03 January 2015

Plowing Under a Carbon-fixing Crop

The common criticism of ocean fertilization by upwelling nutrients from the depths is that it also brings up CO2 from the depths. If one does not explore the issue more fully, it makes one think that upwelling nutrients is counterproductive.

Things look different if one uses push-pump pumps rather than simply upwelling of nutrients. Some of you may recall this argument from my GLOBAL FEVER book from the Univ of Chicago Press, but the following is an excerpt from my more recent THE GREAT CO2 CLEANUP, chapter six:
To avoid competing with the world’s food production and supplies of fresh water, most sequestered carbon must come from new biomass grown in new places. Here I explore how paired ocean pumps might uplift nutrients and then sink the new organic carbon back into the ocean depths.

Instead of sinking only the debris that is heavy enough to settle out, as in iron fertilization, we would be using bulk flow to sink the entire organic carbon soup of the wind-mixed layer (organisms plus the hundred-fold larger amounts of dissolved organic carbon) before its carbon reverts to CO2 and equilibrates with the atmosphere.
        The CO2 later produced in the depths by the sunken carbon soup will reach the surface 400-6,000 years later. Smearing it out over that period greatly reduces the damaging peaks in ocean acidification and global fever.
...If we fertilize via pumping up and sink nearby via bulk flow (a push-pull pump), we are essentially burying a carbon-fixing crop, much as farmers plow under a nitrogen-fixing cover crop of legumes to fertilize the soil. Instead of sinking only the debris that is heavy enough, we would be sinking the entire organic carbon soup of the wind-mixed layer. 
        Algaculture minimizes respiration CO2 from higher up the food chain and so allows a preliminary estimate of the size of our undertaking. Suppose that a midrange 50 g (as dry weight) of algae can be grown each day under a square meter of sunlit surface, and that half is carbon. Thus it takes about 10-4 m2 to grow 1 gC each year. To produce our 30 GtC/yr drawdown would require 30 x 1011 m2 (0.8% of the ocean surface, about the size of the Caribbean).

        But because we pump the surface waters down, not dried algae, we would also be sinking the entire organic carbon soup of the wind-mixed surface layer: the carbon in living cells plus the hundred-fold larger amounts in the surface DOC. Thus the plankton plantations might require only 30 x 109 m2 (closer to the size of Lake Michigan). 
        The space requirement will be more because downpumps will not capture all of the new plankton; it might be less because the relevant algaculture focuses on oil-containing algal species and on harvesting a biofuel crop, not on plowing under the local species as quickly as possible. The ocean pipe spacing, and the volume pumped down, will depend on the outflow needed to optimize the organic carbon production. [The chemostat calculation FYI.] Only field trials are likely to provide a better estimate for the needed size of sink-on-the-spot plankton plantations, pump numbers, and project costs. Though ocean fertilization is usually proposed for low productivity regions where iron is the limiting nutrient, another strategy is to boost the shoulder seasons in regions of seasonally high ocean productivity. For example, ocean primary productivity northeast of Iceland drops to half by June as the nutrients upwelled by winter winds are depleted. Continuing production then depends on recycling nutrients within the wind-mixed layer. However, to the southwest of Iceland, productivity stays high all summer.

       Because not all of the new plankton will be successfully captured and sunk, fertilization will stimulate the marine food chain locally. Most major fisheries have declined in recent decades and, even where sustainable harvesting is practiced, it still results in fish biomass 73% below natural levels. At least for fish of harvestable size, there is niche space going unused.
       Locating the new plankton plantations over the outer continental shelves is more likely to supply a complete niche for many fish species, whereas deep-water plantations will lack variety. (The main commercial catch in deep water is tuna.) Also, down-pumping near the shelf edge would deposit the organic carbon in the bottom’s offshore “undertow” stream, carrying it over the cliff onto the Continental Slope into deeper ocean.
        Note that pumps would be tethered to the bottom so that the ocean currents are always creating a plume downstream: a plume of fertilizer near the surface and a second plume of carbon soup in the depths. (Pumping up from a different depth than pumping down will prevent the interaction that characterizes the oceanographers’ box models.) While the water might come back around in a thousand years, the plumes for the clean-up will only be about twenty years long and well diluted by that time.

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