicer said. “Some of the algal life, the stuff that uses photosynthesis that uses CO2, it’s actually not doing too badly … and there are some animals with calcium carbonate skeletons that can exist there, but they’re very good at shifting ions around inside their bodies.”

But for organisms unaccustomed to living in seawater with high levels of CO2, an acidifying ocean can be detrimental to survival. Ocean acidification is already exacerbating stress on coral reefs around the Indian Ocean and Asia, one study found. Other research shows that increasingly acidic waters reduce the ability of Antarctic phytoplankton to build strong cell walls, which makes them smaller and less able to store carbon, compounding the effects of climate change.

For organisms unaccustomed to living in seawater with high levels of CO2, an acidifying ocean can be detrimental to survival. Image by Milos Prelevic via Unsplash (Public domain).

And things are set to get much worse. Since the industrial revolution began, it’s estimated that global oceanic pH has fallen from about 8.16 to about 8.07. If nations don’t curb carbon emissions, and we continue along a “business-as-usual” path, it’s believed that global average ocean pH levels will drop to roughly 7.67 by 2100, which is about five times the amount of acidification that has already occurred — a level of drastic change that the planet has not experienced for the past 21 million years. The full ramifications of such a drop are unknown.

Of course, acidification is not the only serious risk the world’s oceans face. Humanity is unleashing a tidal surge of other marine threats, ranging from rising sea temperatures to deoxygenation, from overfishing to microplastic pollution. This plethora of interconnected and interacting threats greatly complicates the planetary boundary theory’s capacity to forecast future outcomes. But these multiple threats are sounding warning bells.

In May 2022, the U.N.’s World Meteorological Organization (WMO) released a report on the state of the global climate as of 2021; it found that four key climate change indicators were at all-time highs: greenhouse gas emissions, sea level rise, ocean heating, and ocean acidification.

“All of the things we do, not separately, but together determine whether that ocean will give us all the [life-supporting] stuff that it’s given us in the past, from food to oxygen, to everything else,” Spicer said. “Ocean acidification is a sentinel, if you like — it’s something which has caught people’s imagination [and] it points towards the fact that we are having a huge experiment with our Earth.”

Ocean acidification is already exacerbating stress on coral reefs around the Indian Ocean and Asia. Image by Umeed Mistry / Ocean Image Bank.

Ocean acidification: Is there a critical threshold?

Some scientists continue trying to pinpoint the critical threshold for ocean acidification — when pH sinks so low it threatens the ocean’s ability to function, putting the entire Earth operating system at risk.

The theory of planetary boundaries argues that this will happen when the saturation rate of aragonite — a key form of calcium carbonate that many organisms use in their calcification processes — lowers to the point where it dissolves. More specifically, the theory says that humanity will enter a “zone of uncertainty” when aragonite saturation levels are over 80% of what they were in preindustrial times.

The theory also suggests that the global boundary for ocean acidification has not yet been breached. However, Richardson warns that this threshold is already being crossed at regional levels.

“I prefer to think of it like blood pressure,” she said. “If your blood pressure is over 120 over 80, it’s no guarantee you’re going to have a heart attack. But it does increase the risk, which is why we bring it down. That’s really what we need [to understand] in terms of planetary boundaries and the Earth system.” We need to identify “where we have a pressure point, where the risks are becoming greater and greater, [until they reach a] state that might not be as conducive to human activities as the state we have today,” she said.

Research published in 2015 suggests that we’ve already transgressed four boundaries: climate change, biosphere integrity, land-system change, and the altered biogeochemical cycles of phosphorus and nitrogen. More recently, scientists have indicated that we’ve also entered the danger zone for two other planetary boundaries: the release of novel entities, including plastic pollution, and freshwater destabilization.

Ken Caldeira, an emeritus senior scientist at the Carnegie Institution for Science’s Department of Global Ecology who has focused much of his work on ocean acidification, questions the appropriateness of the planetary boundary framework in that it seems to suggest that “things are OK” up until we cross a certain threshold. Instead, he says, ocean acidification should be viewed as “progressive deterioration.”

“It can be socially advantageous to develop some guardrails to keep us from going too far down these paths, but most of these guardrails are socio-political constructs with some scientific motivation, but the specific values that are used for the guardrails are not strongly supported by science,” Caldeira told Mongabay.

Mangroves and coral in Papua New Guinea. When pH sinks too low it threatens the ocean’s ability to function. Image by Matt Curnock / Ocean Image Bank.

Riebesell said the planetary boundary framework is “justified” in counting ocean acidification as a planetary boundary because of its far-reaching impacts, but agrees that it’s hard to set a global threshold since acidification happens differently in each ecosystem. He notes, for instance that ocean acidification is currently happening more intensely in the Arctic Ocean.

“The Arctic Ocean has already gone into being corrosive for calcium carbonate,” Riebesell said. “So you could call that boundary being crossed because all calcifiers [there] will find it hard to survive under those conditions.”

Kim Currie, a marine chemist who manages the New Zealand Ocean Acidification Observing Network (NZOA-ON), says there’s also a lot of variance between acidification in the open ocean and coastal regions.

“In the open ocean, it’s more or less the same rate of increase,” Currie told Mongabay. “But in coastal areas, that rate varies hugely, because there’s other things that are affecting the acidity [there].” For example, the presence of kelp forests or proximity of agricultural runoff can determine how acidic ocean coasts are, she said.

Currie added that while there may not be a single tipping point for ocean acidification, there will be multiple tipping points for different organisms. “There’s going to be some animals that are more resilient than others,” she said. “There are animals that can cope with pH changes a little bit better than other animals. So you’ll get a bit of natural selection.” Knowing precisely how these various animal responses will impact biodiversity and individual ecosystems is unknown.

The nine planetary boundaries, counterclockwise from top: climate change, biosphere integrity (functional and genetic), land-system change, freshwater change, biogeochemical flows (nitrogen and phosphorus), ocean acidification, atmospheric aerosol pollution, stratospheric ozone depletion, and release of novel chemicals. In 2022, scientists announced the transgression of both the freshwater and novel entities boundaries. Image courtesy of J. Lokrantz/Azote based on Steffen et al. (2015) via Stockholm Resilience Centre.

‘Too many warnings’

No matter how it’s framed, researchers agree that ocean acidification is escalating — and having a worsening effect on the environment. For Riebesell, the way forward is through the lens of solutions.

“My personal perception is that we may have paralyzed our societies by putting out too many warnings — warnings about acidification, ocean warming, ocean deoxygenation,” he said. “We keep sending these messages that our oceans are about to die, but we’re not offering any solutions beyond ‘let’s reduce CO2 emissions.’”

This glass-half-empty view could also be viewed optimistically, since a single solution can positively affect multiple planetary boundaries. Reduce emissions, for example, and that single step takes us back from the brink of at least three planetary boundary thresholds: climate change, ocean acidification, and biosphere integrity.

Since 2019, Riebesell has been leading a project that uses alkaline rock minerals to lower the pH of seawater, making it more alkaline and less susceptible to ocean acidification, while enabling the ocean to better absorb carbon dioxide.

“It’s a very slow process,” Riebesell said. “But the question is, can we enhance that? And if we do, what consequences does this have for marine biota? And that’s what we’re looking into at the moment.”

Another solution to ocean acidification: the planting of seagrass, which research has found to mitigate acidification on calcifying organisms. However, Spicer notes that while seagrass plantings and other efforts are a step in the right direction, they only provide solutions at local and regional levels. Global solutions, he emphasizes, are most needed at the moment.

A ray amidst seagrass. One local and regional solution to ocean acidification is the planting of seagrass, which research has found to mitigate acidification on calcifying organisms. Image by Jordan Robins / Ocean Image Bank.

More than anything else, most researchers agree that lowering emissions is the one and only way humanity solves the acidification problem.

“I love the fact that human beings have tried to find ways of mitigating and adapting,” Spicer said. “But we’re in a huge truck heading towards a wall. And we’re trying to make the truck slightly nicer as we move towards that wall, and the only thing that’s going to stop [us hitting the wall] is stopping emissions.”

Spicer says he believes strongly that transformative change is still possible, especially after seeing how the world responded to the COVID-19 pandemic.

“We can still choose to some extent the size of the car crash, or [to] put that in a positive way,” Spicer said, “we can still choose what sort of world we want to live in to some extent.”

Banner image: A hogfish in the coral reefs of Cuba. Image by abrice Dudenhofer / Ocean Image Bank.

Citations:

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Elizabeth Claire Alberts is a staff writer for Mongabay. Follow her on Twitter @ECAlberts.

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