Thin Soup and a Thin Story
david @ 9:12 am
A firm called planktos.com 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?
The tropics I think would be fraud as a basis for carbon offsets because the fertilization would have happened anyway, eventually, naturally. I guess I could imagine the concept working as advertised in the deep Southern Ocean. Not so easy to fertilize down there, but if you manage to fertilize it, you will accomplish something that wouldn’t have happened anyway.
But the change in carbon chemistry of the ocean and ultimately the atmosphere need to be transparently documented, also, if we are to trade carbon offsets based on iron fertilization. Documenting a change in carbon content of surface waters might be possible in the tropics, but it would be a nightmare in the Southern Ocean, probably impossible to do reliably. Ocean chemistry data is generally cleaner than land data, less susceptible to local variability. In tranquil, well-behaved parts of the ocean like near the Galapagos, it would be probably easier to document changes in the carbon content of the upper ocean than it would be on land. On the other hand, the ocean moves around a lot more than the land does, in general. The Southern Ocean, in particular, is a maelstrom. Tracking a plume of fertilized water to measure the change in carbon content would be a mite trickier.
Southern Ocean surface water also has a harder time changing the CO2 concentration of the atmosphere, because it gets mixed into the interior so quickly. Ultimately it would take centuries to bring the atmospheric CO2 to a new equilibrium value. You would have to wait until your fertilized water filled up the entire deep ocean. I think the long time scale also means that a ton of carbon removed from Antarctic surface waters does not translate to a ton of carbon removed on some reasonable timescale from the atmosphere. The efficiency is much lower than that, and difficult to document.
I would put ocean fertilization on the avoid list, along with planting trees. It's too hard to pin down the actual amount of CO2 removed from the atmosphere by your actions. It's also not a long-term solution, since the ocean leaks. Humankind would have to keep fertilizing the ocean indefinitely in order to preserve the claimed CO2 drawdown. If you’re concerned about climate change, build a windmill. Ocean fertilization does not seem to me suitable to be the basis for a reliable financial commodity, or a practical tool for geo-engineering climate.
David Archer
Blain, S. Effect of natural iron fertilization on carbon sequestration in the Southern Ocean. Nature, doi:10.1038/nature05700, 2007.
Marinov, I. The Southern Ocean biogeochemical divide. Nature, doi:10.1038/nature04883, 2006.
Pacala, S. and S. Socolow, Stabilization Wedges: Solving the Climate Problem for the Next 50 Years with Current Technologies. Science 305: 968-972, 2004.
Zeebe, R. and D. Archer, Feasibility of ocean fertilization and its impact on future atmospheric CO2 levels. Geophys. Res. Letters, doi:10.1029/2005GL022449, 2005.
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