Coral and shellfish under threat as seas turn acidic
Shellfish, coral reefs and other marine animals with exterior
skeletons will find it difficult to cope with the speed at which the
oceans are turning acidic due to rising concentrations of carbon
dioxide, a study has found. Scientists have calculated that the current
rate of ocean acidification today is unprecedented over the past 300
million years, when the seas experienced at least four major mass
extinctions involving rising ocean acidity. In the past 100 years, the
concentration of carbon dioxide in the atmosphere has increased by about
a third. This has resulted in a corresponding decrease in the natural
alkaline state of the oceans, effectively making them more acidic as CO2
in the air dissolves in seawater to create carbonic acid, scientists
said. The oceans have been more acidic than they are now on several
occasions in the past. But the current rate of acidification is many
times faster than, for instance, at the time of the "Permian mass
extinction" 252 million years ago, which wiped out 95 per cent of marine
life on Earth, they said. "We know that life during past
ocean-acidification events was not wiped out - new species evolved to
replace those that died off.
But if industrial carbon emissions continue at the current pace, we
may lose organisms we care about, such as coral reefs, oysters and
salmon," said Bärbel Hönisch of Columbia University's Lamont-Doherty
Earth Observatory in New York. The study, published in the journal
Science, investigated seabed sediments built up over hundreds of
millions of years.
These contain microscopic fossils and the physical signatures of
marine organisms that lived during this period, which can be used to
estimate average levels of pH, the chemical measure of acidity and
alkalinity. Shellfish, reef-building corals and microscopic organisms
such as foraminifera, which are at the base of the marine food chain,
all rely on high concentrations of carbonate ions in seawater, which are
affected by the production of carbonic acid. As acidity increases, then
these organisms will find it more difficult to make the shells and bony
skeletons essential for survival, said Dr Daniela Schmidt of Bristol
University, a co-author of the study.
"We looked at the past 300 million years of the Earth's history and
analysed events that are associated with ocean acidification. None of
these events is a perfect analogy to what is happening now because the
current rate of change is so exceptional," Dr Schmidt said. "But we
estimate it is happening 10 times faster than in the past and this is
important in terms of the biology because the speed puts pressure on the
ecosystem. If it runs too fast, you are not giving the genetics time to
cope." The scientists found that there was only one previous mass
extinction when the rate of acidification remotely matched that of
today.
But today's acidification is still occurring about 10 times faster
than that of the Palaeocene-Eocene Thermal Maximum (PETM) 56 million
years ago, they said. Over the past 100 years, for instance, the rise in
atmospheric carbon dioxide has led to a fall in the pH of the ocean of
0.1 units, which is equivalent to a 30 per cent rise in acidity. By
2100, scientists estimate that the pH of the oceans could fall by a
further 0.3 units, bringing the average ocean pH down to 7.8. This would
mean that the increase in ocean acidification since the Industrial
Revolution would by the end of this century match the acidity changes
that occurred during the PETM, which are believed to have occurred over
a relatively short geological period of 5,000 years.
- The Independent
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