Some hope fertilizing tiny, floating plants in the ocean, prompting them to suck carbon dioxide out of the air, could help solve global warming.

A new experiment confirms this controversial idea has some merit, although important questions remain.

Using an eddy in the Southern Ocean near Antarctica, researchers used iron fertilizer — the sort used to improve lawns — to create a man-made algal bloom. In the weeks that followed, researchers say, this bloom funneled a significant amount of Earth-warming carbon down into the ocean's depths, where it will remain sequestered for some time, unable to contribute to global warming.

This experiment provides some important insight into this potential approach to combating climate change, said Ken Buesseler, of the Woods Hole Oceanographic Institution, writing in Thursday's (July 19) issue of the journal Nature.

A potential solution?

This general approach, modifying the planet to address climate change, is known as geoengineering, and, geoengineering proposals like iron fertilization tend to raise many uncertainties and risks. Other geoengineering ideas have included pumping aerosols into the atmosphere to block out solar radiation or tucking away excess carbon in underground reservoirs. [Top 10 Craziest Environmental Ideas]

Ocean fertilization is a controversial idea, prompting protest from those who fear the unintended environmental impacts it may have.

"Most scientists would agree that we are nowhere near the point of recommending [iron fertilization of the oceans] as a geoengineering tool. But many think that larger and longer [iron fertilization] experiments should be performed to help us to decide which, if any, of the many geoengineering options at hand should be deployed," Buesseler wrote.

Phytoplankton, which includes microscopic marine plants and photosynthetic microbes, blooms naturally in the ocean. However, in seawater, there is only limited iron, an element these organisms need to grow, so by adding iron to seawater, it's possible to make a man-made bloom.

The research vessel used in the iron fertilization experiment, the Polarstern. (Image credit: Alfred Wegener Institute)

In this study, the researchers fertilized an eddy because it offered a largely self-contained system, or "a gigantic test tube," said lead researcher Victor Smetacek, with the Alfred Wegener Institute for Polar and Marine Research in Potsdam, Germany.

By mixing an iron fertilizer into the seawater, the researchers created the equivalent of a good-size spring bloom like those seen in the North Sea or off Georges Bank off the New England coast, which turned the water from blue to turquoise, Smetacek said.

Moving carbon

The team found that after they added the iron, the levels of nutrients, including nitrogen, phosphorus and silicic acid, which algae called diatoms use to construct their glass shells, declined until around 24 days after the fertilizer was added.

Dissolved inorganic carbon, which normally remains in equilibrium with the carbon dioxide in the atmosphere, also declined more quickly than it could be replaced by the carbon dioxide in the atmosphere.

Meanwhile, their measurements revealed particulate organic matter, including the silica the diatoms used to make their shells, and chlorophyll, the green pigment used in photosynthesis, increased within the surface waters.

After day 24, however, the particulate matter — the remains of the algae that had sucked up the carbon — sank, traveling down from the surface layer, falling to depths between 328 feet (100 meters) to the seafloor, about 12,467 feet (3,800 m) below.

If this organic matter settles into the deep ocean, it may not reach the surface for centuries or millennia, depending on ocean circulation, Smetacek said.

Much of the former phytoplankton bits are likely to have settled on the seafloor as "fluff" — "like a layer of fluff that you would find under your bed if you did not vacuum it for a long time," Smetacek told LiveScience in an email. "Eventually, this loose matter flattens into the sediments and a part gets buried; this stuff is sequestered for geological time scales." (Geologists measure time in terms of millennia to many millions, even billions, of years.)

His team estimated that for every iron atom they introduced into the eddy, at least 13,000 carbon atoms were taken up into the biomass of the algae, becoming available for export into deeper water. They also found that at least half of the organic matter associated with the bloom — nearly all of it made up of glass-walled diatoms — sank below, 3,280 feet (1,000 m).

Far from proven

In spite of the experiment's success, Smetacek is cautious about the implications for cleaning up human's greenhouse gas emissions.

"It's a very thorny subject," he said. "What we can say here at this stage is that we need to have more experiments (before) we can make any firms statements on that."

Many questions about the feasibility and safety of this approach remain. Buesseler points out that iron fertilization has the potential to stimulate toxic algae blooms; cause the production of nitrous oxide, a more potent greenhouse gas than carbon dioxide; or to suck oxygen out of water as the algae decompose, a phenomenon that is responsible for creating dead zones, like the one found in the Gulf of Mexico.

The approach also has limited potential, since even used on a large scale, it could only remove a fraction of the excess carbon dioxide humans are emitting.

Iron fertilization has another potentially important application, one unrelated to climate change, Smetacek said, suggesting that it may have the potential to restore an ecosystem in the Southern Ocean, where whales once fed on abundant swarms of krill.

In spite of the loss of whales to whaling, their prey, shrimplike krill, have declined dramatically. Smetacek believes this is because the whales played a crucial role in keeping the waters fertilized with iron, which prompted the blooms of phytoplankton, which feed the krill. He has proposed fertilizing a stretch of Antarctic sea ice with iron to see how it affects krill growth.

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