Reducing atmospheric carbon dioxide, a greenhouse gas linked to global warming, by fertilizing the oceans with iron may not be as attractive a solution as once thought according to a report in Science magazine.
In their observations released on April 4, Ken Buesseler of the Woods Hole Oceanographic Institution and Philip Boyd of the University of Otago in New Zealand report that iron fertilization as a means to "lock up" carbon in the oceans should be explored but note that the most basic question of "Will it work?" needs to be addressed before such a strategy is undertaken on a commercial scale.
The controversial idea gained momentum in the 1980s, not only among climate and ocean scientists but with ocean entrepreneurs and venture capitalists who saw potential for enhanced fisheries. Plankton take up carbon in surface waters during photosynthesis, creating a bloom that other animals feed upon. Carbon from the plankton is integrated into the waste products from these animals and other particles, and settles to the seafloor as "marine snow" in a process called the "biological pump." Iron added to the ocean surface increases the plankton production, so in theory fertilizing the ocean with iron would mean more carbon would be removed from surface waters, where it is exchanged with the atmosphere, and carried to the deep sea.
But will ocean fertilization work? Buesseler and Boyd point to three open-ocean experiments in which iron was used to fertilize large sections of the Southern Ocean, the oceans surrounding Antarctica. The 13-day Southern Ocean Iron Enrichment Experiment (SOIREE), 21-day Eisen or Iron Experiment (EisenEx-1) and the most recent month-long US led Southern Ocean Iron Experiment (SOFeX) all produced significant increases in planktonic biomass and decreases in dissolved inorganic carbon in the water column. However, there was limited evidence that the particles carried large quantities of carbon to the deep ocean. The authors raise concerns over the space and time scales needed for commercial applications and the inefficiency of this process.
Buesseler participated in several iron fertilization experiments, and studied thorium, a naturally occurring element that is "sticky" by nature and serves as an easy-to-measure proxy for carbon export in seawater. Recent thorium experiments he and colleagues conducted in the Arabian Sea and around Antarctica show that many factors affect carbon uptake by plankton in surface waters. For example, biological communities and plankton production vary with location and season, so the balance between carbon uptake by the marine plants and carbon export on sinking particles is highly variable and typically only a small fraction of the carbon sinks to the deep ocean. The studies of the Arabian Sea and Antarctic waters support this new data and demonstrate that simply adding iron to the ocean may not result in enhanced removal of carbon dioxide from surface waters to the deep ocean.
There is growing commercial interest in trying an industrial-scale fertilization experiment. Territorial waters of the Marshall Islands in the South Pacific and off Chile have been proposed for large scale iron fertilization projects, and some businesses have suggested that countries could profit by trading carbon credits with industrialized nations. Others claim increased fisheries will be a positive result of adding iron to the surface waters.
"The experiments enabled us to make an initial determination about the amount of iron that would be required and the size of the area to be fertilized," Buesseler said. "Based on the studies to date, the amount of iron needed and area of ocean that would be impacted is too large to support the commercial application of iron to the ocean as a solution to our greenhouse gas problem."
"If adding iron to the ocean stimulates plankton that simply grow and die in the surface ocean, then it is a lot like proposing to grow grass to curb greenhouse carbon dioxide," Buesseler added. "When the grass is mowed or dies off in the winter, all of the carbon is returned back to the atmosphere. If you can grow a tree, you have a longer-term carbon sink, much like if you created an ocean bloom that resulted in carbon that naturally sinks to the deep ocean, a solution that can impact atmospheric CO2 for many decades or centuries to come. Knowing whether ocean fertilization is more like growing grass or trees is the question yet to be answered."
Adding iron to the ocean is likely to alter the ocean in unforeseen ways, Buesseler noted, and says the issue needs to be discussed by many people, from commercial interests, economists and fisheries biologists to national governments, climate modelers and ocean scientists.
"It may not be an inexpensive or practical option if what we have seen to date is true in further experiments on larger scales over longer time spans," he said. "The oceans are already naturally taking up human-produced carbon dioxide, so the changes to the system are already underway. We need to first ask will it work and then what are the environmental consequences?"
Woods Hole Oceanographic Institution: http://www.whoi.edu
SOFeX Cruise: http://www.mbari.org/expeditions/SOFeX2002/index.htm
Why Dump Iron in the Ocean: http://cafethorium.whoi.edu/Fe/Why-Iron.html
Plankton Corp: http://www.planktos.com/main.htm
1999 Cruise Results: http://www.whoi.edu/media/iron.html
Materials provided by Woods Hole Oceanographic Institution. Note: Content may be edited for style and length.
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