From Wikipedia, the free encyclopedia
Ocean acidification is the name given to the
ongoing decrease in the pH of the
Earth's oceans, caused by their
uptake of anthropogenic carbon dioxide from the atmosphere.
Between 1751 and 1994 surface ocean pH is estimated to have
decreased from approximately 8.179 to 8.104 (a change of
cycle describes the fluxes of carbon dioxide (CO2)
between the oceans, terrestrial
biosphere, lithosphere, and
the atmosphere. Human
activities such as land-use changes, the combustion of fossil fuels, and the production of cement have led to a new flux of
CO2 into the
atmosphere. Some of this has remained there; some has been taken up
by terrestrial plants,
and some has been absorbed by the oceans.
The carbon cycle comes in two forms: the organic carbon cycle
and the inorganic carbon cycle. The inorganic carbon cycle is
particularly relevant when discussing ocean acidification for it
includes the many forms of dissolved CO2 present in the
When CO2 dissolves, it
reacts with water to form a balance of ionic and non-ionic chemical species: dissolved
free carbon dioxide (CO2(aq)), carbonic acid
(H2CO3), bicarbonate (HCO
The ratio of these species depends on factors such as seawater temperature and alkalinity (see the
article on the ocean's solubility pump for more detail).
Average surface ocean pH
|Recent past (1990s)
|2050 (2×CO2 = 560 ppm)
Dissolving CO2 in seawater
increases the hydrogen ion
(H+) concentration in
the ocean, and thus decreases ocean pH. Caldeira and Wickett (2003)
placed the rate and magnitude of modern ocean acidification changes
in the context of probable historical changes during the last 300
Since the industrial
revolution began, it is estimated that surface ocean pH has
dropped by slightly less than 0.1 units (on the logarithmic scale of pH;
approximately a 25% increase in H+), and it is
estimated that it will drop by a further 0.3 to 0.5 units by 2100
as the oceans absorb more anthropogenic CO2.
These changes are predicted to continue rapidly as the oceans take
up more anthropogenic CO2 from the
atmosphere, the degree of change to ocean chemistry, for example
ocean pH, will depend on the mitigation and emissions pathways
society takes. Note
that, although the ocean is acidifying, its pH is still greater
than 7 (that of neutral water),
so the ocean could also be described as becoming less basic.
Although the largest changes are expected in the future, a
report from NOAA scientists found large quantities of
water undersaturated in aragonite are already upwelling close to the
Pacific continental shelf area of North
Continental shelves play an important role in marine ecosystems
since most marine organisms live or are spawned there, and though the study
only dealt with the area from Vancouver to northern California, the authors
suggest that other shelf areas may be experiencing similar
Similarly, one of the first detailed datasets examining temporal
variations in pH at a temperate coastal location found that
acidification was occurring at a rate much higher than that
previously predicted, with consequences for near-shore benthic
Changes in ocean chemistry can have extensive direct and
indirect effects on organisms and the habitats in which they live.
One of the most important repercussions of increasing ocean acidity
relates to the production of shells and plates out of calcium
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,
such as coccoliths.
After they are formed, such structures are vulnerable to dissolution unless the
surrounding seawater contains saturating concentrations of
carbonate ions. The saturation state of seawater for a mineral
(known as Ω) is a measure of the thermodynamic potential for the
mineral to form or to dissolve, and is described by the following
Here Ω is the product of the concentrations (or activities) of the reacting ions
that form the mineral (Ca
divided by the product of the concentrations of those ions when the
mineral is at equilibrium (Ksp), that is, when
the mineral is neither forming nor dissolving. In
seawater, a natural horizontal boundary is formed as a result of
temperature, pressure, and depth, and is known as the saturation
horizon, or lysocline.
Above this saturation horizon, Ω has a value greater than 1, and
CaCO3 does not
readily dissolve. Most calcifying organisms live in such
Below this depth, Ω has a value less than 1, and CaCO3
will dissolve. However, if its production rate is high enough to
offset dissolution, CaCO3 can still occur
where Ω is less than 1. The carbonate compensation
depth occurs at the depth in the ocean where production is
exceeded by dissolution.
Calcium carbonate occurs in 2 common polymorphs: aragonite and calcite. Aragonite is much more
soluble than calcite, with the result that the aragonite saturation
horizon is always nearer to the surface than the calcite saturation
This also means that those organisms that produce aragonite may
possibly be more vulnerable to changes in ocean acidity than those
which produce calcite.
Increasing CO2 levels and the
resulting lower pH of seawater decreases the saturation state of
CaCO3 and raises the
saturation horizons of both forms closer to the surface. This
decrease in saturation state is believed to be one of the main
factors leading to decreased calcification in marine organisms, as
it has been found that the inorganic precipitation of CaCO3
is directly proportional to its saturation state.
Although the natural absorption of CO2 by the
world's oceans helps mitigate the climatic effects of anthropogenic
emissions of CO2, it is believed
that the resulting decrease in pH will have negative consequences,
primarily for oceanic calcifying organisms. These span the food chain from autotrophs to heterotrophs and
include organisms such as coccolithophores, corals, foraminifera, echinoderms, crustaceans and molluscs. As described above,
under normal conditions, calcite and aragonite are stable in
surface waters since the carbonate ion is at supersaturating
concentrations. However, as ocean pH falls, so does the
concentration of this ion, and when carbonate becomes
undersaturated, structures made of calcium carbonate are vulnerable
Research has already found that corals,
experience reduced calcification or enhanced dissolution when
exposed to elevated CO2. The Royal Society of
London published a comprehensive overview of ocean
acidification, and its potential consequences, in June 2005.
However, some studies have found different response to ocean
acidification, with coccolithophore calcification and
photosynthesis both increasing under elevated atmospheric
an equal decline in primary production and calcification in
response to elevated CO2
or the direction of the response varying between species.
Recent work examining a sediment core from the North Atlantic
found that while the species composition of coccolithophorids has
remained unchanged for the industrial
period 1780 to 2004, the calcification of coccoliths has increased
by up to 40% during the same time.
While the full ecological
consequences of these changes in calcification are still uncertain,
it appears likely that many calcifying species will be adversely
affected. There is also a suggestion that a decline in the
coccolithophores may have secondary effects on climate change, by
decreasing the Earth's albedo
via their effects on oceanic cloud cover.
Aside from calcification, organisms may suffer other adverse
effects, either directly as reproductive or physiological effects
acidification of body fluids, known as hypercapnia), or indirectly through
negative impacts on food resources.
Ocean acidification may also force some organisms to reallocate
resources away from feeding and reproduction in order to maintain
internal cell pH (i.e. expenditure of extra energy to run proton pumps).
It has even been suggested that ocean acidification will alter the
acoustic properties of seawater, allowing sound to propagate
further, increasing ocean noise and impacting animals that use
sound for echolocation or communication.
However, as with calcification, as yet there is not a full
understanding of these processes in marine organisms or ecosystems.[35
Leaving aside direct biological effects, 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
This will cause an elevation of ocean alkalinity, leading to the enhancement of
the ocean as a reservoir for CO2 with moderate (and
potentially beneficial) implications for climate change as more
CO2 leaves the atmosphere for the ocean.
"Present day" (1990s) sea surface pH
"Present day" (1990s) sea surface anthropogenic CO2
Vertical inventory of "present day" (1990s) anthropogenic CO2
Change in surface CO
from the 1700s to the 1990s
A NOAA (AOML)
in situ pCO2 sensor, attached
to a Coral Reef Early Warning System station, utilized in
conducting ocean acidification studies near coral reef areas
A NOAA (PMEL) moored
autonomous pCO2 buoy used for
measuring pCO2 and ocean
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Popular media sources:
The following packages calculate the state of the carbonate
system in seawater (including pH):