Slowing global warming is a big job. But some researchers and companies say that job could be done by enlisting the help of small but fantastically numerous--and collectively mighty--marine unicellular organisms called phytoplankton.

Phytoplankton make up the chlorophyll-bearing canopy at the base of the marine food web. As part of the natural biogeophysical cycle, phytoplankton absorb atmospheric carbon dioxide, a heat-trapping gas implicated in global climate change. Some of the carbon is buried when phytoplankton die and settle to the sea bottom.

In theory--one strengthened by some experiments, including one early in 2002--fertilizing phytoplankton could accelerate this natural process, sinking carbon in gigaton quantities. The scientific jury is out on whether such a grand-scale experiment with complex atmospheric processes would work to pull out additional carbon--and if it did, whether the cascade of subsequent effects would ultimately wreak more environmental havoc than the excess carbon would in the first place. [For more, see "The Ocean's Invisible Forest," by Paul G. Falkowski; Scientific American, August 2002.] But those uncertainties haven't stopped entrepreneurs from developing plans to deliver fertilizers to phytoplankton, in case they ever get a green light to do so.

Pump It Down

To understand more about how phytoplankton could trap extra carbon dioxide, it helps to consider the natural process. Using sunlight and nutrients, phytoplankton draw carbon dioxide out of the atmosphere and convert it to organic carbon. As the carbon passes through other consumers in the eat-and-be-eaten cycle, the organisms respire CO₂, returning the gas to the atmosphere. But not all of it. When plankton die and drop into the deep ocean, they take some CO₂ with them. The CO₂ then does not resurface for 1,000 years or so, as part of upwelling currents and other geophysical processes. "Phytoplankton are the engine of the biological pump," says MIT oceanographer Sallie W. Chisholm.

So, the idea is, enhance this biological pump to park more of the CO₂--and, voil¿, humans could temporarily mitigate the effects of global warming and buy some precious time to develop a more permanent fix. There are a number of ventures pursuing variations on this idea.

Ocean Technology Group, based in Australia, for one, envisages delivering the nutrient nitrogen in the form of ammonia to the surface ocean zones to boost phytoplankton growth. Two other companies, Ocean Carbon Sciences and Green Sea Venture, plan to add iron to the oceans. Ocean Carbon Sciences proposes that ships could release an iron-based fertilizer into the shipping lanes, to essentially make up for their negative impact on the local environment. Green Sea Venture would release the iron fertilizer into the waters via small floating, slowly dissolving pellets.

Why Iron?

In the 1980s John H. Martin of Moss Landing Marine Laboratories and other oceanographers were puzzled that ocean areas, such as the equatorial Pacific and the Southern Ocean around Antarctica, had high nutrient levels but low levels of chlorophyll. As Martin found, it is iron--blown from land in dust--that limits phytoplankton growth. Add iron and phytoplankton numbers bloom, Martin and others have shown in tests. Analysis of ancient air trapped in ice cores further revealed a connection between iron levels, global atmospheric CO₂ levels and global climate. This led Martin, who passed away in 1993, to proclaim: "Give me half a tanker of iron and I¿ll give you an Ice Age."

Working from this theory, Green Sea Venture postulates that fertilizing 16 million square miles of the Southern Ocean with 8.1 million tons of iron would zero out the world contribution to atmospheric CO₂ increases from burning fossil fuel--2.2 gigatons of carbon per year. "The potential of the oceans is so great that we ought to do the experiments that allow us to decide whether or not it is a worthwhile undertaking for climate control," says Lee Rice, president of the company. "But the scientific data are not conclusive." No one has yet been able to measure how much carbon sinks into the deep ocean, because the detritus sinks slowly. So that is the focus of an, as-yet-unscheduled, 5,000 square mile fertilization experiment, which the company would help fund. "In addition," says Rice, "there is a real gap between the scientific clarity and how to do this practically, since you do need to verify how much carbon has been sequestered in order to get paid." His investors, he asserts, are willing to wait for science's judgment about whether commercial activity is warranted.

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