The ocean absorbs about 30% of the carbon dioxide (CO2) that is released in the atmosphere. As levels
of atmospheric CO2 increase from human activity such as burning fossil fuels (e.g., car emissions) and
changing land use (e.g., deforestation), the amount of carbon dioxide absorbed by the ocean also
increases. When CO2 is absorbed by seawater, a series of chemical reactions occur resulting in the
increased concentration of hydrogen ions. This process has far reaching implications for the ocean and
the creatures that live there.
The pH scale
The pH scale runs from 0 to 14, with 7 being a neutral pH. Anything higher than 7 is basic (or alkaline)
and anything lower than 7 is acidic. The pH scale is an inverse of hydrogen ion concentration, so more
hydrogen ions translates to higher acidity and a lower pH.
Carbon dioxide and seawater
Carbon dioxide, which is naturally in the atmosphere, dissolves into seawater. Water and carbon dioxide
combine to form carbonic acid (H2CO3), a weak acid that breaks (or “dissociates”) into hydrogen ions
(H+) and bicarbonate ions (HCO3-).
Because of human-driven increased levels of carbon dioxide in the atmosphere, there is more CO2
dissolving into the ocean. The ocean’s average pH is now around 8.1offsite link, which is basic (or
alkaline), but as the ocean continues to absorb more CO2, the pH decreases and the ocean becomes
more acidic.
Coral reef nursery in NOAA Florida Keys National Marine Sanctuary
NOAA unveils 10-year roadmap for tackling ocean, Great Lakes acidification
“Understanding how ocean acidification will affect marine life and the jobs and communities that
depend on it is critical to a healthy ocean and blue economy,” said Kenric Osgood, Ph.D., chief of the
Marine Ecosystems Division, Office of Science and Technology at NOAA Fisheries Service.
Impacts of ocean acidification on shell builders
Ocean acidification is already impacting many ocean species, especially organisms like oysters and corals
that make hard shells and skeletons by combining calcium and carbonate from seawater. However, as
ocean acidification increases, available carbonate ions (CO32-) bond with excess hydrogen, resulting in
fewer carbonate ions available for calcifying organisms to build and maintain their shells, skeletons, and
other calcium carbonate structures. If the pH gets too low, shells and skeletons can even begin to
dissolve.
The pteropod, or "sea butterfly," is a tiny sea snail about the size of a small pea. Pteropods are an
important part of many food webs and eaten by organisms ranging in size from tiny krill to whales.
When pteropod shells were placed in sea water with pH and carbonate levels projected for the year
2100, the shells slowly dissolved after 45 days. Researchers have already discovered severe levels of
pteropod shell dissolutionoffsite link in the Southern Ocean, which encircles Antarctica.
Avery Resor and Catherine O'Hare check on the health of their Gracilaria crop, growing attached to a
long line of rope submerged at Hog Island Oyster Farm.
Farming seaweed can improve water quality
Native seaweed has the potential to be cultivated in California coastal waters and used to alleviate the
effects of local ocean acidification, according to a new study funded by NOAA's California Sea Grant.
Ocean acidification impacts on fish and seaweeds
Changes in ocean chemistry can affect the behavior of non-calcifying organisms as well. The ability of
some fish, like clownfish, to detect predators is decreased in more acidic waters. Studies have shown
that decreased pH levels also affect the ability of larval clownfishoffsite link to locate suitable habitat.
When these organisms are at risk, the entire food web may also be at risk.
While some species will be harmed by ocean acidification, algae and seagrasses may benefit from higher
CO2 conditions in the ocean, as they require CO2 for photosynthesis just like plants on land. There are
some ongoing studies examining if growing seaweed can help slow ocean acidification.
NOAA and cooperative institute scientists are releasing Deep Argo floats off Hawaii to test them in
preparation for the major deployment NOAA and Paul G. Allen Family Foundation will conduct next year
to create the the Atlantic Ocean array in international waters off Brazil.
NOAA invests in new tools to measure the ocean
Four new research projects are giving a boost to NOAA’s ability to measure, track and forecast ocean
acidification, warming and other important ocean health indicators.
Our changing ocean
Estimates of future carbon dioxide levels, based on business-as-usual emission scenarios, indicate that
by the end of this century the surface waters of the ocean could have a pH around 7.8 The last time the
ocean pH was this low was during the middle Miocene, 14-17 million years ago. The Earth was several
degrees warmer and a major extinction event was occurring.
Ocean acidification is currently affecting the entire ocean, including coastal estuaries and waterways.
Billions of people worldwide rely on food from the ocean as their primary source of protein. Many jobs
and economies in the U.S. and around the world depend on the fish and shellfish that live in the ocean.
Current research
Ocean acidification is one aspect of global climate change. Anything we do to mitigate climate change
today will benefit the future of the ocean as well. Over the last decade, there has been much focus in
the ocean science community on studying the potential impacts of ocean acidification. NOAA's Ocean
Acidification Program serves to build relationships between scientists, resource managers, policy
makers, and the public in order to research and monitor the effects of changing ocean chemistry on
economically and ecologically important ecosystems such as fisheries and coral reefs.
Because sustained efforts to monitor ocean acidification worldwide are only beginning, it is currently
impossible to predict exactly how ocean acidification impacts will cascade throughout the marine food
web and affect the overall structure of marine ecosystems. With the pace of ocean acidification
accelerating, scientists, resource managers, and policymakers recognize the urgent need to strengthen
the science as a basis for sound decision making and action.
Dungeness crab harvested alongside some fish on the deck of a fishing boat.
Popular crab might feel the pinch from ocean acidification
A NOAA-funded study has documented that ocean acidification along the U.S. Pacific Northwest coast is
impacting the shells and sensory organs of some young Dungeness crab, a prized crustacean that
supports the most valuable fishery on the West Coast.
EDUCATION CONNECTION
Ocean acidification is a problem that impacts the ocean ecosystem as well as commercial industries like
oyster farms. This topic can be taught in conjunction with lessons about food webs and ecosystems, the
environmental impacts of climate change and CO2 emissions, and chemistry lessons concerning real-life
applications. Students can explore data, including real-time information about carbon dioxide levels in
seawater and in the atmosphere.
Ocean acidification is the decrease in the pH of the Earth's ocean. Between 1950 and 2020, the average
pH of the ocean surface fell from approximately 8.15 to 8.05.[2] Carbon dioxide emissions from human
activities are the primary cause of ocean acidification, with atmospheric carbon dioxide (CO2) levels
exceeding 410 ppm (in 2020). CO2 from the atmosphere is absorbed by the oceans. This produces
carbonic acid (H2CO3) which dissociates into a bicarbonate ion (HCO
−
3
) and a hydrogen ion (H+). The presence of free hydrogen ions (H+) lowers the pH of the ocean,
increasing acidity (this does not mean that seawater is acidic yet; it is still alkaline, with a pH higher than
8). Marine calcifying organisms, such as mollusks and corals, are especially vulnerable because they rely
on calcium carbonate to build shells and skeletons.[3]
A change in pH by 0.1 represents a 26% increase in hydrogen ion concentration in the world's oceans
(the pH scale is logarithmic, so a change of one in pH units is equivalent to a tenfold change in hydrogen
ion concentration). Sea-surface pH and carbonate saturation states vary depending on ocean depth and
location. Colder and higher latitude waters are capable of absorbing more CO2. This can cause acidity to
rise, lowering the pH and carbonate saturation levels in these areas. Other factors that influence the
atmosphere-ocean CO2 exchange, and thus local ocean acidification, include: ocean currents and
upwelling zones, proximity to large continental rivers, sea ice coverage, and atmospheric exchange with
nitrogen and sulfur from fossil fuel burning and agriculture.[4][5][6]
Decreased ocean pH has a range of potentially harmful effects for marine organisms. These include
reduced calcification, depressed metabolic rates, lowered immune responses, and reduced energy for
basic functions such as reproduction.[7] The effects of ocean acidification are therefore impacting
marine ecosystems that provide food, livelihoods, and other ecosystem services for a large portion of
humanity. Some 1 billion people are wholly or partially dependent on the fishing, tourism, and coastal
management services provided by coral reefs. Ongoing acidification of the oceans may therefore
threaten food chains linked with the oceans.[8][9]
The United Nations Sustainable Development Goal 14 ("Life below Water") has a target to "minimize
and address the impacts of ocean acidification".[10] Reducing carbon dioxide emissions (i.e., climate
change mitigation measures) is the only solution that addresses the root cause of ocean acidification.
Mitigation measures which achieve carbon dioxide removal from the atmosphere would help to reverse
ocean acidification. The more specific ocean-based mitigation methods (e.g. ocean alkalinity
enhancement, enhanced weathering) could also reduce ocean acidification. These strategies are being
researched, but generally have a low technology readiness level and many risks.[11][12][13]
Ocean acidification has occurred previously in Earth's history.[14] The resulting ecological collapse in the
oceans had long-lasting effects on the global carbon cycle and climate.
Cause
Spatial distribution of global surface ocean pH (Panel a: the annually-averaged surface ocean pH to be
approximate for the year 1770; Panel b: the difference between pH in 2000 and 1770 in the global
surface ocean).[4]
This diagram of the fast carbon cycle shows the movement of carbon between land, atmosphere, and
oceans. Yellow numbers are natural fluxes, and red are human contributions in gigatons of carbon per
year. White numbers indicate stored carbon.[15]
3:59
Video summarizing the impacts of ocean acidification. Source: NOAA Environmental Visualization
Laboratory.
See also: Oceanic carbon cycle
Present-day (2021) atmospheric carbon dioxide (CO2) levels of around 415 ppm are around 50% higher
than preindustrial concentrations.[16] The current elevated levels and rapid growth rates are
unprecedented in the past 55 million years of the geological record. The sources of this excess CO2 are
clearly established as human driven: they include anthropogenic fossil fuel, industrial, and landuse/land-change emissions. The ocean acts as a carbon sink for anthropogenic CO2 and takes up roughly
a quarter of total anthropogenic CO2 emissions.[17] However, the additional CO2 in the ocean results in
a wholesale shift in seawater acid-base chemistry toward more acidic, lower pH conditions and lower
saturation states for carbonate minerals used in many marine organism shells and skeletons.[17]
Accumulated since 1850, the ocean sink holds up to 175 ± 35 gigatons of carbon, with more than twothirds of this amount (120 GtC) being taken up by the global ocean since 1960. Over the historical
period, the ocean sink increased in pace with the exponential anthropogenic emissions increase. From
1850 until 2022, the ocean has absorbed 26 % of total anthropogenic emissions.*16+ Emissions during
the period 1850–2021 amounted to 670 ± 65 gigatons of carbon and were partitioned among the
atmosphere (41 %), ocean (26 %), and land (31 %).*16+
The carbon cycle describes the fluxes of carbon dioxide (CO
2) between the oceans, terrestrial biosphere, lithosphere,[18] and atmosphere. The carbon cycle
involves both organic compounds such as cellulose and inorganic carbon compounds such as carbon
dioxide, carbonate ion, and bicarbonate ion, together referenced as dissolved inorganic carbon (DIC).
These inorganic compounds are particularly significant in ocean acidification, as they include many
forms of dissolved CO
2 present in the Earth's oceans.[19]
When CO
2 dissolves, it reacts with water to form a balance of ionic and non-ionic chemical species: dissolved free
carbon dioxide (CO
2(aq)), carbonic acid (H
2CO
3), bicarbonate (HCO−
3) and carbonate (CO2−
3). The ratio of these species depends on factors such as seawater temperature, pressure and salinity (as
shown in a Bjerrum plot). These different forms of dissolved inorganic carbon are transferred from an
ocean's surface to its interior by the ocean's solubility pump. The resistance of an area of ocean to
absorbing atmospheric CO
2 is known as the Revelle factor.
Main effects
The ocean's chemistry is changing due to the uptake of anthropogenic carbon dioxide (CO2).*4+*20+: 395
Ocean pH, carbonate ion concentrations (*CO32−+), and calcium carbonate mineral saturation states (Ω)
have been declining as a result of the uptake of approximately 30% of the anthropogenic carbon dioxide
emissions over the past 270 years (since around 1750). This process, commonly referred to as "ocean
acidification", is making it harder for marine calcifiers to build a shell or skeletal structure, endangering
coral reefs and the broader marine ecosystems.[4]
Ocean acidification has been called the "evil twin of global warming" and "the other CO2
problem".[21][22] Increased ocean temperatures and oxygen loss act concurrently with ocean
acidification and constitute the "deadly trio" of climate change pressures on the marine
environment.[23] The impacts of this will be most severe for coral reefs and other shelled marine
organisms,[24][25] as well as those populations that depend on the ecosystem services they provide.
Reduction in pH value
See also: Ocean § pH and alkalinity
Dissolving CO
2 in seawater increases the hydrogen ion (H+
) concentration in the ocean, and thus decreases ocean pH, as follows:[26]
CO2 (aq) + H2O ⇌ H2CO3 ⇌ HCO3− + H+ ⇌ CO32− + 2 H+.
In shallow coastal and shelf regions, a number of factors interplay to affect air-ocean CO2 exchange and
resulting pH change.[27][28] These include biological processes, such as photosynthesis and
respiration,[29] as well as water upwelling.[30] Also, ecosystem metabolism in freshwater sources
reaching coastal waters can lead to large, but local, pH changes.[27]
Freshwater bodies also appear to be acidifying, although this is a more complex and less obvious
phenomenon.[31][32]
The absorption of CO2 from the atmosphere does not affect the ocean's alkalinity.*33+: 2252 This is
important to know in this context as alkalinity is the capacity of water to resist acidification.[34] Ocean
alkalinity enhancement has been proposed as one option to add alkalinity to the ocean and therefore
buffer against pH changes.
Decreased calcification in marine organisms
Various types of foraminifera observed through a microscope using differential interference contrast.
Bjerrum plot: Change in carbonate system of seawater from ocean acidification
Changes in ocean chemistry can have extensive direct and indirect effects on organisms and their
habitats. One of the most important repercussions of increasing ocean acidity relates to the production
of shells out of calcium carbonate (CaCO3).[3] This process is called calcification and is important to the
biology and survival of a wide range of marine organisms. Calcification involves the precipitation of
dissolved ions into solid CaCO3 structures, structures for many marine organisms, such as
coccolithophores, foraminifera, crustaceans, mollusks, etc. After they are formed, these CaCO3
structures are vulnerable to dissolution unless the surrounding seawater contains saturating
concentrations of carbonate ions (CO
2−
3
).
Very little of the extra carbon dioxide that is added into the ocean remains as dissolved carbon dioxide.
The majority dissociates into additional bicarbonate and free hydrogen ions. The increase in hydrogen is
larger than the increase in bicarbonate,[35] creating an imbalance in the reaction:
HCO
−
3
⇌ CO
2−
3
+ H+
To maintain chemical equilibrium, some of the carbonate ions already in the ocean combine with some
of the hydrogen ions to make further bicarbonate. Thus the ocean's concentration of carbonate ions is
reduced, removing an essential building block for marine organisms to build shells, or calcify:
Ca2+ + CO
2−
3
⇌ CaCO3
The increase in concentrations of dissolved carbon dioxide and bicarbonate, and reduction in carbonate,
are shown in the Bjerrum plot.
Decrease in saturation state
Distribution of (A) aragonite and (B) calcite saturation depth in the global oceans[36]
The saturation state (known as Ω) of seawater for a mineral is a measure of the thermodynamic
potential for the mineral to form or to dissolve, and for calcium carbonate is described by the following
equation:
Ω
=
[
Ca
2
+
]
[
CO
3
2
−
]
�
�
�
{\displaystyle {\Omega }={\frac {\left[{\ce {Ca^2+}}\right]\left[{\ce {CO3^2-}}\right]}{K_{sp}}}}
Here Ω is the product of the concentrations (or activities) of the reacting ions that form the mineral
(Ca2+ and CO2−3), divided by the apparent solubility product at equilibrium (Ksp), that is, when the
rates of precipitation and dissolution are equal.[37] In seawater, dissolution boundary is formed as a
result of temperature, pressure, and depth, and is known as the saturation horizon.[3] Above this
saturation horizon, Ω has a value greater than 1, and CaCO
3 does not readily dissolve. Most calcifying organisms live in such waters.*3+ Below this depth, Ω has a
value less than 1, and CaCO
3 will dissolve. The carbonate compensation depth is the ocean depth at which carbonate dissolution
balances the supply of carbonate to sea floor, therefore sediment below this depth will be void of
calcium carbonate.[38] Increasing CO2 levels, and the resulting lower pH of seawater, decreases the
concentration of CO32− and the saturation state of CaCO
3 therefore increasing CaCO
3 dissolution.
Calcium carbonate most commonly occurs in two common polymorphs (crystalline forms): aragonite
and calcite. Aragonite is much more soluble than calcite, so the aragonite saturation horizon, and
aragonite compensation depth, is always nearer to the surface than the calcite saturation horizon.[3]
This also means that those organisms that produce aragonite may be more vulnerable to changes in
ocean acidity than those that produce calcite.[39] Ocean acidification and the resulting decrease in
carbonate saturation states raise the saturation horizons of both forms closer to the surface.[3] This
decrease in saturation state is one of the main factors leading to decreased calcification in marine
organisms because the inorganic precipitation of CaCO
3 is directly proportional to its saturation state and calcifying organisms exhibit stress in waters with
lower saturation states.[40]
Natural variability and climate feedbacks
Further information: Carbonate compensation depth
Already now large quantities of water undersaturated in aragonite are upwelling close to the Pacific
continental shelf area of North America, from Vancouver to Northern California.[41] These continental
shelves play an important role in marine ecosystems, since most marine organisms live or are spawned
there. Other shelf areas may be experiencing similar effects.[41]
At depths of 1000s of meters in the ocean, calcium carbonate shells begin to dissolve as increasing
pressure and decreasing temperature shift the chemical equilibria controlling calcium carbonate
precipitation.[42] The depth at which this occurs is known as the carbonate compensation depth. Ocean
acidification will increase such dissolution and shallow the carbonate compensation depth on timescales
of tens to hundreds of years.[42] Zones of downwelling are being affected first.[43]
In the North Pacific and North Atlantic, saturation states are also decreasing (the depth of saturation is
getting more shallow).*20+: 396 Ocean acidification is progressing in the open ocean as the CO2 travels
to deeper depth as a result of ocean mixing. In the open ocean, this causes carbonate compensation
depths to become more shallow, meaning that dissolution of calcium carbonate will occur below those
depths. In the North Pacific these carbonate saturations depths are shallowing at a rate of 1-2 m per
year.*20+: 396
It is expected that ocean acidification in the future will lead to a significant decrease in the burial of
carbonate sediments for several centuries, and even the dissolution of existing carbonate
sediments.[44]
Measured and estimated values
Present day and recent history
Time series of atmospheric CO2 at Mauna Loa (in parts per million volume, ppmv; red), surface ocean
pCO2 (µatm; blue) and surface ocean pH (green) at Ocean Station ALOHA in the subtropical North Pacific
Ocean.[45]
World map showing the varying change to pH across different parts of different oceans
Estimated change in seawater pH caused by anthropogenic impact on CO
2 levels between the 1700s and the 1990s, from the Global Ocean Data Analysis Project (GLODAP) and
the World Ocean Atlas
Between 1950 and 2020, the average pH value of the ocean surface is estimated to have decreased from
approximately 8.15 to 8.05.[2] This represents an increase of around 26% in hydrogen ion concentration
in the world's oceans (the pH scale is logarithmic, so a change of one in pH unit is equivalent to a tenfold
change in hydrogen ion concentration).[46] For example, in the 15-year period 1995–2010 alone, acidity
has increased 6 percent in the upper 100 meters of the Pacific Ocean from Hawaii to Alaska.[47]
The IPCC Sixth Assessment Report in 2021 stated that "present-day surface pH values are
unprecedented for at least 26,000 years and current rates of pH change are unprecedented since at
least that time.*48+: 76 The pH value of the ocean interior has declined over the last 20-30 years
everywhere in the global ocean.*48+: 76 The report also found that "pH in open ocean surface water has
declined by about 0.017 to 0.027 pH units per decade since the late 1980s".[49]: 716
The rate of decline differs by region. This is due to complex interactions between different types of
forcing mechanisms:*49+: 716 "In the tropical Pacific, its central and eastern upwelling zones exhibited a
faster pH decline of minus 0.022 to minus 0.026 pH unit per decade." This is thought to be "due to
increased upwelling of CO2-rich sub-surface waters in addition to anthropogenic CO2 uptake."*49+: 716
Some regions exhibited a slower acidification rate: a pH decline of minus 0.010 to minus 0.013 pH unit
per decade has been observed in warm pools in the western tropical Pacific.*49+: 716
The rate at which ocean acidification will occur may be influenced by the rate of surface ocean warming,
because warm waters will not absorb as much CO2.[50] Therefore, greater seawater warming could
limit CO2 absorption and lead to a smaller change in pH for a given increase in CO2.[50] The difference
in changes in temperature between basins is one of the main reasons for the differences in acidification
rates in different localities.
Current rates of ocean acidification have been likened to the greenhouse event at the Paleocene–
Eocene boundary (about 56 million years ago), when surface ocean temperatures rose by 5–6 degrees
Celsius. In that event, surface ecosystems experienced a variety of impacts, but bottom-dwelling
organisms in the deep ocean actually experienced a major extinction.[51] Currently, the rate of carbon
addition to the atmosphere-ocean system is about ten times the rate that occurred at the Paleocene–
Eocene boundary.[52]
Extensive observational systems are now in place or being built for monitoring seawater CO2 chemistry
and acidification for both the global open ocean and some coastal systems.[17]