Thin Soup and a Thin Story Sulandırılmış Çorba ve Sulandırılmış bir HikâyeФертилизацията с желязо не е решение на СО2 проблема

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.

Ingilizce’den çeviren Figen Mekik adlı bir firmanın, okyanusları gübreleme yoluyla karbon bedelinin hafifletilmesini sağlayacak bir ürünü çok ilgi görüyor. Okyanusun bazı yerlerinde, planktonların yetişmesi için gerekli tüm malzeme zaten mevcut; eksik olan tek şey eser miktarda demir. Kullandıkları her bir atom demire karşılık, bitkimsi plankton 50,000 atom karbon israf ediyor. Atmosferden karbon emmek için bundan daha güzel bir yöntem olabilir mi?

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