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updated 4/13/2009 4:51:30 PM ET 2009-04-13T20:51:30

An ambitious plan to slow global warming by locking CO2 deep in the ocean has hit a stumbling block, according to a new study that shows the geoengineering technique is not as effective as scientists had previously hoped.

"The amount of carbon dioxide that could be taken up is less than we assumed," said Victor Smetacek, a scientist at Alfred Wegener Institute who co-led the expedition. Don't discount ocean fertilization yet however, say scientists working both inside and outside the research study.

"With other climate change initiatives, ocean fertilization could still play a role in reducing climate change," said Smetacek.

The idea behind ocean fertilization is a simple one. Carbon dioxide is slowly raising the average temperature of the Earth. If some of the carbon dioxide in the atmosphere could be removed the Earth wouldn't heat up as fast.

Scientists have come up with a wide variety of plans to remove CO2 from the atmosphere. Some scientists want to pump C02 deep into the underground. Others want to use carbon nanotubes to turn CO2 into methane or other compounds.

Smetacek and his colleagues want to lock up the excess carbon dioxide inside the world's oceans, or more specifically, inside the bodies of microscopic creatures known as plankton, that would die and fall to the bottom of the ocean.

A certain class of plankton, known as phytoplankton, already remove carbon dioxide from the atmosphere through photosynthesis. If there were more phytoplankton, the German and Indian scientists reasoned, then the microscopic creatures would lock away more carbon dioxide.

Phytoplankton need iron to grow. In the open ocean usable iron is difficult to come by, so adding iron to the ocean would lead to more phytoplankton.

Over the course of two and a half months the team of scientists "administered," as Smetacek says, more than six tons of dissolved iron (the kind found in most home improvement stores) over a 300-square-kilometer (116-square-mile) patch of the southwest Atlantic.

The additional iron certainly encouraged more phytoplankton. The amount of biomass in the test area doubled, which scientists determined during marathon 36 hour sampling sessions.

The scientists created more plankton, but the plankton didn't perform as the scientists had hoped. Instead of dying and sinking to the bottom of the ocean, the additional plankton were eaten, first by copepods, then by ampipods. As the carbon moved up the food chain some of was released back into the atmosphere as carbon dioxide.

The scientists concluded that fertilizing the southwest Atlantic was not a good way to lock away carbon dioxide, but that ocean fertilization needs additional testing before it's discounted.

Other oceans and other materials, like silicon instead of iron, might be better candidates for geoengineering, says Ken Buesseler, a scientist at Woods Hole Oceanographic Institute in Massachusetts who has conducted his own ocean fertilization experiments.

"No one is saying that [ocean fertilization] alone will solve the greenhouse gas problem," said Buesseler. "But if we try many different solutions at the same time it could have a significant impact."

Buesseler says that more experiments are necessary to find the best section of the ocean and the best material to encourage plankton to take oxygen out of the atmosphere and to then sink to the bottom of the ocean.

"There is no one solution to solving global warming," said Buesseler, "But doing nothing doesn't seem very satisfying to me."

© 2012 Discovery Channel

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