Acid Oceans
by Prof Dr Ulf Riebesell of the Leibnitz Institute of Marine Sciences in Kiel, Germany
Complete darkness surrounds us as we slowly descend towards the ocean
floor. The numbers on the instrument panels of our submarine, JAGO, change
rapidly: the depth increases to 160 metres, the temperature cools to 4 degrees
Celsius. Suddenly a mysterious world unfolds in the beam of our headlights.
White and pinkish mounds of coral grow side by side, building reefs as large
as football fields. The colourful formations are a hub of activity. As we peer out
of our port hole, fish, crabs and tiny shrimp pass by.
Like their tropical counterparts, cold water corals provide a habitat for a
myriad of marine life. These hotspots of deep sea biodiversity have only
become known to us in the last ten years. Like pearls on a necklace, reefs of
cold water corals extend along the eastern margin of the Atlantic Ocean over
a length of 5000 kilometres, from northern Norway to the African coast. As
we glide over the reefs admiring their silent beauty, it is hard to imagine that
these pristine ecosystems might soon be lost from our planet. Yet, if human
carbon dioxide emissions continue to increase at present rates, corals in large
expanses of the ocean will soon be living in seawater that corrodes their
calcareous skeletons. The catch word for this process is ocean acidification:
the pH value of seawater (a measure of its acidity) is steadily decreasing. Like
osteoporosis in humans, the corals' calcareous skeletons will dissolve faster
than they can rebuild them.
But how can such emissions endanger life in the ocean? The underlying
process leading to ocean acidification is very simple, much simpler than the
CO2-induced changes in our climate system. Its origin lies in the absorption
of massive amounts of man-made CO2 by the surface ocean. Nearly half of
the amount of the gas that has been released from fossil fuels through human
activities since the beginning of the industrial revolution — over 500 billion
tons of it — has been taken up by the ocean, as the largest habitat on our planet
serves as its largest sink of greenhouse gases: in the long term, it is expected
to absorb 90 per cent of all fossil fuel CO2 released into the atmosphere. The
acidification of seawater will continue to creep into the deep ocean, even long
after emissions dwindle or come to an end. It may be described as a blessing
for our climate system because it dampens CO2-induced greenhouse warming
- but it will prove to be a curse for marine life.
When carbon dioxide dissolves in seawater it forms carbonic acid. Part of it
is neutralized by the carbonate buffer, a chemical reaction that consumes
carbonate ions — the building material used by calcifying organisms to
produce their shells and skeletons. The remaining acid leads to a decrease
in the pH of seawater. The lower the pH value, the greater the concentration
of hydrogen ions and hence the more acidic the water. The ocean’s uptake
of carbon dioxide from fossil fuels has already caused a pH decrease of 0.1
units, which corresponds to a 30 per cent increase in hydrogen ions. If current
trends in CO2 emissions continue, seawater’s pH will decrease by about 0.45
units from pre-industrial times by 2100. This would be lower — and the rate of
change faster — than has occurred for at least the past 400,000, and probably
for the last 20 million, years.
This will affect not only cold water corals, but calcifying organisms in general. As
the concentration of carbonate ions diminishes, the production of calcareous
structures will become increasingly difficult. All calcifying species so far tested
in laboratory simulations show a decrease in calcification in response to ocean
acidification. Calcification is a widespread phenomenon among many marine
organisms, extending from corals to mussels, snails, sea stars and sea urchins,
to tiny calcifying unicellular animals and plants at the base of the marine food
web. Even fish precipitate calcium carbonate to build some of their internal
structures, such as calcareous platelets in their vestibular apparatus. Judging
from current experimental results, there is a high risk that many calcifying
groups may lose their competitive fitness to prevail in an ocean of increasing
acidity. The consequences this may have for the marine food web are
presently unknown.
Looking back in Earth´s history, we can learn a lesson from the fossil record.
When a comet hit the Yucatan Peninsula in northern Mexico 65 million
years ago, massive amounts of calcium sulphate were blasted into the
atmosphere. There it reacted with oxygen and water to form sulphuric
acid. The amounts of sulphuric acid were sufficient to make the surface
ocean corrosive for the calcareous shells and skeletons of surface dwelling
organisms. It probably only took a few years until mixing with deep ocean
waters neutralized acidification at the surface, but it was long enough to
cause the extinction of nearly all planktonic calcifiers. Two million years
went by before corals reappeared in the fossil record. It took a further
20 million years for the species diversity of calcifying groups to recover to
pre-extinction levels.
Research on the effects of current ocean acidification is still in its infancy.
No one knows how the negative responses observed experimentally on
individual organisms will translate to communities and ecosystems. How
will these responses be affected by other stress factors such as changing
temperatures or the availability of nutrients? There is also a major challenge
in determining the ability of sensitive organisms to adapt to ocean
acidification. Despite much uncertainty, it is probably safe to say that
continued ocean acidification will cause the loss of marine biodiversity,
with presently unforeseeable consequences for marine ecosystems
and food webs.
In its 1995 report, the Intergovernmental Panel on Climate Change (IPCC)
published a series of scenarios of projected CO2 emissions for the 21st
century. Its worst case scenario was critically judged at that time as being far
too pessimistic. But records over the past 10 years indicate that the actual
trend in global CO2 emissions is above the one in this scenario. Despite rising
awareness of the problems associated with increasing levels of atmospheric
CO2, our efforts to turn this process around are still lagging behind. Ocean
acidification and the associated risks to marine life provide yet another
incentive to act quickly and decisively in order to reduce global emissions of
carbon dioxide.
Copyright:
OUR PLANET MAGAZINE THE MARINE ENVIRONMENT December 2007 page 11 - reproduced for scientific purposes only.
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