A sea of uncertainty

In 1999 when John Martin infamously asserted 'Give me half a tanker of Iron and I will give you an ice age' the world, perhaps, thought for a second that the problem of ocean acidification could be solved.

Phytoplankton absorb CO2 and sunlight to produce
energy and photosynthesis

His statement was made in response to his hypothesis that an increase in the primary production of planktonic communities found in high nutrient, low chlorophyll (HNLC) areas of the surface ocean would increase the amount of carbon sequestration from the atmosphere. This hypothesis arises as during the glacial period the interactions between iron availability and silicon usage by diatoms in the Southern Ocean are thought to explain, in part, the reduction in atmospheric CO2 concentrations (Siegenthaler, 2005). Iron rich conditions are therefore thought to have caused the long term sequestration of atmospheric CO2 that occurred during the glacial intervals.

Primary productivity in 30–40% of the world's oceans is limited by the availability of iron, particularly in the open-ocean regions of the Southern Ocean, equatorial Pacific Ocean and north Pacific Ocean (Moore et al., 2002). Increasing the amount of iron in these regions has thus been explored due to the potential of added iron to reproduce the interactions that occurred in the oceans during glacial period. 

Phytoplankton are nutrient limited and as a result an increase in the nutrients iron (Fe), nitrogen (N) or phosphorus (P), would increase their primary production, producing large 'blooms' of photosynthesising algae. The hope of studies that research iron fertilisation is that the ocean, with added iron, will sequester CO2 and thus slow the rising levels of warming caused by climate change (Armbrust, 2009) however most studies so far have shown increased productivity (see fig above) however this is limited to consumption and recycling in the surface - there is little deep water sequestration. 

Studies have focused on the use of iron due it being both the most efficient out of the three by 2 and 5 orders of magnitude (respectively) and the cheapest out of the three (The Royal Geographical Society, 2009).

Green patches in the sea are phytoplankton communities 

The IPCC have predicted that by 2100, mean surface ocean pH will be at a level of 0.44 units lower than pre-industrial levels while atmospheric CO2 will measure 965ppm (IPCC, 2000). Using these predictions, Cao and Caldeira (2010) measure the extent to which iron fertilisation could be used to reverse the effects of climate change by bringing us back to pre-industrial temperatures - or as John Martin asserts give us 'an ice age'. Their study found that iron fertilization could only reduce mean surface ocean pH to 0.38 units lower than pre-industrial levels while atmospheric concentrations of CO2 could only be reduced to 833ppm (Cao and Caldeira, 2010). 

Focusing more specifically on the Southern ocean, Zarharlev et al., (2008) concluded that even if there was continuous fertilisation, the uptake of CO2 would only reach a maximum of 1Gt - which is less than 11% of our annual emissions. More recently, Martin et al., (2013) have shown that iron fertilization enhanced net community production but not downward particle flux in the Southern Ocean due to the ability of zooplankton communities to reprocess sinking particles and alter particle size distribution - both of which prevent the export and sequestration of CO2.

Iron fertilisation is therefore limited as a method to address climate change as it can not bring about large scale reductions in surface ocean pH or atmospheric CO2. Further to this, it does not necessarily stimulate the particulate organic carbon (POC) export and sequestration desired under limited Si concentrations. 

In addition to this, there are a number of potential environmental impacts that could result from such large scale fertilisation such as - 

1. Ocean oxygen depletion - anoxia, caused by eutrophication in lakes and coastal regions, can have huge negative impacts on bethnic communities and thus the whole food web of the ocean and as oxygen depletion is an outcome of iron fertilisation the risk of anoxia is high, particularly in the Indian Ocean - the implication of this could be catastrophic. 
2. Nutrient depletion in surrounding waters 
3. Harmful algae blooms could develop
4. The size of the phytoplankton species may change and become larger which would have implications for the food web of the ocean
5. Increased greenhouse gases as a result of increased methanogenesis that occurs in the digestion of phytoplankton

According to the IPCC, ocean iron fertilisation offers a potential method for removing CO2 from the atmosphere due to the potential of phytoplankton to sequester carbon dioxide in the form of particulate organic carbon. Having said that, they also state that ocean iron fertilisation remains largely speculative and many of the environmental side effects need to be assessed. Large scale iron fertilisation could have negative impacts on marine life and human health.

Despite this, however, there are rumblings that a commercialisation of iron fertilisation will occur in order to sell carbon offsets in a future where the price of carbon has increased due to climate treaties having mandated even stricter caps on emissions and governments having issued higher taxes.

As iron fertilisation would likely occur beyond a countries 200 mile exclusive economic zone, regulation would fall under international law. At present, the London Convention treaty that concerns the sea promotes the effective control of all sources of marine pollution and governs the deliberate disposal of waste or other matter at sea for its 82 treaty members. The updated London Protocol agreement specifies that all dumping is prohibited except for some specified wastes such as carbon dioxide from industrial carbon capture processes into sub-seabed geology. Under these two agreements iron fertilisation is not specifically dumping however even if it were it is not unusual for entities seeking to skirt a treaty to register their ships in a nation that is not part of the treaty or does not enforce is very strictly (Powell, 2008).

These weaknesses in International law may be countered by economic markets and the strict regulations imposed on the trading of carbon credits by the Kyoto Protocol and the European Union Emissions Trading Scheme. In an attempt to console, Powell (2008) highlights these regulations mean that carbon credits from iron fertilisation can, at present, only be sold on the voluntary carbon market. 

In my opinion, this isn't consoling at all as, despite it currently being much smaller than other segments of the carbon market, the voluntary market is growing and in addition to this it is not regulated strictly like Kyoto and EU projects - this could be dangerous as improper and inaccurate accounting and dumping of iron could occur. Buesseler et al., (2008) argue that it would be premature to start selling carbon offsets from commercial iron fertilization due to uncertainty over how effectively CO2 is removed from the atmosphere and retained in the ocean for a significant amount of time. In addition to this there are numerous environmental impacts and risks of unregulated dumping.

Buesseler et al., (2008) later wrote "moving forward on OIF should only be done if society is willing to acknowledge explicitly that it will result in alteration of ocean ecosystems and that some of the consequences may be unforeseen". In my opinion I am not sure if gaining societies acknowledgement is enough to permit iron fertilisation (is acknowledgement approval?), nor do I feel that societal acknowledgement is the direction that should be taken 'moving forward'. If the method is ineffective and the risks are too great than the method cannot be used to fix climate change.