The ocean and the atmosphere are constantly seeking balance.
Gases like oxygen, nitrogen, and carbon move between the ocean’s surface and the atmosphere by billions of metric tons every year.
A higher concentration of one gas in the atmosphere leads to more of that gas being taken up by the ocean as the two try to reach a state of balance – or equilibrium. However higher concentrations of carbon, emitted by human activities predominantly through the burning of fossil fuels, have been observed in the atmosphere since the Industrial Revolution.
This has consequently led to an increase in the ocean’s accumulation of carbon globally. This increase in ocean carbon has caused a chain of chemical reactions driving one of the primary environmental threats to marine ecosystems, fisheries and coastal communities – ocean acidification.
Carbon dioxide absorbed at the ocean’s surface binds with water molecules to produce an acid known as carbonic acid (H2CO3), which dissociates into bicarbonate ions (HCO3–) and a free-floating hydrogen ion (H+).
A rise in hydrogen ions is what changes the pH of any liquid, lowering the pH and making it more acidic.
The ocean’s absorption of carbon dioxide from the atmosphere is a natural process that has occurred over Earth’s history. However, today with more carbon in the atmosphere globally, we also see more carbon in the surface ocean that is drawing down the surface ocean’s pH and leading to global ocean acidification.
The ocean’s pH at the sea surface once measured at 8.2 before the 1700’s. Today, it’s estimated at 8.1.
So while, 0.1 decrease in ocean pH may seem insignificant, it means the concentrations of hydrogen ions within the ocean have increased dramatically. The pH scale is logarithmic, meaning that each whole number change on the scale represents a tenfold change in acidity or basicity.
In human blood, a 0.1 decrease in pH can lead to seizures, heart arrhythmia or even a coma. Here, we’re talking about a 0.1 pH decrease across the global surface ocean – a body of water that spans 139 million square miles.
The 0.1 decrease in global ocean pH means our oceans have increased in acidity by 25% in the last 300 years. And with continued growth in atmospheric carbon emissions, this trend will continue.
Based on model simulations scientists estimate that by 2100, the average pH of the ocean’s surface waters could decrease by 0.3 – 0.4. This is equal to a 100% to 150% increase in acidification over the next 75 years.
Scientists are already observing the impacts of a lower pH ocean. Increasingly acidic waters erode away at the calcium carbonate shells and skeletons of scallops, crabs, oysters, clams, mussels and entire coral reefs.
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It accelerates the erosion of coral reefs and decreases their ability to grow (i.e. calcify), leading to the loss of critical infrastructure for storm surge protection and one of the Earth’s most crucial habitats providing shelter to a vast breadth of marine biodiversity.
However, the impacts of ocean acidification are not consistent across the ocean. In some places, it’s exacerbated. In other regions, ocean acidification is being buffered against by complex changes in seawater carbonate chemistry.
Therefore, better observing regional changes related to ocean acidification, seeking long-term solutions and mitigating the effects on key ecosystems is critically important to mitigating the effects of ocean acidification. This is a big job for scientists today who are working hard to identify the intensity and the ecosystem effects of ocean acidification that we can expect in the future. They ask questions like – how does it vary across entire regions, how will it intensify over years and decades, and what ecosystems are most vulnerable?
Scientists at AOML are doing exactly that with research cruises, the deployment of cutting-edge instruments, and advanced modeling efforts.
Research Cruises
Identifying the impacts of ocean acidification across vastly different ecosystems requires a comprehensive view and long term monitoring of changes in ocean chemistry across thousands of miles of U.S. coastline.
To do this, scientists at AOML lead the Gulf and Ocean Monitoring Ecosystems and Carbon Cruises (GOMECC) and East Coast Ocean Acidification (ECOA) cruises every four years to assess changes in seawater chemistry across the Gulf region and greater south Atlantic.
CTD cast in progress aboard GOMECC cruise. Photo Credit: NOAA.
By deploying a CTD at specified stations from offshore to shallow waters along the coast, scientists across institutions collect seawater samples at different depths to holistically assess the partial pressure of carbon dioxide (pCO2) – an indication of how much carbon dioxide is in seawater.
With these cruises returning to specified sites every four years, scientists can assess long-term changes in the seawater chemistry and identify the regions most impacted by increasingly acidic seawater.
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NOAA’s Atlantic Oceanographic and Meteorological Laboratory, 16 June 2025. Full article.
