Salting the Southern Ocean with iron results in increased growth of phytoplankton that take carbon dioxide from the atmosphere and extract the carbon into a form that eventually sinks deep into the ocean.
A remarkable expedition to the waters of Antarctica reveals that iron supply to the Southern Ocean may have controlled Earth's climate during past ice ages. A multi-institutional group of scientists, led by Dr. Kenneth Coale of Moss Landing Marine Laboratories (MLML) and Dr. Ken Johnson of the Monterey Bay Aquarium Research Institute (MBARI), fertilized two key areas of the Southern Ocean with trace amounts of iron. Their goal was to observe the growth and fate of microscopic marine plants (phytoplankton) under iron-enriched conditions, which are thought to have occurred in the Southern Ocean during past ice ages. They report the results of these important field experiments (known as SOFeX, for Southern Ocean Iron Enrichment Experiments) in the April 16, 2004 issue of Science.
Previous studies have suggested that during the last four ice ages, the Southern Ocean had large phytoplankton populations and received large amounts of iron-rich dust, possibly blown out to sea from expanding desert areas. In order to simulate such ice-age conditions, the SOFeX scientists added iron to surface waters in two square patches, each 15 kilometers on a side, so that concentrations of this micronutrient reached about 50 parts per trillion. This concentration, though low by terrestrial standards, represented a 100-fold increase over ambient conditions, and triggered massive phytoplankton blooms at both locations. These blooms covered thousands of square kilometers, and were visible in satellite images of the area.
Each of these blooms consumed over 30,000 tons of carbon dioxide, an important greenhouse gas. Of particular interest to the scientists was whether this carbon dioxide would be returned to the atmosphere or would sink into deep waters as the phytoplankton died or were consumed by grazers. Observations by Dr. Ken Buesseler of Woods Hole Oceanographic Institution and Dr. Jim Bishop of Lawrence Berkeley National Laboratories (reported separately in the same issue of Science) indicate that much of the carbon sank to hundreds of meters below the surface. When extrapolated over large portions of the Southern Ocean, this finding suggests that iron fertilization could cause billions of tons of carbon to be removed from the atmosphere each year. Removal of this much carbon dioxide from the atmosphere could have helped cool the Earth during ice ages. Similarly, it has been suggested that humans might be able to slow global warming by removing carbon dioxide from the atmosphere through a massive ocean fertilization program.
This report provides support for the idea that dissolving large quantites of iron into the ocean could be used as a technique to slow or perhaps even reverse the build-up of carbon dioxide in the atmosphere. The report of the Woods Hole scientists is particularly interesting because it suggests that some of the carbon pulled from the atmosphere by ocean iron fertilization will stay out of the atmosphere for long enough to be worthwhile as an approach for preventing global warming.
The controversial idea of fertilizing the ocean with iron to remove carbon dioxide from the atmosphere gained momentum in the 1980s. Climate and ocean scientists, as well as ocean entrepreneurs and venture capitalists, saw potential for a low-cost method for reducing greenhouse gases and possibly enhancing 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 to the deep ocean. Once in the deep ocean, the carbon would be "sequestered" or isolated in deep waters for centuries. The oceans already remove about one third of the carbon dioxide released each year due to human activities, so enhancing this ocean sink could in theory help control atmospheric carbon dioxide levels and thus regulate climate.
However; estimates have been produced, suggesting only US$1-5 per tonne of carbon fixed. This compares very well with US$50-200 for other proposed sinks, so both commercial and political interest is thus high.
A controversial report by the National Academy of Sciences in 1992 looked at iron fertilization, among other geoengineering options. Although the NAS noted some caveats, it concluded that iron fertilization does have one attractive feature: a relatively cheap price tag. Running 360 ships full-time to fertilize 46 million square kilometers of ocean would cost somewhere between $10 billion and $110 billion a year.
Understanding of the impact on greenhouse gas balances through ocean fertilisation by the addition of iron or nutrients, such as nitrates and phosphates, is still limited (Annex B). With regard to iron fertilisation, there are substantial uncertainties about the overall response of the Southern Ocean ecosystem, and it is possible fertilisation could lead to increased emissions of other greenhouse gases, nitrous oxide and methane, significantly offsetting any increased uptake of carbon dioxide. Estimated costs of ocean fertilisation are highly uncertain. One source estimates are £3 to £37/tC but this may be rather optimistic due to uncertainties over the effectiveness of the process. The other option, ocean fertilisation by nutrients appears to be even less practical than that for iron. Costs are also uncertain but are likely to be higher at £30 to £120/tC.
But those estimates will become more precise as additional research work more accurately measures how much of the carbon fixed by ocean plants ends up sinking into the ocean depths.
The ferilization of oceans with iron would need to be done continuously to maintain a continued rate of extraction of carbon from the atmosphere to counteract carbon dioxide emissions from fossil fuel burning. Cessation of seeding would very quickly lead to a return to lower levels of plankton.
Projections from this experiment indicate that if the polar oceans were completely seeded in such a fashion, atmospheric CO2 would decrease by about 10%. This would substantially mitigate the greenhouse effect caused by CO2. Such plankton growth has other benefits as well. One potential benefit may be that the increase in plankton would lead to an increase in the populations of other ocean fauna, such as whales and dolphins, that feed on plankton. Another benefit, again, is that it is relatively inexpensive. A continual iron-seeding program would cost only about $10 billion a year. Yet another benefit is that plankton growth stops about a week after seeding, so if the plankton were determined to have a detrimental effect, the effort could be quickly disbanded.
But why store carbon in the oceans?
Faced with the stark reality, even the International Panel on Climate Change has admitted that we may have to consider what it calls ‘carbon management strategies’ to complement reductions in greenhouse gas emissions. One option is to store the excess carbon on land; this is already being done in deep geological formations, abandoned mines and the like.
But it is the oceans that have the greatest natural capacity to absorb and store carbon. On an annual basis, the surface of the ocean absorbs about 30% of the carbon in the atmosphere, less during El Niño years. But over very long timescales, of thousands of years, as much as 85% is absorbed by the oceans. The ocean contains an estimated 40,000 billion tons of carbon, as compared to 750 billion tons in the atmosphere and about 2200 billion tons on land. This means that, were we to take all the atmospheric CO2 and put it in the deep ocean, the concentration of CO2 in the ocean would change by less than 2%.
Iron ocean fertilization could be a bone for fish growth as the fish ate away at the much larger quantities of plankton. So part of the cost of this proposal might be paid for in larger fish catches. If a property rights system could be worked out for the fertilized ocean fisheries then a portion of the sales of harvested fish could go toward paying for the ocean fertilization. Though a carbon tax on all oil, natural gas, and coal extraction could be levied as well.
|Share |||Randall Parker, 2004 April 19 12:53 AM Engineering Environmental|