Thin Soup and a Thin Story

A firm called is getting a lot of airplay for their bid to create a carbon offset product based on fertilizing the ocean. In certain parts of the ocean, surface waters already contain most of the ingredients for a plankton bloom; all they lack is trace amounts of iron. For each 1 atom of iron added in such a place, phytoplankton take up 50,000 atoms of carbon. What could be better?

Phytoplankton biomass does not last forever, any more than tree biomass does. The trick therefore is to get the carbon to sink out of the surface ocean into the depths, generally in the forms of snot and poop. Once it reaches a depth of a kilometer or so, it can decompose to CO2 again but the water will be isolated from the atmosphere for decades, maybe centuries.

There have been iron fertilization experiments of the ocean before, many of them, in the equatorial Pacific, the Southern Ocean, and the North Pacific. These are places where the ocean chemistry is right for iron fertilization, that is, where there is available nitrogen as nitrate or ammonia, and phosphorus. The experiments uniformly find that phytoplankton growth is stimulated by iron. But most studies have not found an increase in the rate of organic carbon sinking into deeper waters.

If could be, however, that a sustained fertilization will allow the snivelers and the poopers time to get their acts in gear and start exporting carbon more efficiently. This was the conclusion of a recent analysis of natural iron fertilization by the Kerguelen Plateau in the Southern Ocean (Blain et al, 2007). Previous iron fertilization experiments were generally single-pulse additions of iron dissolved in acid. The iron lasted a few days before sinking out on particles or mixing down. If iron were released into the ocean in the form of floating time-dissolving pellets, the steady stream of iron would probably be a more effective fertilizer than the single dumps were.

Once the CO2 concentration of the upper ocean is depleted by growth and sinking of phytoplankton, the timescale for gas exchange with the atmosphere is about a year for a one-hundred meter ocean mixed layer, typical of the tropics. Tropical surface waters, one could argue, will still be at the surface a year from now, so there is plenty of time for them to replenish their CO2 concentration by sucking it out of the atmosphere.

The problem with the tropics is that if tropical surface waters are destined to remain at the surface for a while, they are also probably destined to ultimately scrounge the iron they need, to use the available nitrogen and phosphorus. The water might duck into the thermocline for a few decades, but it will ultimately resurface and be subject again to photosynthetic plankton and iron fertilization from falling dust. Marinov et al (2006) showed that a stimulation of phytoplankton production in one part of the ocean usually acts to depress production elsewhere. So what’s the point of paying for a carbon offset to fertilize a water parcel now, when nature would fertilize it soon anyway? That’s against the rules of offsets; it has to be something that wouldn’t happen anyway.

The one part of the ocean where fertilization of the ocean does not depress the fertility elsewhere is the deep Southern Ocean. Here the water sinks to the abyss, rather than taking a leisurely tour through the upper ocean. But now the practical picture looks different. Instead of the benign tropics, you have sea ice, waters mixed to hundreds of meters down (bad for phytoplankton) and total darkness for much of the year. Fertilize that!

Modelers have long ago concluded that iron fertilization of the ocean can play only a small role in managing the carbon cycle in the coming century. Part of the issue is that the Southern Ocean also covers only a very small area of the surface ocean, just a few percent. Model experiments where the Southern Ocean is completely fertilized show a drawdown of maybe 15 ppm by the year 2100 [Zeebe and Archer, 2005]. We could change a light bulb and do better than that.

Perhaps however the total potential drawdown from ocean sequestration is the wrong question to ask. The total rate of biological export production in the ocean is probably of the order of 15 Gton C / year, and the fertilization enhancement could be at most maybe 1 Gton C / year. That can’t slay the 7 Gton C / year fossil fuel CO2 dragon all by itself, but could it help? Nowadays we’ve given up the idealistic search for a single solution, and we’re building the future out of wedges [Pacala and Socolow, 2004], or what the more dignified IPCC Working Group III calls a “portfolio of solutions”. Would carbon offsets by fertilizing the ocean be at least realistic?

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