OCEAN ACIDIFICATION FROM
CLIMATE CHANGE
The world’s oceans soak up much of the carbon
dioxide produced by living organisms, either as dissolved gas, or
in the skeletons of tiny marine creatures that fall to the bottom
to become chalk or limestone. Oceans currently absorb about one
metric tonne of CO2 per person per year. It is estimated that the
oceans have absorbed around half of all carbon dioxide generated
by human activities since 1800 (120,000,000,000 tonnes).
But in water, carbon dioxide becomes a weak carbonic
acid, and the increase in the greenhouse gas since the industrial
revolution has already lowered the average pH (the laboratory measure
of acidity) of seawater by 0.1 units on the 14-point scale, to 8.2.
Predicted emissions could lower it by a further 0.5 by 2100, to
a level not seen for millions of years.
There are concerns that increasing acidification
could have a particularly detrimental effect on corals and other
marine organisms with calcium carbonate shells. 16% of the world’s
coral reefs have died from bleaching since 1998, and increased acidity
may also directly affect the growth and reproduction of fish as
well as the plankton on which they rely for food.
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 2004 surface ocean pH is estimated to have decreased from approximately
8.25 to 8.14 (Jacobson, 2005).
oceans and the Carbon cycle
In the natural carbon cycle, the atmospheric concentration
of carbon dioxide represents a balance of fluxes between the oceans,
terrestrial biosphere 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 carbon dioxide into the atmosphere.
Some of this has remained in the atmosphere (where it is responsible
for the rise in atmospheric concentrations), some is believed to
have been taken up by terrestrial plants, and some has been absorbed
by the oceans.
When carbon dioxide dissolves, it reacts with
water to form a balance of ionic and non-ionic chemical species
: dissolved free carbon dioxide, carbonic acid, bicarbonate and
carbonate. 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).
Acidification in oceans due to carbon dioxide
Dissolving carbon dioxide also increases the hydrogen
ion (H+) concentration in the ocean, and thus reduces ocean pH.
The use of the term "ocean acidification" to describe
this process was introduced in Caldeira and Wickett (2003). Since
the industrial revolution began, ocean pH has dropped by approximately
0.1 units (on the logarithmic scale of pH), and it is estimated
that it will drop by a further 0.3 - 0.4 units by 2100 as the ocean
absorbs more carbon dioxide caused by human activities (Caldeira
and Wickett, 2003; Orr et al., 2005).
Although this oceanic absorption will help ameliorate
the climatic effects of human-caused emissions of carbon dioxide,
it is believed that it will have negative consequences for oceanic
calcifying organisms. These use the calcite or aragonite polymorphs
of calcium carbonate to construct cell coverings or skeletons. Calcifiers
span the food chain from autotrophs to heterotrophs and include
organisms such as coccolithophores, corals, foraminifera, echinoderms,
crustaceans, and some mollusks, especially pteropods. Aside from
calcification (and specifically calcifiers), organisms may suffer
other adverse effects, either directly as reproductive or physiological
effects (e.g. carbon dioxide-induced acidification of body fluids,
known as hypercapnia), or indirectly through negative impacts on
food resources. However, as yet there is not a full understanding
of these processes in marine organisms or ecosystems.
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 under-saturated, structures
made of calcium carbonate are vulnerable to dissolution. Research
has already found that corals (Gattuso et al., 1998), coccolithophore
algae (Riebesell et al., 2000) and pteropods (Orr et al., 2005)
experience reduced calcification or enhanced dissolution when exposed
to elevated carbon dioxide. The Royal Society of London published
a comprehensive overview of ocean acidification, and its potential
consequences, in June 2005 (Raven, et al., 2005).
ecological consequences of ocean Acidification
While the full ecological consequences of these
changes in calcification are still uncertain, it appears likely
that calcifying species will be adversely affected. There is also
some evidence that the effect of acidification on coccolithophores
(among the most abundant phytoplankton in the ocean) in particular
may eventually exacerbate climate change, by reducing the earth's
albedo as well as oceanic cloud cover (Ruttiman, 2006). Present
evidence suggests that dramatic changes in the biogeochemistry of
the marine environment over the next 100-200 years can be avoided
only with early and deep reductions in carbon dioxide emissions.
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