CDR: Ocean fertilization

Initially this method was perceived to be the solution to our budding CO2 problem with John Martin in 1988 declaring, 'Give me half a tanker of iron, and I'll give you an ice age'.  However, after numerous small-scale studies and growing knowledge regarding aquatic ecosystems this technique has been rendered obsolete with two UN conventions explicitly banning large scale ocean fertilization and a growing opinion that continuing investigation is a waste of resources (Strong et al., 2009). But before I get to the problems with this method, let me first outline what it involves:

How does it work?

This method proposes that by increasing primary production in the ocean surface waters, we can increase the amount of CO2 drawn down from the atmosphere.  Carbon fixing, photosynthetic organisms such as phytoplankton are nutrient limited, thus this increase in primary production can be achieved by supplying additional nutrients such as nitrogen (N), phosphorus (P) and Iron (Fe) to surface waters.

Martin's idea was born from the then contemporary studies exploring changes atmopsheric CO2 concentrations between the last glacial maximum (~200ppm) compared to the last interglacial and the pre-industrial levels of the current interglacial (~280ppm) (Martin, 1990).  The Vostock ice core indicated a negative relationship between atmospheric CO2 levels and Fe, as indicated in the diagram below.  This suggests that during periods of low CO2, iron was more widely distributed as dust from arid areas, palaeo-productivity studies indicate that during these time productivity was also high, supporting the Iron hypothesis that much ocean productivity is iron limited and clearly interacts with atmospheric CO2 concentrations.

The majority of studies and reviews focus mainly on Fe fertilization for two reasons:

• Iron is significantly more effective at increasing CO draw down, being 2 and 5 orders of magnitude more effective that P and N, respectively (Royal Society, 2009).
• N and P are significantly more expensive due to high demand for food production (Royal Society, 2009) ... also if you check out Cordell et al.'s (2009) story of phosphorus you'll see this particular resource is a hot commodity at the moment and arguably finite if we don't change our ways.

How effective would it be? - not very!

As previously mentioned, there have been numerous small-scale experiments but scaling up provides very little additional understanding as to global impact of large-scale implementation (Strong et al., 2009).  Subsequently, modelling has been employed to establish the likely impacts.  

The general consensus is that fertilization would be insufficient in mitigating global warming .  The study by Zaharlev et al. (2008) calculated that even if the Southern ocean were continuously fertilised such that there were no Fe limitation, enhanced uptake of CO2 peaked at less that 1Gt of CO2, less than 11% of annual emissions.  This mean that this technique alone could never really mitigate the high concentration of CO2 that we've put into the atmosphere, not now nor if annual emissions continue to rise!

Environmental risks

Now, I've spent A LOT of time reading about how terrible eutrophication can be (admittedly this has been in relation to terrestrial water bodies, but still - ), the effects can be devastating to aquatic ecosystems on all trophic levels!  The complexity of the ocean trophic structure far exceeds that of any terrestrial water body, making the effects of ocean fertilization fairly uncertain. Thats aid there are two likely effects that I believe to be worth mentioning:

• Ocean oxygen depletion: anoxia is a common symptom of eutrophication in lakes and coastal waters, with severe impacts on benthic communities and thus structuring of the entire food web. Sarmiento and Orr (1991) modelled various scenarios of CO2 increase and iron fertilization of the Southern Ocean.  The model indicated that oxygen depletion would occur with the biggest risk of anoxia in the Indian Ocean.  Palaeo-records have revealed that there have been numerous nutrient related oceanic anoxic events throughout the Earth's history, many of which have been so extreme as to induce marine mass-extinction (Kaiho and Hasegowa, 1994; Mort et al., 2007) ... we don't need to cause another!
• Increased greenhouse gases  (Fuhrman and Capone 1991): 'but I the aim was to reduce greenhouse gases?', I hear you ask.  This is where you begin to appreciate just how complex the Earths system is.  Increased productivity  would likely see an increase in methanogenesis that occurs during digestion of phytoplankton by zooplankton.  Whilst anoxia would also increased the production of methane by archaebacteria that dwell in anoxic waters.  Fe fertilisation would also cause an increase in N cycling, resulting in an increased production of N2O.  

Conclusion

Categorically banned under UN convention, large-scale ocean fertilization is considered to be ineffective.   It is unable to remove a high enough concentration of atmospheric carbon dioxide to be a viable option whilst simultaneously enhancing global warming through the increased production of other greenhouse gases.