The depths of the ocean are slowing climate change, but at a cost. Seawater acidity is increasing as the oceans absorb carbon dioxide from fossil fuels.

Average surface ocean pH is now 30% more acidic than in pre-industrial times. In a business-as-usual scenario, by 2100, ocean water could be over one-and-a-half times more acidic.

The effects of ocean acidification can be grouped into “known knowns, known unknowns, and unknown unknowns,” says Wiley Evans, a chemical oceanographer leading ocean acidification research for the Hakai Institute. Evans is riffing on an old Donald Rumsfeld quote because, although the underlying chemistry is straightforward, the full consequences of ocean acidification are uncharted. “A lot of what is unknown is at the species level,” he says. Only a handful of organisms have been thoroughly tested for their response to rising acidity. “And then there’s the ecosystem response – how impacts on one organism is going to affect the whole food web, and the ecosystems that those organisms reside in. So pretty quickly it snowballs into a lot of unknowns.”

Biologists, chemical and physical oceanographers, geneticists, and other experts are now working together to investigate these unknowns. Research into ocean carbonate chemistry has been done since the 1970s, but only in 2003 did the term ocean acidification come into use. It’s now an “exploding field,” says Evans. “It really is interdisciplinary, it relies on the physics and the biology, and the ecosystem level [research], and so ecologists and modellers and everyone is really working together to try and understand the breadth of the problem.”

The dynamic ocean environment makes the research complex. “The pH of the ocean changes constantly,” says Mark Spalding, president of the Ocean Foundation, an environmental organization based in Washington, DC. “It fluctuates daily, it fluctuates seasonally, it fluctuates with El Niño events, it fluctuates when you have an upwelling from the deep ocean.” Ocean acidity also differs by location. Natural factors dominate the exchange of atmospheric carbon dioxide into the ocean, but just as with atmospheric carbon, additional emissions have tipped the scales.

Ocean acidification also has to be disentangled from other co-occurring climate change stressors such as warming water and deoxygenation. Each effect is worrying on its own; combined, these multiple stressors are greater than the sum of their parts.

Despite this complexity, ocean acidification is caused by simple chemical reactions.

“It’s pretty much just like high school level chemistry,” says Evans. “As you increase carbon dioxide in the atmosphere, it’s not all going to stay there, it’s going to want to move into an area of lower concentration, and CO2 gas will dissolve into seawater.”

When carbon dioxide in the air reacts with seawater it forms carbonic acid. This carbonic acid then breaks down into bicarbonate and a hydrogen ion.

The pH of a solution (how acidic or basic it is) is determined by the concentration of hydrogen ions – the more hydrogen ions, the more acidic. Ocean water hasn’t become an actual acid, but its alkalinity is declining.

Surplus hydrogen ions then bond with carbonate ions also found in seawater to form more bicarbonate.

Impact on shellfish

Many types of shelled organisms build their shells by combining carbonate ions with calcium to form calcium carbonate. But because some of the carbonate ions have already bonded with hydrogen ions, shellfish, mullusks, corals, and other shelled creatures face a scarcity of carbonate ions with which to build their shells.

“The first time we really noticed this was happening was with oysters in the Pacific Northwest of the United States,” says Spalding. “The largest oyster farming business in the world … saw a dramatic loss in production as a result of some acidic events.” Since then, ocean acidification has become an ongoing concern for shellfish farmers, particularly for operations in developing countries lacking the technology to monitor and react to pH changes.

Shellfish are most vulnerable to acidity as tiny, newly hatched larvae, when they have to exert a tremendous amount of energy to build their first shell. The scarcity of carbonate can either prove fatal, or the shellfish “end up with very deformed shells and thinner shells, and become more susceptible to predators,” says Spalding.

The same threat applies to urchins, snails, seastars, and corals, although the exact chemical process can vary. In a study conducted near a volcanic seep which emulated future CO2 levels, “triton shell” sea snails were found to have their shell thickness halved, and in some cases so dissolved their body tissue was exposed.

The most urgent example of shell corrosion may be to creatures near the base of the food chain called pteropods, commonly known as sea butterflies.

Some species are predicted to benefit from ocean acidification, but this isn’t necessarily a good thing

“They look like minuscule snails with wings,” says Spalding, “and they’re having a very, very hard time forming their shells.” Field observations have recently confirmed that pteropod shells in an area of increased acidity in the Gulf of Alaska are dissolving. “If the ocean continues to have its chemistry change … we could have a rippling effect where the food resource for other animals in the ocean, and up the food chain, has a rippling collapse,” says Spalding.

For corals, ocean acidification is only one ingredient: “warmer waters, thanks to climate change, nutrient pollution, oxygen deprivation, and then you add in ocean acidification, and you’ve got a horrible recipe for collapse of coral systems,” says Spalding. “The current predictions are losses of 90% or more of our coral reefs by 2030.”

At rock bottom on the marine food chain are diatoms – microscopic, single-celled algaes encased in shells of transparent silica. They live near the surface of the ocean, transforming sunlight to sugars via photosynthesis. When they die, a percentage fall to the ocean floor, sequestering carbon.

Iron uptake in diatoms

Diatoms are immensely important to the marine food web, and to all life on earth – cycling about 20% of the oxygen we breathe. Research published in Nature in 2018 suggests ocean acidification could threaten diatom populations – but not by hindering shell formation. Increased acidity, for an unknown reason, prevents the uptake of iron that diatoms need to proliferate. Diatom numbers are already in decline, and rising surface ocean temperatures are suspected. A crash in diatom populations could cripple the ocean’s ability to cycle carbon dioxide, accelerating further climate change.

Fish are not off the hook in the struggle to adapt to a more acidic environment, either. Tests conducted in the National Oceanic and Atmospheric Administration’s Fisheries research lab showed salmon exposed to future pH levels were less responsive to the smell of salmon skin extract, which would normally warn the fish of a predator attack and prompt them to hide or flee. Testing of the salmons’ nose and brain tissue indicated that the fish could still detect the scent, but were not interpreting and acting on the trigger.

Ocean acidification may have caused events in the earth’s history that go well beyond concerns over food security

Other behaviours relying on scent – reproduction, navigation, and finding food – are also likely affected. Similar research has shown elevated CO2 concentrations to blunt prey detection in sharks, and affect the ability of reef fish to discern healthy reef habitat.

Some species are predicted to benefit from ocean acidification, but this isn’t necessarily a good thing. Research from the Canary Islands suggests the algae Vicicitus globosus, found from temperate regions to the tropics, will thrive in rising CO2 levels. Toxic blooms of V.globosus are already known to cause fish kills, and now, researchers say, “may pose an emergent threat to coastal communities, aquaculture and fisheries.”

Krill probably OK

Other organisms may shrug off ocean acidification altogether. Krill are small crustaceans found in all oceans, and an essential food source for marine mammals and seabirds in the Southern Ocean. In a 2018 study published in Communications Biology, adult Antarctic krill, in acidity simulating near-future conditions, were able to go about their business undisturbed. The krill’s resilience is attributed to a special structure in their gills able to balance body fluid pH. Krill eggs and embryos, however, don’t have this ability, so the results are not definitive.

The sum of these effects (and several more) put future food security in doubt. Evans says that the capacity of the oceans to feed us could be threatened when we need it most. “We need to divest from cattle production, as they’re major contributors to CO2 emissions, and we’ll be relying more on aquaculture and fishing … and so we essentially could be in a bottleneck, where some of these species that we’re going to try and grow are going to have difficulty.”

Read more: Help the Kelp: Warming waters and sea urchins are decimating kelp forests

Ocean acidification may have caused events in the earth’s history that go well beyond concerns over food security. Research published in Science in 2015 puts forward the theory that modern day ocean acidification has chilling parallels with the Permo-Triassic mass extinction, also known by its cheerful nickname, the Great Dying.

Around 252 million years ago, volcanoes belched huge quantities of carbon dioxide into the atmosphere over tens-of-thousands of years. This in turn triggered a spike in ocean acidity that the research suggests killed up to 96% of marine life. During the Great Dying, CO2 levels in the ocean shot up about as quickly as they are today.

“Climate change is physics, and ocean acidification is chemistry,” says Spalding, “but they’re both the same carbon dioxide molecules. Our ocean is our biggest sink for our carbon emissions, but there are limits. It can only take so much.”

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