Studies have shown that fertilisation in HNLC regions (high-nitrate and low-chlorophyll, approximately 30% of the world’s oceans) could sequester 0.5 Gt of carbon per year. An even better rate could be achieved in oligotrophic waters by stimulating primary productivity of phytoplankton. However, using OIF leads to large-scale changes in ocean chemistry and the ocean ecosystem. The changes also occur over a long period of time, as the OIF is supposed to work in a scale of at least a hundred years.
The application of iron to the ocean directly leads to an increase in phytoplankton and thereby to an increased production of organic matter, which then sinks into deeper ocean levels. It is important that the sequestered CO2 remains in the ocean over a longer period of time, which means it has to stay in depth below the zone where winter mixing occurs. Only the CO2 that reaches deep waters can be seen as sequestered. However, a certain part will not reach this depth and can still affect the surface layer with increased availability of nutrients, inorganic C and an oxygen deficiency. Therefore processes in mid-depth water still need to be studies in more detail.
As a side effect the production of DMS (dimethylsulfite), which is a source of atmospheric sulfur, can be enhanced. As discussed in a previous post, sulfur can lead to cloud formation and thereby enhance albedo, which is another positive effect in terms of climate change mitigation.
With the increased availability of iron more of the macronutrients in the surface layer of the ocean can be used. This in turn leads to a change in the nutrient composition, which can later on lead to a limitation of further primary production. This again can have impacts e.g. on fishing resources.
As another downstream effect OIF could release N2O and methane, which are even more potent greenhouse gases than CO2.
Changes in source water in coastal areas can either have positive or negative effects. One positive effect is that the increased nutrient availability can lead to increased primary production. On the other hand, coastal hypoxia can increase or become more frequent, which has already been witnessed in previous OIF experiments. Cullen and Boyd (2008) further suggest, that all changes go on gradually, increasing with repeated fertilisation processes. However, the expected changes can so far not be quantified and we still rely on estimated values. Further research is still needed, before OIF can be seen as a promising and secure geoengineering method. The problem with predictions is the time scale, as experiments today can not show us the impact if iron fertilisation in 100 years. Furthermore there are likely to be other unpredictable effects, which result from the modification of the ocean system. The articles stresses the point, that currently uncertainties are still too high to make sell carbon offset, as it was also suggested. However, Cullen and Boyd (2008) see OIF as a promising method, which on the other hand bears risks.
I think they have a point. From what I have read, ocean fertilisation sounds as a promising option at first sight. However, in my opinion the uncertainties about side effects and long-term ecological consequences are still too high to modify the ocean system on such a large scale. It may be frustrating to repeat this with almost every suggested technique, but again further research is necessary to make a decision about the use of OIF.
Literature:
Cullen, J.J., Boyd, P.W. (2008) ‘Predicting and verifying the intended and unintended consequences of large-scale ocean iron fertilization’ Marine Ecology Progress Series, 364, 259-301.
Available online: http://www.int-res.com/abstracts/meps/v364/p295-301/
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