Why the Rust Swam Away

Scrubbing rust stains out of the sink the other day, I suddenly remembered the controversial idea of easing global warming by dosing the seas with iron. I knew that somebody had done something to test the idea, and that the test hadn't fulfilled the hopes. Then I forgot about it again, until volume 143, number 1943, of the journal New Scientist emerged from the mailbox amidst a clot of catalogs. It contained an article showing that the idea was both sound and useless.

Nowadays just about everyone who reads a newspaper understands how a buildup of carbon dioxide (among other gases) is keeping Earth's heat from escaping back out to space. The gases are acting much like the glass in a greenhouse, and their presence is as much a result of human activity as is greenhouse glass. Humankind is burning fossil fuel by the megaton, and the burning is releasing carbon dioxide that had been locked away by millions of years of plants' growing, dying, and being buried.

It's occurred to many people that if plants once held extra carbon dioxide, maybe they could be encouraged to take it up again. Top candidates for the job are the fast-growing oceanic microplants, the phytoplankton. Some scientists suspected the only reason phytoplankton weren't sopping up all the extra carbon dioxide already was that they ran out of other necessary substances---fertilizers, in effect, such as the nitrates and phosphates gardeners need to add to their vegetable gardens.

Yet huge areas of the oceans seem to have plenty of those crucial nutrients without supporting an accompanying bloom of phytoplankton. Various explanations were offered, including a suggestion from the late John Martin of California's Moss Landing Marine Laboratory that an insufficiency of iron might limit how much phytoplankton a section of seawater could support.

Martin's idea earned some support, but more scoffing. Dissolved iron could be measured everywhere in the sea, even if sometimes in very small amounts. Therefore, it could not be the limiting factor.

That argument sounded convincing until analytical techniques improved so much that researchers were able to recognize that, in some places, the iron their instruments were finding was only the molecular trail of their own steel research vessels. Martin's idea began to seem worth trying.

A little over a year ago, oceanographers from the U.S. and Britain sprinkled iron that had been chelated (treated so it was more immediately useful for plants) over eight square kilometers of the equatorial Pacific Ocean. The Gulf of Alaska had been considered as one site for the experiment, but in November, well, more southerly seas seemed better for plankton growth and oceanographers' survival.

Phytoplankton numbers tripled within three days, showing that iron deficiency had indeed been limiting their growth. But only a day or two later, the phytoplankton bloom was being eaten away by swarms of zooplankton. No one has yet figured out where these current-borne little sea animals came from; they didn't have time to breed in response to the suddenly enriched pastures. But somehow, there they were.

From the point of view of removing carbon dioxide from the atmosphere, all was not yet lost. Anyone who has had to clean out a stable appreciates that plant-eating animals convert a lot of their food into carbon-containing wastes. In the ocean, zooplankton wastes take the form of fecal pellets. Quantities of the pellets sink to the sea floor, and thus can remove a fair amount of carbon from circulation.

However, the scientists carefully monitoring the experiment discovered that another kind of organism multiplied exuberantly in the aftermath of the iron addition. Bacteria immediately attacked the fecal pellets, releasing lots of carbon dioxide in the process.

Thus, adding iron to barren waters led to a great flurry of life, but left the global greenhouse intact. Next summer, I'll worry about that. Honest.