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18 Oceanus Magazine Vol. 46, no. 1, 2008 www.whoi.edu/oceanusThe first part of biogeo-chemist John Martin’s famous prediction—“give me half a tanker of iron, and i’ll give you an ice age”—has been proved partly right: iron is the only thing standing in the way of plankton blooms in some regions of the ocean. But still far from certain is the second part—that this could actually help draw heat-trapping carbon diox-ide from the air, sequester it in the deep ocean, and lower global temperatures. it’s based to a large degree on evidence linking cold periods in past ice ages with high accumulations of iron-rich dust, but a direct cause-and-effect has been hard to prove.in projecting the future of iron fertilization, scientists use several lines of evidence to augment what they have learned from the 12 small-scale iron-addition experiments they have conducted since 1993 (see Page 10). With computer models, they add unlimited amounts of imaginary iron into a simulated ocean. From samples cored from antarctic gla-cial ice and the ocean floor, they track roller-coaster levels of iron, carbon dioxide, and temperature through eight separate ice ages. and by examining places where blooms occur naturally, they glean hints for how the ocean responds to iron coming from natural sources.These three approaches help predict an upper limit for what future iron fertiliza-tion attempts might achieve, and the pre-dictions are large. ice-core records show regular, naturally occurring periods over the past 500,000 years in which high levels of iron-rich dust in the atmosphere coincided with decreased atmospheric carbon dioxide levels of up to 100 parts per million (ppm). (For comparison, atmospheric carbon dioxide levels have increased by 100 ppm since the industrial Revo-lution.) global computer models give a high end of around 83 ppm for cO2 reduction from fertilizing the oceans’ iron-limited regions. and some parts of the ocean are up to 10 to 100 times more effec-tive at using naturally oc-curring iron to stimulate plankton blooms than any human-designed experi-ments have yet done.But ice-core evidence cannot prove whether iron-rich dust caused the cO2 declines, or whether both resulted from simi-lar causes. The upper limit predicted by the models would require draining the southern Ocean of all the remain-ing nutrients that plank-ton also need to grow, an outcome that humans probably can’t engineer and in any case wouldn’t want to. and natural fer-tilizations appear funda-mentally different from artificial experiments—right down to the type of iron involved.among the speak-ers at a conference on ocean iron fertilization convened at Woods Hole Oceanographic institution (WHOi)in sep-tember 2007, three scientists took a clos-er look at each of these approaches: Jorge sarmiento of Princeton university, Robert anderson of Lamont-Doherty earth Ob-servatory, and stéphane Blain of cnRs/ université de la Méditerranée, France. Learning from computer modelsWhen modelers work on iron fertiliza-tion, the experiments happen inside their computers, giving them the freedom to think big. instead of scaling up from small iron additions, they can turn the problem around, asking how much iron plankton would require to use up all the nutrients in surface waters. Their models simulate as many real-world physical, biological, and chemical laws as the scientists can program and the computer processors can handle.at the heart of the question lies the fact that the oceans’ supply of nutrients is more or less fixed. Most of the nutrients come from the subsurface ocean and well up to the surface where phytoplankton grow. growing plankton combine these nutrients with iron and carbon dioxide according to a fairly standard recipe. When the nutrients are gone, further iron addition can have no effect on carbon dioxide levels. so model-ers can calculate iron’s maximum effect in much the same way as a chef can calculate a reci-pe’s yield.The answer, accord-ing to sarmiento, is sur-prisingly small in every proposed fertilization region except the south-ern Ocean: about 4 ppm drawdown in atmospheric carbon dioxide for the equatorial Pacific, and a little less for the northeast Pacific. “The southern Ocean is the place where you can really reduce cO2 in the atmosphere by quite a significant amount, [on the order of] 70 ppm,” or about 150 bil-lion tons of carbon, sarmiento said. The southern Ocean has so much po-tential because its nutrient levels are three Lessons from Nature, Models, and the PastOther lines of evidence inform the debate on ocean iron fertilizationBiogeochemist John Martin promulgated the “iron hypothesis” in the 1990s, saying that iron in the ocean could stimulate phytoplankton blooms that would help draw carbon dioxide from the air into the ocean and lower global temperatures. “The Southern Ocean is the place where you can really reduce CO2 in the atmosphere by quite a significant amount, [on the order of] 70 ppm, or about 150 billion tons of carbon.”—Jorge Sarmiento,Princeton UniversityCourtesy of Lynn McMasters, Moss Landing Marine LaboratoriesWOODs HOLe OceanOgRaPHic insTiTuTiOn 19to five times higher than those of other iron-deprived waters. adding iron to these nutrient-rich waters stimulates plankton to take up cO2 dissolved in the ocean. When the plankton die, or are eaten and excreted, their particles sink, and carbon is transport-ed toward the depths, where it is less likely to be recycled back into the atmosphere.But there are problems with fertilizing the southern Ocean, sarmiento reported, citing model results from Princeton, MiT, ucLa, stanford university, and Los ala-mos national Laboratory. First are the practical obstacles: pack ice, six months of darkness each year, and horrendous gales. as fierce winds blow waters across the sea surface, deeper waters upwell to replace them, bringing carbon dioxide upward, back in contact with the atmosphere. “it’s going to be really tough with a real-istic iron model to see the full realization of that 70 ppm that we get when we force the nutrients to zero artificially” (with models), sarmiento said.in addition, sarmiento pointed out that currents in different parts of the southern Ocean make a crucial difference in how deeply carbon might be sequestered. close to the antarctic continent, cold surface wa-ter sinks to the bottom of the ocean to be-gin a centuries-long creep northward. This is the ideal place


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