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Source: The Hindu
(http://www.hinduonnet.com/2008/06/19/stories/2008061955591100.htm)
Opinion
 -
News Analysis
  

For a pilot project on deep sea storage of CO2




Wallace S. Broecker




One of the world’s leading climate scientists challenges
Greenpeace’s opposition to storing CO2in the depth of the
oceans.




Most of us who are concerned about global warming agree that an
important part of any strategy designed to stem the ongoing build-up
of greenhouse gases in the atmosphere will be to capture and store
CO2. Potential storage sites include spent oil fields, saline
aquifers, layered basalts and the deep ocean.

“Point pollution”



While Greenpeace accepts the inevitability that CO2 will be captured
and stored, it strongly opposes storage in the deep sea. As it is
clear that virtually all the CO2 released to the atmosphere as a
result of fossil fuel burning will ultimately find its way to the deep
sea, its objection is focused on the “point pollution”
created by purposeful injections of CO2. The fear is that such an
activity will put at risk benthic biota — the community of
creatures and plants in the deep sea — living in the vicinity of
the injection sites.
 In February 2007, I contacted Bill Hare, a senior scientist at
Greenpeace, asking him to reconsider his organisation’s stance
against experiments to evaluate the environmental consequences of CO2
injected into the deep sea. I pointed out that if marine disposal
proves to be economically favourable, and if push comes to shove,
forces more powerful than Greenpeace will probably intervene and deep
sea disposal will commence without adequate testing and evaluation.
 Hare agreed to reconsider this matter in consultation with members of
his and other like-minded organisations. In June 2007, he reported
back that no change in policy would be made.
 What is known about deep ocean storage?
 First, in order to ensure that the injected CO2 has adequate time to
mix throughout the deep sea, injection should be at depths greater
than 3,500 metres — that is, the depth below which
“liquid” CO2 becomes more dense than sea water.
 Experiments conducted by Peter Brewer, of the Monterey Bay Aquarium
Research Institute, not only confirm that this is the case but also
demonstrate that the CO2 injected rapidly reacts with sea water to
form a solid clathrate, which is more dense than both liquid CO2 and
sea water. Hence, the injected CO2 would end up on the sea floor as a
slush. This would gradually dissolve, releasing the CO2 to the
surrounding sea water, where it would react with the dissolved
carbonate and borate ions to become chemically bound in the form of
bicarbonate ion. As the concentration of carbonate and borate ions is
small, the neutralisation would take place gradually as the CO2-rich
sea water mixed into the surroundings.


Prime taget for storage



We know that, based on radiocarbon measurements, the residence time of
water in the abyssal Atlantic is in the order of 200 years. For the
Indian Ocean, it is about 800 years, and for the Pacific about 1,000
years. As the deep Pacific has the largest volume, and is adjacent to
earthquake-prone land areas where below-ground storage could not be
safely done, it will be a prime target for storage.
 A conservative upper limit on the storage capacity of the deep
Pacific would be to require that the CO2 concentration in the water
returning to the surface not be allowed to exceed the concentration in
cold surface water at equilibrium with the atmosphere. Were this the
limit to be adopted, then the capacity of water deeper than 1,500
metres in the Pacific would be about 480 gigatons of CO2, or about 130
gigatons of carbon for each 100 parts per million rise in atmospheric
CO2 content.


Need for study



We know enough to say with confidence that deep ocean disposal of CO2
is certainly feasible, but unless small-scale pilot experiments are
conducted, information necessary to assess the impact on the macro
abyssal biota will remain obscure. The injections could be made from
ships equipped for deep sea drilling, and if the CO2 were tagged with
radiocarbon, its dispersal away from the sea floor clathrate pile
could be sensitively monitored.
 Studies of the costs associated with ocean disposal would also be
conducted. The CO2 would have to be sent through pipelines from its
collection point to a port, where it would be loaded on tankers that
would carry it to a floating ocean station, from which it would be
piped to the abyss.
 Putting aside the opposition by the environmental community, ocean
disposal will become a viable option only if the costs are competitive
with those associated with storage in hyper-saline continental
aquifers.


Do the homework



As any strategy designed to stem the build-up of greenhouse gases will
have adverse environmental consequences, we must seek to minimise
their impact. To the extent that we could capture and store CO2
produced by fossil fuel burning, we would reduce the acidification of
the surface ocean, and hence the additional stress on coral reef
communities. To date, there is no indication that the projected rise
in upper ocean CO2 content will have adverse impacts on fish. If so,
assuming the limit described above were to be observed, then once
spread through the deep sea, the injected CO2 would not adversely
impact on benthic biota.
 However, I sympathise with those who claim that the benthic world is
a fragile one. Hence, before we poke it with CO2, we should do our
homework. Therefore, I challenge Greenpeace to relax its stand and
allow a pilot project to proceed. — © Guardian Newspapers
Limited, 2008
(Wallace S. Broecker is the Newberry professor in the Department of
Earth and Environmental Sciences at Columbia University, New York.)












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