Image via NASA Earth Observatory:  “Sea-viewing Wide Field-of-view Sensor
(SeaWiFS) image shows chlorophyll concentrations in the N.E. Pacific Ocean.
Note the patch of high chlorophyll concentration (yellow and red pixels)
toward bottom center of the image. This patch was created by the SERIES Project
(Subarctic Ecosystem Response to Iron Enrichment Study).“
“Terraforming,” for readers who avoid the science fiction shelves in their local bookstores and libraries, is the word coined to describe changing another planet’s characteristics wholesale, obviously with the goal of making that planet more habitable for humans.* The photo to the right might be considered experimental terraforming of our own planet.
As the photograph’s NASA link explains, the image depicts a patch of sea off the Alaskan coast with strikingly different chlorophyll concentrations than the surrounding ocean:
By “seeding” the seawater with trace amounts of iron—the equivalent of one geritol tablet per hundred tons of sea water—scientists stimulated a fairly large phytoplankton bloom. In this patch, the chlorophyll levels are elevated from mid-ocean values of near 0.3 milligrams per cubic meter (blue pixels in this image) to more than 3 milligrams per cubic meter (yellow and red pixels)…
Similar experiments have occurred  in the South Pacific; taken together, they largely validate the so-called “iron hypothesis” advanced by Dr. John Martin, which asserted that the “desolate zones” of the open ocean where next to no phytoplankton (microscopic chlorophyll-based life) is found can be explained by these regions’ near-complete lack of dissolved iron, rather than by predation or the lack of other trace elements.
How does this relate to terraforming? Because, as Dr. Martin put it, “Give me a half tanker of iron, and I will give you an ice age.”  Inevitably, the surge of biomass produced by iron fertilization of a desolate zone will first remove carbon dioxide from the atmosphere and surface water, and then, upon death, settle into the deep ocean. This net removal of carbon dioxide from the atmosphere would contribute to global cooling, just as net additions of carbon dioxide contribute to global warming.
Personally, this seemed like a pretty cool (no pun intended) idea to me: if global warming ever gets too far out of hand, we may have an ace in the hole with Dr. Martin’s suggestion. As the linked biography suggests, though, the idea was quickly controversial; it is also clear that Dr. Martin himself was not advocating the immediate start-up of an iron fertilization program. Rather, he just wanted more research done, and the photo above is a tribute to his success on that score.
What are the possible drawbacks? Some of the experiments have shown less than expected carbon flow to the deep sea, so that the method’s efficacy is not always high; it would take more iron to work, with whatever drawbacks (beyond sheer cost) there are to iron level “spikes” in the upper ocean. Perhaps more importantly, the deep sea environment could change drastically; removing enough carbon dioxide to reverse historical industrial carbon dioxide additions to the atmosphere might well  raise deep sea pH (acidity) levels by 0.1 pH,** and deep sea organisms are very sensitive to such changes. As Richard Monastersky of Science News writes  in his 1995 account of the “IRONEX” experiment off the Galapagos, the decay of plant matter would rob the upper ocean of oxygen, and might release significant amounts of methane, an even more potent greenhouse gas than carbon dioxide. Even good friends of John Martin like Sally Chisholm  of MIT are among the leading skeptics of the “geritol tablet” option to fighting global warming.
So now I’m not sure about it, except as an emergency measure. I suppose the controversy may someday be, is the emergency already severe enough to warrant trying mass iron fertilization as a last-ditch way out, and who pays for it, and who approves doing it. It was interesting to read about, though. For me at least.
* Among the foremost science fiction novels dealing with the possibility are Kim Stanley Robinson’s Mars Trilogy (Red Mars , Green Mars , Blue Mars ). Robinson attempts to make the prospect seem legitimately controversial even for sterile Mars, and largely succeeds; as the titles indicate, that controversy is won by the terraformers.
**It’s been a while, but I believe this equates to about a 25% increase in acidity; not a gargantuan increase, but not as negligible as “0.1” looks at first glance. As the link indicates, “reverse” means to return atmospheric carbon dioxide to twice the pre-industrial level, and not the attempt to return all the way to that level.
UPDATE, 3/24/06: broken link at “occurred” edited to point to “SOIREE: A Phytoplankton Party in the South Pacific .” SOIREE stands for “Southern Ocean Iron Enrichment Experiment .”
EDIT, 12/3/12: new photo image uploaded to, linked from Photobucket account, captioned.