Four alternative methods of ocean fertilization are known to the authors, viz.:
1. Oceanographer John Martin disclosed a method for fertilizing High Nutrient Low Chlorophyll (HNLC) areas of the ocean with iron in order to stimulate phytoplankton growth so as to absorb atmospheric CO2 (Martin,1990). The practicality of the method has been convincingly demonstrated experimentally. However the method is limited to HNLC regions of the ocean where there is already a high nitrogen concentration.
2. Another method of artificial ocean mixing would be to use an Ocean Thermal Energy Conversion (OTEC) device to bring deep nutrient rich waters to the surface. Such a device has been in operation in Hawaii for a number of years. The nutrient rich waters are fed into on-shore fish farms.
3. An apparatus for artificial ocean mixing has been proposed whereby a flexible vertical pipe with a valve pumps deeper water to near the surface by wave action. (Kithil and Boeing: Ocean Carbon Biogeochemistry Workshop, Woods Hole Oceanographic Institute, July 10-13,2006). A similar idea has been proposed by James Lovelock.
4. Ian Jones of the University of Sydney has proposed using fixed-nitrogen products such as urea, made on land, to fertilize low nitrogen regions of the ocean.
Of these only the first is likely to prove both economic and viable. There have been a number of experiments that have demonstrated that distributing iron in the ocean does indeed lead to an increased phytoplankton productivity. Whether this leads in turn to greater sequestration of atmospheric CO2 remains in question. Absence of experimental evidence for this is largely the result of the relatively short time scale of oceanographic cruises.
An excellent review of this work has been given by Falkowsky (2002) who points out that some serious difficulties with the method remain. To quote him:
Major disruptions to the marine food web are a foremost concern. Computer simulations and studies of natural phytoplankton blooms indicate that enhancing primary productivity could lead to local problems of severe oxygen depletion. The microbes that consume dead phytoplankton cells as they sink toward the sea floor sometimes consume oxygen faster than ocean circulation can replenish it. Creatures that cannot escape to more oxygen rich waters will suffocate.
Such conditions also encourage the growth of microbes that produce methane and nitrous oxide, two greenhouse gasses with even greater heat-trapping capacity than CO2.
None of the other three methodologies can deliver the quantity of nutrient required at sufficiently low cost. In order to compare with the 220 m3/s yield of a Nutrient Megapump, calculated under Nutrient Megapump, an ocean wave device would need to intersect the waves across 4 km of ocean. This is assuming 100 percent efficiency. The fact that, despite several decades of work in this field, no viable offshore wave power device has yet been contrived makes this solution look rather fanciful.
Nevertheless, Method 4, artificial fertilization by means of land based chemicals may provide a useful adjunct to the method proposed here, for fine tuning the Nutrient Megapump's artificial marine ecosystem.
The major problem with both the iron method and the urea method is that they do not address the issue of phosphorous depletion. Phosphorus is essential in the production of nucleic acids and its absence would provide a major limitation to plankton growth.
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