February 1, 2001

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One of the most promising places to sequester carbon is in the oceans, which currently take up a third of the carbon emitted by human activity, roughly two billion metric tons each year. The amount of carbon that would double the load in the atmosphere would increase the concentration in the deep ocean by only two percent.

Two sequestration strategies are under intense study at the Department of Energy's Center for Research on Ocean Carbon Sequestration (DOCS), where Jim Bishop of Berkeley Lab's Earth Sciences Division is codirector with Livermore Lab's Ken Caldeira. One is direct injection, which would pump liquefied carbon dioxide a thousand meters deep or deeper, either directly from shore stations or from tankers trailing long pipes at sea.


"At great depths, CO2 is denser than sea water, and it may be possible to store it on the bottom as liquid or deposits of icy hydrates," Bishop explains. "At depths easy to reach with pipes, CO2 is buoyant; it has to be diluted and dispersed so it will dissolve."

What happens to carbon dioxide introduced into the ocean in this way may soon be field-tested in Hawaii. Over a two-week period researchers plan to inject 40 to 60 metric tons of pure liquid CO2 over 2,500 feet deep in the ocean near the Big Island.

One variable they will be measuring is acidity. Water and carbon dioxide form carbonic acid, "but once diluted in sea water, carbonic acid is not the dominant chemical species," Bishop says, "because of seawater's high alkalinity and buffering capacity." If calcium carbonate sediments are involved, acidity is even less. "Think of Tums," he suggests.

Fertilizing the ocean

The other major approach to sequestration is to "prime the biological pump" by fertilizing the ocean. Near the surface, carbon is fixed by plant-like phytoplankton, which are eaten by sea animals; some eventually rains down as waste and dead organisms. Bacteria feed on this particulate organic carbon and produce CO2, which dissolves, while the rest of the detritus ends on the sea floor.

"There are areas of the ocean that are rich in nutrients like nitrogen and phosphorus but poor in phytoplankton," says Bishop. "Adding a little iron to the mix allows the plankton to use the nutrients and bloom. The energy for the process is supplied by sunlight. Already commercial outfits are dropping iron filings overboard, hoping to increase fisheries -- meanwhile claiming they are helping to prevent global warming."


In fact, Bishop explains, "if the excess fixed carbon in plants is eaten by fish near the ocean surface, the net effect is no gain. And in every part of the ocean there are open mouths."

No one really knows where the carbon trapped by fertilization ends up. In one iron-fertilization experiment in warm equatorial waters, chlorophyll increased 30-fold in a week, and there was increased carbon sedimentation down through 100 meters. But the bloom shortly dissipated, the fate of the carbon in deeper waters wasn't followed, and long-term effects weren't measured.

In a more recent experiment in cold Antarctic Ocean waters the plankton bloom persisted much longer. Seven weeks after the experiment ended a distinct pattern of iron-fertilized plankton was still visible from space -- "which means the fixed carbon was still at the surface."

Bishop says that "people who want to add iron think the particulate matter will fall straight to the bottom; I have sampled natural plankton blooms, and I have not seen that happen. These guys have a potentially effective method of sequestering carbon, but as yet there is no scientific basis for their claims."

Fishing for data

One of Bishop's specialties is measuring the ocean's particulate organic and inorganic matter at different depths to determine variations with ocean circulation patterns and biological regimes. He and his colleagues lower units overboard that pump and filter thousands of liters of seawater through meshes of different size, trapping biological products and dissolved minerals. "In my former life I was a garbage collector," he jokes. "But this way we can tell how the ecosystem functions."

Because ships are expensive, carbon data is sparse. Much more data is needed to calibrate ocean circulation models such as those developed by Livermore's Ken Caldeira, who uses computers to visualize the possible consequences of both major strategies of sequestration.

Of these circulation models Bishop remarks that "any oceanographer would say, 'that looks like the ocean' -- because decades of ship observations has given us knowledge of how the ocean circulates." But representations of how fast things are happening at middle depths could be off by a factor of four, he says, "and we have no way of knowing unless we get more data. We can't afford enough ships, but we can do the job with floats."

Flying and gliding through the depths

Project Argo, a consortium of scientists from a dozen countries, is now deploying some 3,000 floats around the globe to measure the temperature and salinity of the upper 2,000 meters of the ocean. "They sink, stay down for two weeks, come up and send their position and data to a satellite," says Bishop. "They're like 3,000 points of light. They'll give us the mid-depth circulation."


Bishop and colleagues at the Scripps Institution of Oceanography and from private industry have developed a modified float dubbed SOLO, supported by the National Oceanographic Partnership Program (a consortium of universities and other research organizations) and the National Oceanic and Atmospheric Administration. SOLO is equipped with instruments for measuring organic and inorganic carbon, both in solution and tied up in particles -- a "robotic carbon observer" with more sophisticated timing and positioning systems and faster data transfer than the Argo floats.

Four SOLOs will be launched this year in well-studied areas of the northern Atlantic and Pacific, where weather ships were once located. "Weather ships gave us more than just weather. When satellites came in, we lost an important source of time-dependent measurements of how ocean chemistry changes, seasonally and day- to-day."

Unlike trees, Bishop notes, the plants that fix carbon in the ocean typically live and die in a single day. "This makes it really hard and expensive to follow the variability of plants using a ship. When the weather gets bad all work on the ship stops -- yet biology goes on unobserved." But after floats are placed in currents from ships or planes, they "fly as balloons do, for seasons."

A more versatile platform is a robotic "glider" that can be launched from a harbor. Bishop says that a miniature submarine glider dubbed Spray, developed at Scripps and Woods Hole, "is an exciting new vehicle that we are learning how to use this year. You tell it where to go, what to do, and to come back when it's done."

Equipped with big batteries and a Global Positioning System receiver, Spray can be programmed by satellite to maintain a heading or adjust course and depth. Thus, Bishop says, "Gliders can effectively monitor what happens after ocean fertilization, swimming back and forth through the fertilized patch, measuring biomass and iron concentration at various depths."

Assessing change

DOCS is working to develop new platforms and instruments to gather sufficient data on fertilization, direct CO2 injection, and other methods for ocean carbon sequestration, data that will help researchers understand the implications for the oceans and the global environment. The most important questions are possible changes in biogeochemical processes -- and how the public responds to them.

"We need to define the sequestration strategies and find out if they can really work, and if there are problems associated with them," says Bishop. "One of our most important tasks is to establish DOCS as a consistent, unbiased voice on the scientific issues. We want to be an educational center, not an engineering center."

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