Natural lab shows sea’s acid path
By Richard Black
Environment correspondent, BBC News website
Natural carbon dioxide vents on the sea floor are
showing scientists how carbon emissions will affect
Dissolved CO2 makes water more acidic, and around the
vents, researchers saw a fall in species numbers, and
snails with their shells disintegrating.
Writing in the journal Nature, the UK scientists
suggest these impacts are likely to be seen across the
world as CO2 levels rise in the atmosphere.
Some of the extra CO2 emitted enters the oceans,
acidifying waters globally.
Studies show that the seas have become more acidic
since the industrial revolution.
The only way of reducing the impact of ocean
acidification is the urgent reduction in CO2 emissions
Plymouth Marine Laboratory
Research leader Jason Hall-Spencer from the University
of Plymouth said that atmospheric CO2 concentrations
were now so high that even a sharp fall in emissions
would not prevent some further acidification.
“It’s clear that marine food webs as we know them are
going to alter, and biodiversity will decrease,” he
told BBC News.
“Those impacts are inevitable because acidification is
inevitable – we’ve started it, and we can’t stop it.”
Corals construct their external skeletons by
extracting dissolved calcium carbonate from seawater
and using it to form two minerals, calcite and
aragonite. Molluscs use the same process to make their
As water becomes more acidic, the concentration of
calcium carbonate falls. Eventually there is so little
that shells or skeletons cannot form.
The oceans are thought to have absorbed about half of
the extra CO2 put into the atmosphere in the
This has lowered its pH by 0.1
pH is the measure of acidity and alkalinity
The vast majority of liquids lie between pH 0 (very
acidic) and pH 14 (very alkaline); 7 is neutral
Seawater is mildly alkaline with a “natural” pH of
The IPCC forecasts that ocean pH will fall by “between
0.14 and 0.35 units over the 21st Century, adding to
the present decrease of 0.1 units since pre-industrial
Around the vents which Dr Hall-Spencer’s team
investigated, in the Mediterranean Sea near the
Italian coast, CO2 bubbling into the water forms a
sort of natural laboratory for studying the impacts of
acidified water on marine life.
Globally, the seas now have an average pH of about 8.1
– down about 0.1 since the dawn of the industrial age.
Around the vents, it fell as low as 7.4 in some
places. But even at 7.8 to 7.9, the number of species
present was 30% down compared with neighbouring areas.
Coral was absent, and species of algae that use
calcium carbonate were displaced in favour of species
that do not use it.
Snails were seen with their shells dissolving. There
were no snails at all in zones with a pH of 7.4.
Meanwhile, seagrasses thrived, perhaps because they
benefit from the extra carbon in the water.
These observations confirm that some of the processes
seen in laboratory experiments and some of the
predictions made by computer models of ocean
ecosystems do also happen in the real world.
“I can’t count the number of times that scientific
talks end with ‘responses have not yet been documented
in the field’,” said Elliott Norse, president of the
Marine Conservation Biology Institute (MCBI).
“This paper puts that to rest for several ecologically
important marine groups.”
The Intergovernmental Panel on Climate Change (IPCC)
suggests that without measures to restrain carbon
dioxide emissions, ocean pH is likely to fall to about
7.8 by 2100.
This suggests that some of the impacts seen around the
Mediterranean vents might be widespread.
“I think we will see the same pattern in other parts
of the world, because we’re talking about keystone
species such as mussels and limpets and barnacles
being lost as pH drops,” said Dr Hall-Spencer.
The IPCC suggests that some areas, notably the
Southern Ocean, might feel the impacts at lower
concentrations of CO2.
Last month, scientists reported that water with CO2
levels high enough to be “corrosive” to marine life
was rising up off the western US coast.
Bottom water naturally contains more CO2 than at
shallower depths. This scientific team argues that
human emissions have pushed these levels even higher,
contributing to pH values as low 7.5 in waters heavily
used by US fishermen.
“If [pH 7.8] is a universal ‘tipping point’, then it
indicates that sections of the western coast waters
off North America may have passed this threshold
during periods when this upwelling of waters high in
CO2 occurs,” commented Carol Turley from Plymouth
Marine Laboratory (PML), who was not involved in the
Mediterranean Sea study (PML is not affiliated with
Organisms such as coral are also damaged by rising
temperatures, and studies are ongoing into the
combined effect of a warming and acidifying ocean.
There is much to learn. And during the coming week,
scientists will announce the inauguration of the
European Project on Ocean Acidification (Epoca), a
four-year, 16m euro (£12.5m) initiative aiming to find
Studying the impacts may prove easier than doing
anything about them.
“The reason that the oceans are becoming more acidic
is because of the CO2 emissions that we are producing
from burning fossil fuels,” observed Dr Turley.
“Add CO2 to seawater and you get carbonic acid; it’s
simple chemistry, and therefore certain.
“This means that the only way of reducing the future
impact of ocean acidification is the urgent,
substantial reduction in CO2 emissions.”
Tony Tweedale, 2f4
50 Lady Lawson St.
EH3 9DW, Edinburgh