The hypothesis of these experiments is that fertilizing the oceans with iron could sufficiently boost photosynthetic rates of floating patches of microscopic plants called phytoplankton to remove large amounts of carbon dioxide from Earth's atmosphere, said Richard Barber, a professor of biological oceanography at Duke's Nicholas School of the Environment and Earth Sciences.
According to the hypothesis, extra carbon dioxide absorbed during photosynthesis would be converted into plant tissue that would then sink deep into the ocean, removing the gas from circulation for long periods. But, so far, "that result has not been shown," Barber said.
Barber will discuss the subject at a symposium beginning at 9 a.m. PT on Friday, February 13, 2004 during the American Association for the Advancement of Science's 2004 annual meeting in Seattle.
CO2 is among several gases believed to contribute to warming Earth's climate by trapping extra amounts of the sun's heat -- something like a greenhouse. The "iron hypothesis" method of scrubbing the atmosphere of much of its carbon dioxide was proposed in 1990 by John Martin, an oceanographer at Moss Landing Marine Laboratory in California, who died before he could test the idea.
Barber, a long time colleague of Martin's, served as chief scientist on an initial cruise, to the equatorial Pacific, to evaluate how marine life would respond to extra iron.
In that and subsequent cruises, he and other scientists have documented that phytoplankton photosynthetic rates did quickly increase in response to iron fertilization. The unanswered question is what ultimately happens to the assimilated CO2.
On land there is no question about photosynthesis removing CO2 from atmospheric circulation and channeling that carbon into growing tissue. "As a tree grows it can have 20 or 40 years worth of carbon stored in its trunk," Barber said. "You could certainly have six months of carbon in grass."
At sea, however, scientists have learned that phytoplankton are quickly consumed by equally diminutive marine animals, called zooplankton. In a normal ocean environment, without iron fertilization, "the coupling between the plants that grow and the animals that eat them is very tight," Barber said. Plants "rarely get beyond the first day."
For the iron hypothesis to work successfully, photosynthesized carbon in plant material would have to quickly sink to depths between 100 and 300 meters, beyond the range of grazing zooplankton, he said.
However, scientists are finding that iron fertilization also serves to boost zooplankton populations. "They increase in numbers very quickly, but not initially," Barber said.
Results so far suggest iron-fertilized phytoplankton have only one or two weeks to sink or be eaten. After that, zooplankton numbers "increase in a way that they overgraze, just like overgrazing a field," he added.
Once phytoplankton are eaten, carbon assimilated into their tissue would be quickly returned to the atmosphere as carbon dioxide, he said.
If Martin's original hypothesis is ultimately shown to remove significant carbon from the atmosphere, a separate question is whether regulators could find a way to incorporate iron fertilization into a system of "carbon credits." Barber will also discuss that issue at the Seattle meeting.
Under the carbon credit concept included in the framework of the Kyoto Protocol, a factory emitting excess carbon dioxide, for example, could cancel out those emissions by paying for more trees to be planted.
Such an inventory system requires a way of defining and monitoring how much carbon is likely to be stored in a growing forest and for how long, Barber said. He questioned whether that concept could be applied to plants at sea absorbing carbon dioxide and then quickly sinking.
"It would be a monitoring problem which would be very, very difficult to carry out at the present time with present technology," Barber said.