Ocean Acidification

Ocean Acidification Threatens Marine Food Web and Coastal Economies

The ocean has absorbed an estimated 26 percent of the carbon dioxide (CO2) released into the atmosphere since the start of the Industrial Revolution in 1750. This uptake of CO2 from the atmosphere is a major contributor to the decline in seawater pH known as ocean acidification. Both human-caused ocean acidification and human-caused climate change are due to rising CO2 concentrations in the atmosphere. The global ocean’s absorption of CO2 may have slowed several of the impacts of climate change that humans would have otherwise experienced on land due to a warmer atmosphere, but this “buffering” service comes at a heavy cost.

Acidification is having a significant impact on the chemical composition of seawater, especially in colder basins like the Arctic where CO2 dissolves more readily. This change in ocean chemistry is threatening the healthy development of many marine species, and poses risks to humans who depend on oceans for their livelihoods. The U.S. is no exception: in the Pacific Northwest, acidification has already cost the farmed oyster industry almost $110 million—jeopardizing 3,200 jobs—and 16 coastal regions are expected to experience rapid acidification by mid-century.

The following primer presents an overview of ocean acidification science, including the chemistry behind human-caused acidification, its impacts and the actions being taken to address it.

Observed and Projected Ocean Acidification

This graph shows the correlation between rising levels of CO2 in the atmosphere from the Mauna Loa, Hawaii CO2 record with rising CO2 levels in the nearby ocean. As more CO2 accumulates in the ocean, the pH of the ocean decreases. Source: NOAA.

Since the beginning of the industrial era, the pH of surface seawater has decreased from about 8.2 to 8.1. While the ocean is “basic” because it is above 7 on the pH scale, dissolved CO2 acidifies the ocean by increasing the concentration of hydrogen ion. The pH change from 8.2 to 8.1 corresponds to a 26 percent increase in acidity (or hydrogen ion concentration) and is likely the fastest acidification rate in 300 million years. Without concerted action to reduce global CO2 emissions, oceanic pH could drop to 7.8 by the end of this century. That would be a huge change, representing a 150-percent increase in acidity. Such an alteration in the marine environment could have devastating results, for any marine species that extracts calcium carbonate to build its shell or skeleton, and for the people who depend upon them.

The Known Effects of Excess CO2 in the Ocean

Changing Ocean Chemistry

The ocean’s absorption of excess CO2 is changing its chemical composition, which affects marine ecosystems. Excessive uptake of CO2 most directly impacts the marine carbonate system (a system based on the carbonate ion, CO32–, and its uses), which controls seawater acidity. As CO2 dissolves in seawater (H2O), it forms carbonic acid (H2CO3), a weak acid that breaks apart into bicarbonate (HCO3–) and hydrogen ion (H+). By definition, more hydrogen ions (H+) result in increased acidity (lower pH). The rate of acidification, however, is limited by the presence of carbonate ion (CO32–), which binds up most of the newly formed hydrogen (H+), forming more bicarbonate (HCO3–). At the current pH of 8.1, approximately 90 percent of the carbon in the ocean occurs in the form of bicarbonate ion (HCO3–), 9 percent in the form of carbonate ion (CO32–), and only about 1 percent of the carbon is in the form of dissolved CO2. Under normal seawater conditions, more than 99.99 percent of the hydrogen (H+) ion will combine with carbonate ion (CO32–) to form bicarbonate (HCO3–). But as the ocean absorbs excess CO2, there is not enough carbonate ion (CO32–) to go around.

In other words, as humans release more CO2 into the atmosphere, the ocean’s ability to absorb CO2 becomes exhausted and more CO2 remains in the air.

Source: NOAA.

Human-caused climate change is also warming the ocean. During the past 50 years, the ocean has stored an estimated 93 percent of excess heat energy due to climate change. As the ocean warms, CO2 solubility in seawater decreases, reducing the volume of CO2 the oceans can absorb from the atmosphere. Both the reduced CO2 solubility caused by ocean warming and the limited presence of carbonate ions (CO32–) have reduced the ocean’s current ability to protect against atmospheric climate change, or its ‘buffering capacity’, to only 70 percent of what it was at the beginning of the industrial era. This may be reduced to only 20 percent by the end of the twenty-first century.

Affected Marine Ecosystems

Marine ecosystems are highly susceptible to acidification. The carbonate ion (CO32—) that is used to bind excess hydrogen ion (H+) is critical to some marine animals that extract carbonate ion and calcium ion (Ca2+) to form the solid crystals of calcium carbonate (CaCO­3) that build their shells. The extra energy that these animals must expend to extract calcium carbonate leaves them less energy to feed, grow to adulthood, reproduce and in some cases, hide from predators.

Among the species already showing serious stress from a decline in seawater pH are many shellfish we enjoy eating, including oysters, clams, mussels and scallops, as well as the favorite prey of Pacific pink salmon, a tiny sea snail known as a pteropod. More than a billion people worldwide currently rely on food from the ocean as their primary source of protein. Species that may be harmed beyond repair in coming decades include essential parts of the marine food web like corals, crustaceans and plankton. All of these species are essential to healthy fisheries and are major contributors to tourism and related jobs in coastal communities.

Important calcifying organisms on coral reefs: (A) reef-building corals, (B) coralline algae, (C) Halimeda algae, (D) Penicillus algae, (E) reef sand composed of benthic foraminifera. Source: NOAA.

Coral reefs, in particular provideessential ecosystem services to countless marine species. If they decline—due to accelerating acidification, destructive fishing practices, pollution and warming waters—other marine life will weaken with them, resulting in less vibrant and less productive oceans. Dead and dying coral reefs, in turn, will result in increased economic hardship for coastal populations.


Decreased skeletal growth in reef-building corals and coralline algae is one of the best-known consequences of ocean acidification. This chart shows the change in calcification rate within coral ecosystems as atmospheric CO2­ concentrations increase, showing 280 parts per million (ppm) in the pre-industrial era, 390 ppm around present day, and 560, two times the pre-industrial value. Source: NOAALangdon et al., 2003.

Regional Variation

Certain regions will be impacted more than others. The northeastern Pacific Ocean, including the Arctic and sub-Arctic seas, is particularly susceptibleto significant shifts in pH and available calcium carbonate (CaCO3) ion. This is because the colder, upwelling seawater in this region absorbs more CO2, leading to much faster changes in pH than marine scientists anticipated. A recent studyfound that rising ocean acidity has already cost the oyster industry almost $110 million in the Pacific Northwest and is expected to threaten the shellfish industry in Northeastern and Mid-Atlantic waters. According to the study, U.S. coastal communities earn over $1 billion in revenue from mollusks, but 16 out of 23 of these coastal regions will face “rapid acidification” by mid-century. Other analyses show that large areas of the oceans along the US west coast, theBering Sea and the western Arctic Ocean will also become difficult for calcifying animals within the next 50 years. For example, a recent study found that even crustaceans in their larval and juvenile stages, such as New England lobster and Alaskan red king crab may soon be affected by rapid declines in seawater pH, which has implications for lucrative U.S. seafood exports.

Advancing Our Understanding of Acidification

Efforts to measure pH changes in seawater are accelerating. European scientists were among the first to deploy many sophisticated sensors in cold waters, a project involving more than 100 scientists with the European Project on Ocean Acidification (EPOCA). In the U.S., oceanographers with the National Oceanic and Atmospheric Administration (NOAA) have been measuring ocean CO2 absorption for more than 30 years. They have also been studying and monitoring how rapidly changing ocean chemistry—driven by this absorption—affects marine ecosystems.

Location of planned U.S. ocean acidification monitoring and research sites and affiliated NOAA labs. Source: NOAA.

Now NOAA and various U.S. state scientists, together with aquaculture operators and fishermen—such as those working in Washington, Oregon, California, Maine and Maryland—have begun research collaborations to determine whether and how fast acidification is happening in their own coastal waters. However, much more research is needed. Scientists have also begun to actively communicate about the urgent need to reduce atmospheric greenhouse gases and begin planning for imminent ocean changes.

Brief Review of Scientific Research To-Date

The IPCC Fifth Assessment Report (AR5), Working Group I, Chapter 3

“The observed decrease in ocean pH of 0.1 since the beginning of the industrial era corresponds to a 26% increase in the hydrogen ion concentration [H+] concentration of seawater (Orr et al., 2005bFeely et al., 2009). The consequences of changes in pH, CO32–, and the saturation state of CaCO3 minerals for marine organisms and ecosystems are just beginning to be understood.”

“When CO2 reacts with seawater, it forms carbonic acid (H2CO3), which is highly reactive and reduces the concentration of carbonate and can affect shell formation for marine animals such as corals, plankton and shellfish. This process could affect fundamental biological and chemical processes of the sea in coming decades (Fabry et al., 2008;Doney et al., 2009).”

“A global mean decrease in surface water pH of 0.08 from 1765 to 1994 was calculated based on the inventory of anthropogenic CO2 (Sabine et al., 2004), with the largest reduction (–0.10) in the northern North Atlantic and the smallest reduction (–0.05) in the subtropical South Pacific. These regional variations in the size of the pH decrease are consistent with the generally lower buffer capacities of the high latitude oceans compared to lower latitudes (Egleston et al., 2010).”

“Estimates of future atmospheric and oceanic carbon dioxide concentrations indicate that, by the end of this century, the average surface ocean pH could be lower than it has been for more than 50 million years (Caldeira and Wickett, 2003).”

The Third U.S. National Climate Assessment

“As human-induced emissions of carbon dioxide (CO2) build up in the atmosphere, excess CO2 is dissolving into the oceans where it reacts with seawater to form carbonic acid, lowering ocean pH levels (“acidification”) and threatening a number of marine ecosystems (Doney et al., 2009). Currently, the ocean absorbs about a quarter of the CO2 humans produce every year (Le Quéré et al., 2009). Over the last 250 years, the oceans have absorbed 560 billion tons of CO2, increasing the acidity of surface waters by 30% (Caldeira and Wickett, 2003Feely et al., 2009Orr et al., 2005). Although the average oceanic pH can vary on interglacial timescales (Caldeira and Wickett, 2003), the current observed rate of change is roughly 50 times faster than known historical change (Hönisch et al., 2012Orr, 2011). Regional factors such as coastal upwelling (Feely et al., 2008), changes in discharge rates from rivers and glaciers (Mathis et al., 2011), sea ice loss (Yamamoto-Kawai et al., 2009), and urbanization (Feely et al., 2010) have created ‘ocean acidification hotspots,’ where changes are occurring at even faster rates.”


For more background about acidification and about US federal, regional and state-specific collaboration efforts to address the ecological and economic impacts of acidification:



Federal government acidification experts

  • Dr. Richard A. Feely of the Ocean Climate Research team
  • Dr. Libby Jewett, Director of NOAA’s Ocean Acidification Program,