- When compared with historical samples, corals show that the Salish Sea and California Current System are acidifying faster than anticipated because of greenhouse gas emissions. Models indicate that at this rate, carbon dioxide levels in the oceans will continue rising faster than concentrations in the atmosphere.
- Increasingly acidic seas pose growing risks to sensitive marine life, from clams and oysters to any organism with a spine, as well as economically important fisheries and the communities that depend on them.
- British marine ecologist Stephen Widdicombe calls the threat existential. Our continued failure to cut emissions can only lead to “a world where uncontrolled climate change including ocean acidification leaves us with an ocean that is less productive, less diverse and less able to provide humans with the wealth of services that we currently all benefit from,” he said.
In 1888, researchers aboard the R/V Albatross began the world’s first concentrated marine research expeditions off California’s Pacific coast. The team collected untold plant and animal specimens, including orange cup corals from the Salish Sea, which they carefully preserved and stored in collections at the Smithsonian Institution.
These specimens have now become rare physical evidence of ongoing changes in the chemistry of the Pacific Ocean as seawater absorbs the carbon dioxide released into the atmosphere by burning fossil fuels.
Researchers recently analyzed these 130-year-old samples, which come from a time before the Industrial Revolution’s greenhouse gas impacts had really kicked in. Then they compared them with new specimens collected in the same locations by a team aboard another research vessel, the R/V Rachel Carson, in 2020.
They discovered that this region is acidifying, far faster than models have predicted, findings they recently published in the journal Nature Communications.
“Ocean acidification is not a distant or abstract phenomenon. It is already underway, it is amplified in some regions, and it has real consequences for ecosystems and coastal economies today,” study lead author Mary Margaret Stoll, from the University of Washington, told Mongabay.
This research focused on the Salish Sea and the cold California Current System, an interweave of four currents predominantly flowing south from the Canadian province of British Columbia to the Mexican state of Baja California. Prevailing winds push water away from the coast along these currents, drawing water up from the deep, bringing nutrient-rich sediments along with it. These support exceptional biodiversity and lucrative fisheries.
The waters off the North America Pacific coast form one of the most productive marine ecosystems on Earth. But now, industrial carbon emissions, deforestation and other human activities are changing ocean chemistry, lowering the pH and making waters more acidic in a process known as ocean acidification.
This threatens the long-term survival of economically and ecologically important marine creatures, including clams, sea urchins, oysters, and both shallow- and deep-water corals, as well as certain plankton, which form the base of the ocean food web.
If current trends continue, ocean surface waters could reach acidity levels about 150% higher than pre-industrial levels — higher than they’ve been for more than 20 million years — according to the U.S. National Oceanic and Atmospheric Administration.
Ocean chemistry
CO₂ absorbed by the ocean reduces the availability of carbonate ions in seawater, making it harder for marine organisms to form calcium carbonate shells. That leaves crustacean species with thinner, more fragile shells and corals with weaker skeletons.
This ocean chemistry imbalance continues to worsen with rising atmospheric CO₂. The discovery that acidity is increasing faster than greenhouse gas levels in the atmosphere is worrying, Stoll said.
“What concerns me most from our study is that acidification in the Salish Sea and the California Current System isn’t just keeping pace with atmospheric CO₂, it’s actually amplified,” she said.
CO₂ concentrations in the atmosphere rose by about 120 parts per million (ppm) since 1888, to more than 414 ppm by 2020. Meanwhile dissolved CO₂ in the Salish Sea rose even faster, by the equivalent of around 172 ppm, the study found.
As part of this new study, the researchers created models for the California Current System and Salish Sea. If global emissions continue at their current trajectory, the researchers predict that ocean CO₂ will increase in those seas 20% faster than atmospheric CO₂ in waters 50-75 meters deep (164-246 feet) and 60% faster in waters 100-150 m deep (328-492 ft) during this century. The researchers based this on both coral analysis and on projected atmospheric CO₂ if we don’t curb current emissions levels.
More acidic waters are already impacting Pacific Northwest shellfish farmers. “Ocean acidification is real, is driven by fossil fuel emissions and is dramatically impacting the health of our oceans,” said Alan Barton, who manages the Whiskey Creek Shellfish Hatchery in Oregon, one of the largest oyster larvae producers on the U.S. West Coast. “We’ve got the bank statements to prove it. Whiskey Creek almost went out of business because of it, and continues to battle it every single day,” he added.
The bigger picture
Earth’s oceans have so far protected us from the more severe effects of climate change by absorbing about 31% of fossil fuel emissions since the 1800s, according to previous research published in the journal Science.
But there’s no such thing as a free lunch: This increase corresponds almost exactly to 30% higher ocean acidity than during the early years of the Industrial Revolution, when steel and textile manufacturing, petroleum refining and other carbon-intensive industries emerged and emissions rose. Prior to that, global greenhouse gas emissions were low.
While the California Current System supports exceptional biodiversity and lucrative fisheries, nutrient-rich seawater from these depths also carries a sting in its tail: It’s loaded with CO₂ absorbed decades ago.
The water that upwelled along the U.S. West Coast in summer 2025 last saw the surface around 30-50 years ago, Barton said, when it sank into the mid-depths of the ocean near the North Pole. “This means it carries the signal of fossil fuel emissions from sometime around the 1980s.”
This has serious implications for future emissions. “Even if we dramatically all reduce carbon emissions today, we will still have 30-50 more years of increasingly impacted water coming on the conveyor belt of ocean circulation each year,” Barton said.
While the Salish Sea is small, it functions as an “incredibly valuable” global early-warning system because its waters are already carbon-rich, Stoll said. So the effects of ocean acidification are playing out faster than average there, she said, noting that the better we can understand what is happening already, the better we can anticipate and plan for a future with inevitably more acidic oceans.
Time-capsule corals
The 54 historical corals collected by the R/V Albatross had sat, untouched, in the Smithsonian for more than a century. Handling them “felt like holding a time capsule of the ocean before large-scale fossil fuel emissions began,” Stoll said. They offered a “uniquely tangible” way to assess changes to ocean acidity over time.
To find out how much the acidity level has changed since the 1880s, the researchers tested the boron present in the old corals and compared those findings with 42 modern corals taken from the same locations. Boron ratios in water are directly linked to pH — the level of acidity or alkalinity — and the changes they found corresponded to an acidity increase of about 24%.
“These corals don’t just record chemistry, they bridge more than a century of science and exploration,” Stoll said. “That continuity was one of the most meaningful aspects of the project.”
The research method used in this “very important and well conducted study … offers us the opportunity to jump aboard a time-machine back into recent history,” said Stephen Widdicombe, a British marine ecologist and co-lead of the U.N.-endorsed Ocean Acidification Research for Sustainability Programme. Widdicombe wasn’t involved in the study.
Stoll and her team used data on currents, temperatures, wind and chemistry to create digital reconstructions of how the California Current System might respond if CO₂ emissions aren’t reduced. Their findings suggest a pH as low as 7.5 at a depth of 75 m by the end of the 21st century.
This has serious implications for marine life. The difference between an average ocean surface pH of around 8.1 — which it was in 1990 — and 7.5 may not seem like much, but on a logarithmic scale like pH, that small shift amounts to almost triple the acidity.
Impacts on marine life
The organisms that are especially prone to ocean acidification include oysters, clams, sea snails, crab larvae and corals.
Oysters offer a good example why. They’re born as tiny, shell-less, free-swimming larvae, and usually form their shells within their first 48 hours. But if the water pH is too low, it’s impossible for them to do so. An entire generation can die within days.
That’s already occurring. “In today’s coastal ocean, the chances for spawning events to coincide with favorable water pH are much lower than in decades past,” Barton said. “This often leads to failed spawn events and poor survival of juvenile oyster seed.”
Impacts may also go well beyond the structural integrity of shells and corals. A growing body of research suggests that in acidic waters, senses are dulled for both crabs and fish. Their sight, hearing and smell are affected, reducing their ability to hunt, avoid prey and reproduce.
Short-sighted Dungeness crabs (Metacarcinus magister) are among those affected. Scent sensors on their antennae help them find food and ocean acidification seems to blunt their function. Those exposed to dissolved CO₂ in a lab setting — at levels similar to those expected by the end of this century — only responded to prey normally when its scent was 10 times more pungent than when prey was in less acidic waters, according to a 2023 study published in the journal Global Change Biology.
Other research showed that clownfish larvae were attracted — instead of repelled — by the smell of predators when they were reared in waters with CO₂ at levels that are predicted by the end of this century. Still another study found that in CO₂-infused waters, cuttlefish eyesight deteriorated, reducing their ability to hunt and find appropriate habitat.
This puts some lucrative fisheries at risk, including the Dungeness crab fishery, spanning California, Washington and Oregon, which generated more than $209 million during the 2023-2024 season. Regional shellfish aquaculture, including the Whiskey Creek hatchery, contributes an average $270 million to the U.S. economy annually.
So far, Dungeness crab populations are looking pretty good. The 2022-2023 season even broke catch volume records, while the 2023-2024 season broke revenue records. Barry Day, a crab fisher based in California’s Half Moon Bay, said he hasn’t noticed any changes to crab shells so far. “The just-under-size crab are good the next year, as it has always been.”
For oyster larvae, though, it’s a different story. U.S. Pacific hatcheries, including Whiskey Creek, suffered mass die-offs of oyster larvae in the 2000s. Now, Barton said he has to constantly monitor and chemically buffer incoming water to artificially adjust the pH. These days it’s almost always lower than optimal for larvae to develop and maintain shells efficiently. “We, more than really most anyone else, are feeling the real-world effects of ocean acidification today, not in some distant predicted future,” Barton said.
Looking ahead
While decarbonization of energy systems is progressing in some countries, human CO₂ emissions, largely from fossil fuel use and deforestation, are still rising, up 1.1% in 2025.
Stoll said she hopes that the study’s projections can be integrated into long-term fisheries management planning and that the findings can help improve cultural, economic and ecological decision-making and policy formation.
The threat to ocean life from fossil fuel emissions — and by extension to human prosperity — is existential, Widdicombe said. We don’t need more research to tell us that the time to act “was yesterday, or rather decades ago,” he added. Our continued failure to cut emissions can only lead to one place, he said, “a world where uncontrolled climate change including ocean acidification leaves us with an ocean that is less productive, less diverse and less able to provide humans with the wealth of services that we currently all benefit from.”
Experts say there’s only one way to avoid this future. “Ultimately, cutting CO₂ emissions remains the most important action to slow acidification globally,” Stoll said.
Banner image: Vermillion rockfish and soft coral observed during an expedition in Channel Islands National Marine Sanctuary. Image courtesy of Marine Applied Research and Exploration, NOAA.
Citations:
Stoll, M. M., Deutsch, C. A., Jurikova, H., Rae, J. W. B., Frenzel, H., Gothmann, A. M., … Gagnon, A. C. (2025). A century of change in the California Current: upwelling system amplifies acidification. Nature Communications, 16(1), 9661. doi:10.1038/s41467-025-63207-6
Gruber, N., Clement, D., Carter, B. R., Feely, R. A., van Heuven, S., Hoppema, M., … Wanninkhof, R. (2019). The oceanic sink for anthropogenic CO2 from 1994 to 2007. Science, 363(6432), 1193-1199. doi:10.1126/science.aau5153
Durant, A., Khodikian, E., & Porteus, C. S. (2023). Ocean acidification alters foraging behaviour in Dungeness crab through impairment of the olfactory pathway. Global Change Biology, 29(14), 4126-4139. doi:10.1111/gcb.16738
Nilsson, G. E., Dixson, D. L., Domenici, P., McCormick, M. I., Sørensen, C., Watson, S.-A., & Munday, P. L. (2012). Near-future carbon dioxide levels alter fish behaviour by interfering with neurotransmitter function. Nature Climate Change, 2(3), 201-204. doi:10.1038/nclimate1352
Xie, J., Sun, X., Li, P., Zhou, T., Jiang, R., & Wang, X. (2023). The impact of ocean acidification on the eye, cuttlebone and behaviors of juvenile cuttlefish (Sepiella inermis). Marine Pollution Bulletin, 190, 114831. doi:10.1016/j.marpolbul.2023.114831
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