Ocean acidification is the gradual decrease in the pH level of Earth's oceans. From 1950 to 2020, the average pH of the ocean's surface dropped from about 8.15 to 8.05. Human activities, such as burning fossil fuels, release carbon dioxide (CO₂) into the atmosphere. This CO₂ is absorbed by the ocean, where it reacts with water to form carbonic acid (H₂CO₃). This acid breaks apart into bicarbonate ions (HCO₃⁻) and hydrogen ions (H⁺). The increase in hydrogen ions lowers the ocean's pH, making it more acidic. However, seawater remains alkaline, as its pH is still above 8. Marine organisms that build shells and skeletons, such as mollusks and corals, are especially affected because they rely on calcium carbonate for their structures.
A change in pH by 0.1 means there is a 26% increase in hydrogen ions in the ocean. The pH scale is logarithmic, so a change of 1 unit equals a tenfold increase in hydrogen ions. The pH and availability of carbonate in seawater vary based on depth and location. Colder and higher latitude waters absorb more CO₂, which can increase acidity and lower carbonate levels in these areas. Other factors, such as ocean currents, upwelling zones, proximity to rivers, sea ice, and atmospheric nitrogen and sulfur from human activities, also influence how CO₂ is exchanged between the ocean and atmosphere.
Lower ocean pH can harm marine life. Scientists have observed reduced shell and skeleton formation, weaker immune systems, and less energy for reproduction in some species. Coral reefs, which support food and livelihoods for about one billion people, are especially at risk. Ocean acidification may disrupt ocean food chains, affecting ecosystems that provide resources for humans.
The main solution to ocean acidification is reducing CO₂ emissions, which is a key goal of climate change efforts. Removing CO₂ from the atmosphere could also help reverse ocean acidification. Some ocean-based methods, such as increasing ocean alkalinity or using weathering processes, are being studied. However, these methods are not yet widely used and carry risks.
Ocean acidification has occurred in Earth's history before. These past events caused major changes in ocean life and the planet's climate.
Cause
In 2021, atmospheric carbon dioxide (CO₂) levels reached about 415 ppm. This is about 50% higher than preindustrial levels. According to the National Oceanic and Atmospheric Administration in 2023, CO₂ levels have increased from around 280 parts per million (ppm) before the industrial era to over 410 ppm today. This rise is mainly due to human activities, such as burning fossil fuels and cutting down forests. The current high levels and fast growth of CO₂ are unlike any seen in the past 55 million years. The main sources of this extra CO₂ are human-caused, including emissions from fossil fuels, industry, and changes in land use. One major source is fossil fuels, which are burned for energy. When burned, CO₂ is released into the air as a byproduct of combustion. This process contributes greatly to the rising CO₂ levels in Earth’s atmosphere. The ocean absorbs about a quarter of all human-caused CO₂ emissions. However, this extra CO₂ changes the ocean’s chemistry, making seawater more acidic and reducing the availability of carbonate minerals that many marine organisms use to build their shells and skeletons.
Since 1850, the ocean has absorbed up to 175 ± 35 gigatons of carbon. More than two-thirds of this, or 120 gigatons of carbon, was taken in by the global ocean since 1960. Over time, the ocean’s ability to absorb CO₂ has grown faster as human-caused emissions have increased. From 1850 to 2022, the ocean absorbed 26% of all human-caused CO₂ emissions. Total emissions during this period were 670 ± 65 gigatons of carbon, with 41% remaining in the atmosphere, 26% absorbed by the ocean, and 31% taken up by land.
The carbon cycle describes how carbon dioxide moves between the oceans, land, atmosphere, and Earth’s rocky layers. This cycle includes both organic materials, like cellulose, and inorganic materials, such as CO₂, carbonate ions, and bicarbonate ions, which are grouped as dissolved inorganic carbon (DIC). These inorganic compounds play a key role in ocean acidification because they are forms of dissolved CO₂ in the ocean. When CO₂ dissolves in water, it forms a balance of chemical substances: dissolved CO₂, carbonic acid, bicarbonate, and carbonate. The proportions of these substances depend on factors like seawater temperature, pressure, and salinity, as shown in a Bjerrum plot. These dissolved inorganic carbon forms move from the ocean’s surface to its deeper layers through a process called the solubility pump. The ability of an ocean area to absorb atmospheric CO₂ is measured by the Revelle factor.
Main effects
The ocean's chemical makeup is changing because it absorbs human-caused carbon dioxide (CO₂). Over the past 270 years, the ocean has taken in about 30% of the CO₂ released by humans. This has caused ocean pH levels, the amount of carbonate ions ([CO₃²⁻]), and the ability of seawater to form calcium carbonate minerals (Ω) to decrease. This process, called "ocean acidification," makes it harder for marine organisms that build shells or skeletons, like corals, to survive. This can harm coral reefs and the larger ocean ecosystems that depend on them.
Ocean acidification is sometimes called the "evil twin of global warming" and "the other CO₂ problem." Rising ocean temperatures and oxygen loss happen at the same time as acidification, creating what scientists call the "deadly trio" of challenges for marine life. These changes will most strongly affect coral reefs, shell-forming marine animals, and people who rely on the services these ecosystems provide.
When CO₂ dissolves in seawater, it increases the number of hydrogen ions (H⁺) in the ocean, which lowers the pH. This chemical reaction is described as:
In shallow coastal and shelf areas, several factors influence how CO₂ moves between the air and ocean, and how pH changes. These include biological processes like photosynthesis and respiration, as well as the movement of deep water to the surface (upwelling). Ecosystem activity in freshwater sources that flow into coastal waters can also cause large, but localized, changes in pH.
Freshwater bodies are also becoming more acidic, though this is a more complex and less obvious process.
The ocean absorbing CO₂ from the atmosphere does not change its alkalinity. Alkalinity is the ability of water to resist acidification. Adding alkalinity to the ocean has been suggested as a way to help balance pH changes.
Changes in ocean chemistry can harm marine life and their habitats. One major effect is on the ability of organisms to produce shells made of calcium carbonate (CaCO₃), a process called calcification. Many marine animals, such as coccolithophores, foraminifera, crustaceans, and mollusks, rely on this process to build their shells. These shells can dissolve if the surrounding water does not have enough carbonate ions (CO₃²⁻) to support them.
Most of the extra CO₂ absorbed by the ocean does not stay as dissolved CO₂. Instead, it breaks down into bicarbonate ions and hydrogen ions. The increase in hydrogen ions is greater than the increase in bicarbonate, creating an imbalance. To restore balance, some carbonate ions in the ocean combine with hydrogen ions to form more bicarbonate. This reduces the amount of carbonate ions in the ocean, which are essential for shell formation.
These changes are shown in the Bjerrum plot, which illustrates how different forms of carbon are distributed in seawater.
Disruption of the food chain is another possible effect. Many marine animals depend on calcium carbonate-based organisms at the bottom of the food chain for food and shelter. This can harm the entire food web and reduce fish populations, which could affect human livelihoods.
The saturation state (Ω) of seawater for a mineral measures how likely the mineral is to form or dissolve. For calcium carbonate, this is calculated by dividing the product of the concentrations of calcium (Ca²⁺) and carbonate ions (CO₃²⁻) by the solubility product (Ksp), which is the value at which precipitation and dissolution are balanced. In seawater, the depth where calcium carbonate begins to dissolve is called the saturation horizon. Above this depth, Ω is greater than 1, and calcium carbonate does not dissolve easily. Most shell-forming organisms live in these areas. Below the saturation horizon, Ω is less than 1, and calcium carbonate dissolves. The depth where the amount of calcium carbonate dissolving equals the amount being supplied to the ocean floor is called the carbonate compensation depth. Higher CO₂ levels and lower pH reduce carbonate ion levels and the saturation state of calcium carbonate, increasing its dissolution.
Calcium carbonate exists in two main forms: aragonite and calcite. Aragonite dissolves more easily than calcite, so the depth where aragonite begins to dissolve (aragonite saturation horizon) is shallower than the depth for calcite. This means organisms that produce aragonite may be more vulnerable to ocean acidification than those that produce calcite. As ocean acidification lowers the saturation state of calcium carbonate, the saturation horizons for both forms move closer to the surface. This makes it harder for marine organisms to form shells, as the ability to form calcium carbonate is directly tied to its saturation state.
Currently, large areas of water with low aragonite levels are rising to the surface near the Pacific continental shelf off the coast of North America, from Vancouver to Northern California. These shelves are important for marine life, as many species live or reproduce there. Similar effects may be happening in other regions.
At great depths in the ocean, calcium carbonate shells begin to dissolve due to high pressure and low temperatures. The depth where this occurs is called the carbonate compensation depth. Ocean acidification will increase this dissolution and move the carbonate compensation depth upward over decades to centuries. Areas where deep water rises to the surface (downwelling zones) are affected first.
In the North Pacific and North Atlantic, the saturation state of calcium carbonate is also decreasing, meaning the depth where calcium carbonate dissolves is becoming shallower. As CO₂ mixes into deeper ocean layers, the carbonate compensation depth in the open ocean is also becoming shallower. In the North Pacific, the carbonate compensation depth is moving upward by 1–2 meters each year.
In the future, ocean acidification is expected to greatly reduce the amount of calcium carbonate that settles on the ocean floor as sediment for several centuries. It may also cause existing calcium carbonate sediments to dissolve.
Measured and estimated values
Between 1950 and 2020, the average pH of ocean surface water is estimated to have decreased from about 8.15 to 8.05. This change means the acidity of the world’s oceans has increased by about 26%. The pH scale is logarithmic, so a change of one pH unit equals a tenfold change in acidity. For example, between 1995 and 2010, the acidity in the top 100 meters of the Pacific Ocean from Hawaii to Alaska increased by 6%.
The IPCC Sixth Assessment Report (2021) stated that current ocean surface pH levels are the highest in at least 26,000 years. Since the late 1980s, the pH of deep ocean water has decreased everywhere globally. The report also noted that the pH of open ocean surface water has declined by about 0.017 to 0.027 units per decade.
The rate of pH decline varies by region. In the tropical Pacific, areas with strong upwelling (when deep, CO₂-rich water rises to the surface) saw a faster pH drop of -0.022 to -0.026 units per decade. This is believed to be caused by increased upwelling of CO₂-rich water and human-caused CO₂ absorption. In contrast, areas like warm pools in the western tropical Pacific experienced slower acidification, with a pH decline of -0.010 to -0.013 units per decade.
The speed of ocean acidification may depend on how quickly the ocean surface warms. Warmer water absorbs less CO₂, which could slow the rate of pH change for a given increase in CO₂. Differences in warming between ocean regions are a major reason for varying acidification rates.
Today’s ocean acidification rates are similar to a major climate event 56 million years ago, called the Paleocene–Eocene Thermal Maximum (PETM). During this event, ocean temperatures rose by 5–6°C, and deep-sea life suffered a major extinction. Today, the rate of carbon entering the atmosphere and ocean is about ten times faster than during the PETM.
Modern systems are being developed to monitor ocean CO₂ levels and acidification in open oceans and coastal areas.
Ocean acidification has occurred in Earth’s history during major extinction events, such as the Capitanian mass extinction, the end-Permian extinction, the end-Triassic extinction, and the Cretaceous–Paleogene extinction. Three of the five largest mass extinctions were linked to rapid increases in atmospheric CO₂, likely from volcanic activity or the release of gas hydrates. High CO₂ levels reduced calcium carbonate saturation, which may have caused mass extinctions. The end-Triassic extinction is the best example of ocean acidification causing marine life loss, as evidence shows volcanic activity lowered carbonate levels, and extinction events matched this in the fossil record. Ocean acidification is also linked to the end-Permian and end-Cretaceous extinctions.
The PETM, 56 million years ago, is a key example of ocean acidification. Massive carbon release into the ocean and atmosphere caused widespread dissolution of carbonate sediments. Studies suggest pH dropped by 0.3 units during the PETM. However, research on the carbon cycle during this event indicates the carbon release was slower than today’s human-caused emissions. More studies are needed to understand how pH changes affected marine life during the PETM.
Predicted future values
Ocean acidification is happening much faster today than it did in Earth's history. This quick change makes it harder for ocean life to adjust slowly over time and stops natural processes that might help reduce acidification. Scientists predict that ocean pH levels could drop to the lowest they have been in 300 million years. The speed at which pH levels are changing is also greater than any time in that same period. These changes are unlike anything seen in Earth's past. Combined with other changes in the ocean’s chemical balance, this drop in pH could harm marine ecosystems and reduce the benefits the ocean provides, such as food and clean water, as early as the year 2100.
How much ocean chemistry will change, including pH levels, depends on how well countries work to reduce greenhouse gas emissions. Scientists use models called Shared Socioeconomic Pathways (SSP) to study different future scenarios based on how societies develop and use resources.
In a scenario with very high emissions (SSP5-8.5), models predict that ocean pH could drop by up to 0.44 units by the end of this century compared to the late 1800s. This would bring pH levels as low as about 7.7, which means hydrogen ion concentrations in the ocean could increase two to four times more than they have already.
Impacts on oceanic calcifying organisms
The full effects of ocean acidification on calcifying species are complex, but it is likely that many of these organisms will be harmed by the changes. Ocean acidification makes it harder for shell-forming organisms to obtain carbonate ions, which are needed to build their hard, outer shells. These calcifying organisms are found throughout the food chain, from plants that make their own food to animals that eat other organisms. Examples include coccolithophores, corals, foraminifera, echinoderms, crustaceans, and molluscs.
All marine ecosystems will face changes in ocean acidification and other chemical changes in the ocean. Ocean acidification may cause some organisms to use more energy to maintain their shells, which could reduce their ability to grow or reproduce. For example, the oyster Magallana gigas has been shown to experience changes in metabolism and shell growth due to the energy required to balance pH imbalances.
Under normal conditions, calcite and aragonite are stable in surface ocean waters because the concentration of carbonate ions is higher than needed for seawater. However, as ocean pH decreases, the amount of carbonate ions also decreases. This makes calcium carbonate less stable, leading to weaker shells and increased risk of shell breakdown. Studies show that corals, coccolithophores, coralline algae, foraminifera, shellfish, and pteropods experience reduced shell growth or increased shell breakdown when exposed to higher levels of carbon dioxide. Even with conservation efforts, some shellfish populations may not recover.
Some studies show different responses to ocean acidification. For example, coccolithophores may increase both their calcification and photosynthesis under higher carbon dioxide levels, while other species may see equal decreases in both processes. Similarly, the sea star Pisaster ochraceus shows faster growth in more acidic waters.
Reduced calcification due to ocean acidification could weaken the ocean's ability to store carbon from the atmosphere in the deep ocean and seafloor sediments, which is part of the biological pump. Acidification may also reduce the size of Antarctic phytoplankton, making them less effective at storing carbon. These changes are studied using frameworks like the Adverse Outcome Pathway (AOP).
A coccolithophore is a single-celled, eukaryotic phytoplankton (a type of algae). Changes in their calcification may affect climate because fewer coccolithophores could reduce ocean cloud cover, lowering Earth’s albedo (reflectivity). A study from 2008 found that coccolithophore species in the North Atlantic remained the same over 224 years (1780–2004), but their shell size increased by 40% during this time.
Warm water corals are declining, with losses of about 50% over the last 30–50 years due to ocean warming, acidification, pollution, and physical damage from human activities. These threats are expected to worsen.
The internal compartments of corals, called the coelenteron, are critical for shell growth. When the surrounding seawater has normal levels of aragonite (a form of calcium carbonate), corals grow their shells quickly. If aragonite levels are too low, corals must work harder to maintain balance inside their bodies, slowing shell growth. If aragonite levels are too low, corals may stop growing entirely. By 2050–2060, about 70% of North Atlantic cold-water corals may live in water that is corrosive to their shells.
Ocean acidification primarily reduces the density of coral shells, not their size. Some coral species could lose more than 20% of their shell density by the end of this century.
An experiment on the Great Barrier Reef showed that reducing seawater carbon dioxide levels (increasing pH) to preindustrial levels increased coral calcification by 7%. Conversely, raising carbon dioxide levels (lowering pH) to levels expected by 2050 reduced calcification by 34%.
However, a study of corals in Queensland and Western Australia from 2007 to 2012 found that corals are more resilient to pH changes than previously thought, due to their ability to regulate internal conditions. This suggests that marine heatwaves, which cause coral bleaching, are a bigger threat to coral reefs than acidification.
In some areas, carbon dioxide bubbles up from the seafloor, changing the pH of seawater. Studies of these carbon dioxide seeps show varied responses among organisms. Coral reefs near seeps in Papua New Guinea have seen reduced coral diversity due to lower pH, but in Palau, coral diversity remains stable, though erosion of coral skeletons increases at lower pH.
Pteropods and brittle stars are important parts of Arctic food webs but are harmed by acidification. Pteropods’ shells dissolve as acidification increases, and brittle stars lose muscle mass when regrowing limbs. Pteropods need aragonite to build shells, but acidification has reduced the availability of carbonate ions, making it harder for them to survive. In Arctic waters, organic matter breakdown has worsened acidification, with some areas now undersaturated in aragonite.
Brittle star eggs die quickly when exposed to Arctic acidification conditions. In experiments, larvae of a temperate brittle star survived less than 0.1% when exposed to pH levels reduced by 0.2 to 0.4 units for more than eight days.
Other impacts on ecosystems
Ocean acidification can harm marine life in many ways. It may slow or stop the formation of hard parts in organisms, such as shells or bones. It can also harm them indirectly by reducing food supplies or directly by affecting reproduction or bodily functions. For example, high levels of carbon dioxide in the ocean can increase the acidity of body fluids in some animals, a condition called hypercapnia. Studies show that increased acidity can lower the metabolic rates of jumbo squid and weaken the immune systems of blue mussels. Atlantic longfin squid eggs hatch more slowly in acidic water, and their statoliths (a part of the inner ear used for balance) are smaller and misshapen in low-pH environments. However, research on these effects is still ongoing, and scientists do not yet fully understand how ocean acidification impacts marine life or ecosystems.
Another way ocean acidification may affect ecosystems is through changes in how sound travels in water. Acidification can alter the acoustic properties of seawater, allowing sound to travel farther and increasing ocean noise. This can interfere with animals that rely on sound for communication or navigation, such as whales and dolphins.
Ocean acidification may also increase the frequency of harmful algal blooms, which can produce toxins like domoic acid, brevetoxin, and saxitoxin. These toxins can accumulate in small marine animals, such as anchovies and shellfish, leading to illnesses in humans, including amnesic, neurotoxic, and paralytic shellfish poisonings. While harmful algal blooms are dangerous, some photosynthetic organisms, such as seagrasses, may benefit from higher carbon dioxide levels. Research suggests that increased seagrass photosynthesis can raise local water pH, which may help calcifying algae grow their hard parts more effectively.
Ocean acidification can also harm marine fish larvae by affecting their sense of smell, which is important for their early development. Orange clownfish larvae live near coral reefs surrounded by vegetative islands and use their sense of smell to find suitable habitats. They also rely on their sense of smell to distinguish between their parents and other fish to avoid inbreeding. In an experiment, clownfish were raised in seawater with a pH similar to today’s oceans (8.15 ± 0.07). When exposed to lower pH levels (7.8 ± 0.05 or 7.6 ± 0.05), the larvae reacted differently to environmental cues, and at the lowest pH, they showed no response at all. However, a 2022 study found that the effects of ocean acidification on fish behavior have decreased significantly in recent years.
European eel embryos, which are critically endangered but important for aquaculture, are also affected by ocean acidification. These eels spend most of their lives in freshwater but travel to the Sargasso Sea to spawn. During this life stage, they are vulnerable to acidification because their bodies are not fully developed and cannot regulate pH effectively. A 2021 study found that exposure to predicted future ocean conditions could harm the development of European eel embryos in the Sargasso Sea. Under extreme acidification, their survival and growth in hatcheries may also be negatively affected.
Research shows that the combination of ocean acidification and rising ocean temperatures has a greater harmful effect on marine life than either stressor alone. Warmer ocean temperatures, along with increased phytoplankton growth from higher carbon dioxide levels, can worsen ocean deoxygenation. Deoxygenation reduces the availability of oxygen in the water, which stresses marine organisms and limits nutrient availability over time.
Studies have shown that the combined effects of ocean acidification, warming, and deoxygenation significantly harm marine ecosystems. Experiments using mesocosms (controlled environments that mimic natural conditions) confirmed that these stressors severely disrupt marine food webs. For example, the negative effects of thermal stress on marine life outweigh any potential benefits from increased productivity of plants and animals due to higher carbon dioxide levels.
Impacts on the economy and societies
Ocean acidification slows the process of forming shells and skeletons in saltwater, which causes coral reefs to grow more slowly and become smaller. These reefs support about 25% of marine life. The effects of acidification are widespread, affecting fisheries, coastal areas, and even the deepest parts of the ocean. Acidification not only harms coral but also the many marine animals that depend on coral reefs for survival.
Acidification threatens the fishing industry and tourism businesses that rely on the ocean. Future acidification may harm ocean-related services and affect the livelihoods of up to 470 to 870 million of the world’s poorest people, depending on how much greenhouse gases are released into the atmosphere.
About 1 billion people rely on fishing, tourism, and coastal management services provided by coral reefs. Continued acidification may disrupt ocean food chains that support life in the sea.
In the Arctic, acidification harms organisms that build shells or skeletons, such as pteropods and brittle stars. These organisms are the foundation of Arctic food webs. Because Arctic food webs are simple, with few steps from small organisms to large predators, the loss of these base-level species could harm the entire ecosystem. For example, pteropods are an important food source for larger animals like fish, seabirds, and whales. Removing them could threaten the survival of these animals and the fisheries that depend on them.
The shellfish industry is an important part of the United Kingdom’s economy. In 2013, shellfish made up 37% of the total value of fish caught. England and Scotland are the largest producers of shellfish in the UK. Each year, fishers in these regions catch about 66,000 tons and 61,000 tons of shellfish, respectively. Wild-caught shellfish are worth about 203 million pounds annually. However, ocean acidification is slowing the growth of many shellfish species, causing significant economic losses in the UK.
It is predicted that by 2100, ocean acidification could reduce the UK’s shellfish production by 14 to 28%. This would result in economic losses of about 23 to 88 million pounds. These losses vary by region because of differences in the types of shellfish caught and how sensitive different species are to acidification. To protect shellfish resources and the economy, solutions may need to be developed at the local, national, or international level.
In 2007, the value of fish caught in US commercial fisheries was $3.8 billion. Of this, 73% came from calcifying organisms and their predators. Acidification harms other marine life as well. For example, slower growth in calcifying species like American lobsters, ocean quahogs, and scallops reduces the amount of shellfish available for sale and consumption. Red king crab fisheries are also at risk because crabs build shells. Baby red king crabs exposed to higher acidification levels experienced 100% death after 95 days. In 2006, red king crabs made up 23% of the total allowable catch. A decline in their population could seriously harm the crab fishing industry.
Possible responses
Reducing carbon dioxide emissions is the main way to solve ocean acidification. Some methods to reduce carbon dioxide include capturing it directly from the air or using plants and technology to store it underground. These methods can also slow the speed of ocean acidification.
Other methods remove carbon dioxide from the ocean by adding nutrients, moving water from deep to the surface, growing seaweed, helping ecosystems recover, increasing ocean alkalinity, breaking down rocks, or using electricity. These methods use the ocean to store carbon dioxide, but they may harm marine life. Research on these methods has increased since 2019.
Together, ocean-based methods could remove 1–100 gigatons of carbon dioxide each year. Their costs range from $40 to $500 per ton of carbon dioxide. For example, breaking down rocks could remove 2–4 gigatons of carbon dioxide yearly at a cost of $50–$200 per ton.
Some methods add minerals to the ocean to balance its pH, which can help marine life near the area where the minerals are added. These methods include increasing ocean alkalinity and using electricity. However, the effects of these minerals spread to distant waters over time, so they are called "local ocean acidification mitigation." These methods could work on a large scale but are expensive, risky, and not yet fully developed.
Ocean alkalinity enhancement is a method that adds minerals like limestone or olivine to the ocean. This increases the ocean’s ability to absorb carbon dioxide. The process mimics natural rock weathering, turning carbon dioxide into bicarbonate or calcium carbonate. Calcium carbonate can be buried in the deep ocean, storing carbon for a long time.
Enhanced weathering is a type of ocean alkalinity enhancement. It involves spreading finely crushed rocks into the ocean or on land, which increases alkalinity. This process can also help absorb carbon dioxide.
Adding alkalinity to the ocean helps reduce acidification but may affect marine life in unknown ways. For example, breaking down some rocks could release harmful metals.
Ocean alkalinity enhancement is costly and energy-intensive because of mining, crushing, and transporting materials. The cost is estimated at $20–$50 per ton of carbon dioxide.
About 30% of carbon emissions since the Industrial Revolution are stored as bicarbonate in the ocean.
Experiments use materials like limestone, olivine, and electricity to increase ocean alkalinity. Another method uses electricity during desalination to capture carbon dioxide from seawater.
Electrochemical methods, such as electrolysis, can remove carbon dioxide directly from seawater. These methods also increase ocean alkalinity by creating substances that absorb carbon dioxide. The hydrogen produced during this process can be used for energy or other purposes.
However, using electrolysis for carbon capture is expensive and uses a lot of energy. Research is still ongoing to understand its environmental effects. Possible problems include harmful chemicals in wastewater and reduced carbon levels in ocean water, which could harm marine life.
Policies and goals
As more people learn about ocean acidification, governments have created policies to improve monitoring of this issue. In 2015, ocean scientist Jean-Pierre Gattuso said, "The ocean has not been given enough attention in past climate talks. Our study shows strong reasons for major changes at the UN climate conference in Paris."
International efforts, like the Wider Caribbean's Cartagena Convention (which started in 1986), may help regional governments support areas that are highly affected by ocean acidification. Many countries, such as those in the Pacific Islands and Territories, have created regional policies, including National Ocean Policies, National Action Plans, and Joint National Action Plans on Climate Change and Disaster Risk Reduction. These efforts aim to help achieve Sustainable Development Goal 14. Ocean acidification is now being included in these plans.
The UN Ocean Decade has a program called "Ocean acidification research for sustainability." This program was proposed by the Global Ocean Acidification Observing Network (GOA-ON) and its partners. It has been officially approved as part of the UN Decade of Ocean Science for Sustainable Development. The program, called OARS, builds on GOA-ON's work and has these goals: to improve ocean acidification science, to increase observations of ocean chemistry changes, to study the effects on marine ecosystems at local and global levels, and to provide information to help leaders reduce and adapt to ocean acidification.
Ocean acidification is included as one of seven Global Climate Indicators. These indicators are measurements that describe climate changes without focusing only on rising temperatures. They cover important areas like temperature, atmospheric gases, oceans, water, and the cryosphere. Scientists and communication experts, led by the Global Climate Observing System (GCOS), identified these indicators. They were approved by the World Meteorological Organization (WMO). These indicators are used in the WMO's annual report on the global climate, which is sent to the Conference of Parties (COP) of the United Nations Framework Convention on Climate Change (UNFCCC). The European Commission's Copernicus Climate Change Service (C3S) also uses these indicators in its annual "European State of the Climate" report.
In 2015, the United Nations adopted the 2030 Agenda and 17 Sustainable Development Goals (SDGs), including a goal focused on the ocean, SDG 14. This goal calls for "conserving and using oceans, seas, and marine resources sustainably." Ocean acidification is directly addressed in Target 14.3, which states: "Minimize and address the impacts of ocean acidification, including through better scientific cooperation at all levels." This target has one indicator: "Average marine acidity (pH) measured at agreed representative sampling stations."
The Intergovernmental Oceanographic Commission (IOC) of UNESCO was chosen to manage the SDG 14.3.1 Indicator. In this role, IOC-UNESCO is responsible for creating the method to measure the indicator, collecting data each year, and reporting progress to the United Nations.
In the United States, the Federal Ocean Acidification Research and Monitoring Act of 2009 helps coordinate government efforts, such as NOAA's "Ocean Acidification Program." In 2015, the U.S. Environmental Protection Agency (USEPA) refused a request to regulate carbon dioxide under the Toxic Substances Control Act of 1976 to reduce ocean acidification. The EPA stated that ocean acidification risks were being "more efficiently and effectively addressed" through domestic actions, such as the Presidential Climate Action Plan, and through international efforts to reduce emissions, deforestation, and promote clean energy and energy efficiency.
History
Research on ocean acidification and efforts to raise awareness about the issue have continued for many years. The basic scientific work started with the development of the pH scale by Danish chemist Søren Peder Lauritz Sørensen in 1909. By the 1950s, experts understood that the ocean played a major role in absorbing carbon dioxide from fossil fuels, but this knowledge was not widely recognized by the broader scientific community. For much of the 20th century, the focus was on the positive effect of the ocean absorbing carbon dioxide, which helped reduce the effects of climate change. The idea that absorbing too much carbon dioxide could cause problems developed later, after important events occurred. The ocean’s ability to absorb heat and carbon dioxide remains vital as the main way to slow climate change.
In the early 1970s, scientists around the world began discussing the long-term effects of fossil fuel carbon dioxide building up in the ocean. These discussions led to debates about how this might affect marine life. By the mid-1990s, scientists studying coral reefs were concerned about the rising levels of carbon dioxide and the changes in ocean pH and carbonate ions.
By the end of the 20th century, the balance between the ocean’s benefits in absorbing heat and carbon dioxide and the harm to marine life became clearer. In 2003, when plans were made for the "First Symposium on the Ocean in a High-CO₂ World" in Paris in 2004, many new studies on ocean acidification were published.
In 2009, members of the InterAcademy Panel urged world leaders to "Recognize that reducing the buildup of carbon dioxide in the atmosphere is the only practical way to reduce ocean acidification." They also emphasized the need to "Reinvigorate action to reduce stressors, such as overfishing and pollution, on marine ecosystems to increase resilience to ocean acidification."
For example, research in 2010 found that between 1995 and 2010, acidity in the top 100 meters of the Pacific Ocean from Hawaii to Alaska increased by 6 percent.
In July 2012, Jane Lubchenco, head of the U.S. National Oceanic and Atmospheric Administration, stated that ocean surface waters are changing faster than early calculations suggested. She said this is another reason to be seriously concerned about the amount of carbon dioxide in the atmosphere and the additional emissions we continue to produce.
A 2013 study showed that ocean acidity is increasing 10 times faster than during any of Earth’s major evolutionary crises.
The "Third Symposium on the Ocean in a High-CO₂ World" was held in Monterey, California, in 2012. A summary for policy makers from the conference said that "Ocean acidification research is growing rapidly."
In a report published in Science in 2015, 22 leading marine scientists stated that carbon dioxide from burning fossil fuels is changing ocean chemistry faster than at any time since the Great Dying (Earth’s most severe known extinction event). Their report said that the 2°C maximum temperature increase agreed upon by governments is too small to prevent "dramatic impacts" on the world’s oceans.
A study from 2020 found that ocean acidification not only harms marine life but also affects human health. It can lower food quality, cause respiratory issues, and harm human health.