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35
OCEAN ACIDIFICATION
The oceans act as a natural reservoir for CO
2
. The dissolved
CO
2
reacts with the seawater to form hydrogen ions. The up-
take of anthropogenic carbon since 1750 has led to the ocean
becoming more acidic, with an average decrease in pH of 0.1
units. However, the effects of observed ocean acidification on
the marine biosphere are yet mostly undocumented. Progres-
sive acidification of the oceans due to increasing atmospheric
carbon dioxide is expected to reduce biocalcification of the
shells, bones and skeletons most marine organisms possess.
Though the limited number of studies available makes it dif-
ficult to assess confidence levels, potentially severe ecological
changes would result from ocean acidification, especially for
corals both in tropical and cold water, and may influence ma-
rine food chains from carbonate-based plankton up to higher
trophic levels.
The oceans are naturally alkaline, with an average pH of around
8.2, although this can vary up to 0.3 units depending on location
and season. Atmospheric carbon dioxide dissolves naturally in
the ocean, forming carbonic acid (H
2
CO
3
), a weak acid. The hy-
drogen ions released from this acid lower the pH. These reactions
are part of a natural buffer system, but recent studies have shown
that the huge amounts of CO
2
created by burning fossil fuels are
over-stretching the rate by which the natural process can neu-
tralise this acidity. The pH of the oceans has decreased 0.1 unit
compared to pre-industrial levels, which equals an increase of 30
per cent in hydrogen ions. While records show that the pH of the
seas can vary slightly over time and in certain areas, the contin-
ued increases in atmospheric CO
2
are expected to alter ocean pH
values within a very short time – an effect greater than any experi-
enced in the past 300 million years (Caldeira
et al
., 2003).