A few readers push back on this post:
As co-author of a marine science book who gives frequent public talks, I get asked about the iron sulfide engineering idea a lot. The downward flux issue is real, but there’s another major problem with the idea that’s even easier to articulate: what happens when those uber-blooms of plankton die off? Even assuming the carbon sequestration worked perfectly, you’ve now filled large swaths of the Southern Ocean with countless tons of dead plant matter. Bacteria will bloom to decompose it, creating enormous anoxic “dead zones” where pelagic fish and other organisms our species enjoys eating/admiring cannot live. Similar phenomena can be observed near major fertilizer runoff sites. The whole point of averting climate change is to prevent the ocean from turning into a sludgy toxic mess! This idea’s side effects are the very problems it means to combat.
Another is on the same page:
You ended your post about Victor Smetacek saying, “Further experiments, however, were halted due to protests from environmentalists.” But you did a disservice to them in not bothering to explain in part why they complained. As was noted in Scientific American:
A similar cruise and experiment in 2009 failed despite dumping even more iron fertilizer over an even larger area of the Southern Ocean. The eddy chosen for that experiment lacked enough silicon to prompt these particular diatoms to grow. Instead, the experiment yielded bloom of algae, which was readily and rapidly eaten by microscopic grazers. As a result, the CO2 in the algal bloom returned to the atmosphere.
In fact, these iron-seeding experiments could backfire by producing toxic algal blooms or oxygen-depleted “dead zones,” such as the one created in the over-fertilized waters at the mouth of the Mississippi River. At present, scientists have no way to ensure that the desired species of silica-shelled diatoms bloom. In short, Smetacek says, the type of bloom—and therefore the ability to sequester CO2—”cannot be controlled at this stage”.
This could be a great way to sink carbon, but we’ve gotten ourselves into problems before assuming a fix will be fine without paying attention to what might go wrong and I can’t blame people for urging caution in going forward here.
Update from a reader:
Not to pile on, but one more issue: Scaling. Thus far, we have data on ocean iron fertilization (OIF) only on a single-trial basis. We have to rely on models to extrapolate from the data at the global scale. However, here is a paper from Nature which makes several assumptions extremely favorable to OIF.
Briefly, I will highlight the favorable assumptions: It assumes a high (RCP 8.5) emissions scenario (thus relative impacts are maximized), continuous fertilization from 2020-2100, instantaneous deployment at full scale, and a full release of iron limitation from all phytoplankton south of 40° latitude.
The sum of this wildly optimistic model? A net loss of -.15°C (Table 2). Even under a low-emissions scenario (or low-sensitivity scenario favored by skeptics) OIF’s maximum potential is to buy us an extra decade at an unknown ecological price.
The Dish 