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.
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 favorable, 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.
As a scientist, I seek rational decisions. So let me begin by outlining 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.
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.
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.
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.
Wallace S Broecker is the Newberry professor in the Department of Earth and Environmental Sciences at Columbia University, US, and is a scientist at Columbia's Lamont-Doherty Earth observatory.