The idea of fertilizing the ocean with iron to capture carbon is one of the more colorful ideas to arise directly from a basic understanding of biogeochemistry. Ocean phytoplankton are iron-limited and alleviating that iron limitation makes them grow enough to cause some of the fixed carbon to get locked away in the deep ocean. There was substantial debate about a decade ago about the feasibility of this idea for fighting climate change and I seem to remember that most of the scientists involved decided that it was an interesting but not practical idea.
A re-promotion of the idea was making the internet rounds today due to this profile of a prominent advocate—practicioner even—of iron fertilization. I had forgotten the details of the debate so it was fun to revisit them. The profile is written in such a way to make it seem like iron fertilization is a great idea that is being held back only by environmentalists scared of geo-engineering. Unfortunately for the author–or perhaps to their credit in being objective–the counterarguments to this viewpoint are apparent in the article itself. The author writes:
iron fertilisation could potentially sequester as much as 1 billion metric tons of carbon dioxide annually, and keep it deep in the ocean for centuries. That is slightly more than the CO2 output of the German economy, and roughly one-eighth of humanity’s entire greenhouse gas output.
This sounds good until it is put to a simple cost benefit analysis. The one-eighth figure likely comes from a modeling study, and is also discussed in an editorial in Nature arguing against iron fertilization back in 2009. (I note that the authors here are better described as ‘scientists’ than ‘environmentalists.’ ) In that editorial they write:
A model published in 2008 (K. Zahariev et al. Prog. Oceanogr. 77, 56–82; 2008), which is as convincing as any available, found that even if the entire Southern Ocean were fertilized forever with iron sufficient to eliminate its limitation of phytoplankton production, less than 1 gigatonne of carbon a year of CO2 of probable future emissions (currently about 8 gigatonnes a year) would be sequestered, and that amount for only a few years at best.
So there you have the cost and the benefit. The benefit: we may reduce 1/8th of our emissions, a substantial and impressive amount, but nowhere near enough to stop global warming. The cost: we risk fucking up an entire ocean. The ‘environmentalists’ were ridiculed in the the pro-fertilization piece for having that attitude, but it hardly seems outrageous to worry that fertilizing an entire ocean to stop 1/8th of our emissions could have unintended consequences, and the Nature editorial shows that scientists, myself included feel the same way.
There are too many examples of fragile food webs tied together as trophic cascades and biocontrol agents gone wrong to not worry about unintended consequences of a vast ecological manipulation in a system that is not totally understood. We are still barely getting a handle on the consequences of doubling carbon in the atmosphere. Is it wise to perform a similar experiment with another element in the ocean? The pro-fertilization piece crescendos toward this point at the end:
The ocean is no longer a vast, unknowable wilderness, whose mysterious gods must be placated before it can be crossed. Instead, it’s become the first viable arena for large-scale manipulation of the planetary environment.
What could possibly go wrong?