NRC Ocean Iron Fertilisation – rebuttal for Iron Salt Aerosol

NRC Ocean Iron Fertilisation – rebuttal for Iron Salt Aerosol

NRC Ocean Fertilization Rebuttal


This rebuttal addresses the claims made in the Ocean Fertilisation section of a document titled Climate Intervention: Carbon Dioxide Removal and Reliable Sequestration, published in 2015 by the National Research Council of the National Academies (NRC).

While their claims may well have been valid regarding ship-based, propeller-spread ocean iron fertilisation, we suggest different conclusions for an Iron Salt Aerosol program where important factors such as the effects of ocean cooling can be taken into account. We also add significant information on geo-chemical sequestration processes that take place at the ocean bottom.

Below we present each discrete NRC claim with a corresponding response from the ISA Team, lead by Franz Oeste.



  1.  Transport of carbon from ocean surface to ocean bottom
  2.  Carbon sequestration at ocean bottom
  3.  Chemical sequestration processes


NRC Claims / ISA Team answers

Most organic matter will be respired back to CO2

NRC: An iron-fertilized increase in sinking organic matter will not necessarily translate directly into a comparable increase in the rate of long-term ocean inorganic carbon sequestration. Much of the sinking organic matter flux due to an iron fertilization–induced bloom will be respired back to CO2, nutrients, and dissolved iron by bacteria and zooplankton in the upper few hundred meters of the water column, and ocean circulation will carry the resulting excess CO2 back to the ocean surface, where it can be released back to the atmosphere on relatively short timescales of a few years to decades, unless there is sufficient iron available to support biological transformation of the excess CO2 back into organic matter (Robinson et al., 2014).

ISA Team (three-part answer):

1.      Transport of carbon from ocean surface to ocean bottom

The diagram below is a simplified schematic of the movement of carbon and iron in the ocean:

Phytoplankton emit dimethyl-sulphide, which attracts krill – the first link in its predator food chain. (Nevitt, 2000; Wright et al., 2011; Endres & Lohmann, 2012, Savoca & Nevitt 2014, Foretich et al. 2017). The krill’s diurnal movement results in a vertical faeces-drop up to 1 km below the ocean surface (Brierley AS, XXX). Upward movement of the high nutrient deep-water above and below the krill swarm (Houghton et al., 2018; Pyper 2008) provides additional fertilization, creating increased organic carbon production by phytoplankton. This nutrition includes the very important faeces and urine of whales, seals, birds and fish (Ratnarajah et al. 2014; Turner 2015; Anonymus, 2016). The food chain transforms a large fraction of the organics originally derived from phytoplankton into refractory organic carbon (humic acids and humins), which remain stable in ocean water for centuries before eventually becoming incorporated into sediments. However, other organic material undergoes oxidative processes that result in increasing levels of CO2 with depth.


2.      Carbon sequestration at ocean bottom

Ocean circulation carries relatively small quantities of CO2 from the ocean bottom back to the surface because nearly all CO2 reaching the ocean bottom is sequestered in the sediments or crust. The increased concentration of deep ocean CO2 increases the acidity of deep waters, accelerating the chemical weathering reaction with the ocean crust peridotite rock, i.e. chemically sequestering nearly all[?] that CO2 as carbonate rock (via carbonic acid).

If an expanded ISA program becomes necessary in order to increase the rate of atmospheric CO2 removal then a larger ocean biomass will need to be allowed to grow. This is because a thriving predator food chain provides the only efficient mechanism for transforming surface carbohydrates to forms of carbon that minimise oxidation of ocean waters, i.e fast sinking faecal pellets and refractory organic carbon.

Fast chemical sequestration of organic carbon into the ocean bottom further reduces the oxygen consumption within the deep ocean, and we suggest that the oceans’ crusts and sediment caps easily have the capacity to incorporate all forms of carbon at a combined rate of 10Gt C/yr, possibly much higher.


3.      Chemical sequestration processes

Dissolved hydrogen carbonate, carbonic acid and refractory organic substances become sequestered by chemical mechanisms in the ocean crust, where at least four prerequisites for that are met:

  1. warm temperatures,
  2. high alkalinity,
  3. CO2-reducing environment (hydrogen from the serpentinization reaction of peridotite rock with ocean water),
  4. a permanent flow of ocean water through the crust (comparable in volume to all the worlds rivers combined).

The in/outflow in point 4. is driven by convection currents and occurs mainly near the subduction zones, mid-ocean ridges, and seamount tops where:

  • warm temperatures induce physical precipitation of carbonate rock,
  • high alkalinity induces chemical precipitation of carbonate rock,
  • hydrogen production induces chemical and biological transformation of hydrogen carbonate and carbonic acid carbon into organic carbon,
  • transformation of hydrogen carbonate to organic carbon induces further alkalinity and precipitation of carbonate rock,
  • chemical transformation of peridotite rock to serpentine rock (which needs a lot of water) either concentrates the remaining salts into brine or precipitates them. This additionally induces the precipitation of carbonate rock,
  • microbial life living in the ocean crust produces a huge mass of humic carbon that remains stable for thousands of years.


Iron scavenging

NRC: Therefore, an important factor is the degree to which the iron released at depth during organic matter respiration remains in the water column or is removed to the sediments through scavenging and particle export. Rapid iron scavenging would imply that ocean fertilization would need to be continued essentially indefinitely to result in permanent carbon disposal from the atmosphere.

ISA Team: Iron scavenging is the norm, as zooplankton feed on phytoplankton and predators move the iron-containing organic nutrients down the food chain. The end-point for most iron in ocean water is in faecal material that drops rapidly to the ocean floor. In this way iron continually becomes permanently sequestered, along with the carbon it has helped assimilate into organics at the surface. Bioavailable iron also enters the oceans from chemical weathering of crust and continents, but not in enough quantity to fertilise phytoplankton sufficiently to induce the required flux of CO2 into the ocean to solve climate change. We therefore agree that an ocean iron fertilization program would need to be continued as long as atmospheric CO2 remains at a level that is destabilising the global climate. Iron Salt Aerosol is already a natural and benign constituent of the troposphere where it induces cooling cloud formation and depletes powerful warming agents. We therefore suggest that a low concentration, widely dispersed Iron Salt Aerosol application would be the best method to maintain a healthy level of iron in surface waters. (Akin to giving vitamin C daily to all the sailors on a ship.) Such a program would be relatively low cost to maintain over many decades. ($1 per ton of CO2 equivalent.)


Iron released in subsurface waters

NRC: Alternatively, if a substantial amount of the added iron that sinks with and is released from respired organic particles is not scavenged from subsurface waters, it could limit the escape of the excess CO2 to the atmosphere when the subsurface water returns to the ocean surface and could extend the duration of enhanced ocean carbon sequestration due to iron fertilization.

ISA Team: Yes, this is what happens. Some of the iron and other nutrients that is released from respired or otherwise oxidised organic particles is transported by vertical currents and ocean life to the surface, nutrifying phytoplankton and thereby helping to prevent CO2 outgassing.


Low long-term sequestration

NRC: Enhanced long-term carbon sequestration, typically defined as a duration of more than 100 years, would also occur from the small fraction of sinking particles that reach intermediate or deep waters (greater than 1,000 m).

ISA Team: In a healthy ocean this is not a small fraction. A healthy predator food chain results in a large fraction of organics eventually being converted to rapidly dropping faeces. Additionally, with strong overturning ocean circulation activated by polar surface cooling, additional transport of oxygen and CO2 to the deep ocean increases the transformation of organic compounds to carbonic acid. Carbonic acid is readily sequestered as carbonate rock in the ocean crust, and the lower the pH of the ocean bottom water entering the alkaline crust, the faster that precipitation reaction takes place.


Calculating the effects of ocean fertilisation

NRC: Because of the large natural background levels and variability of subsurface dissolved inorganic carbon, the direct measurement of small changes in ocean carbon sequestration at depth from ocean iron fertilization experiments is challenging. Furthermore, it is not possible in the field to track the subsequent fate of water parcels for sufficiently long time to quantify the rate of return to the surface ocean. Therefore, estimates of the efficiency of iron fertilization on ocean carbon sequestration are restricted so far to numerical model studies that require a number of assumptions about biological dynamics and iron biogeochemistry. With these caveats in mind, modelling studies indicate that the potential upper limit for a sustained ocean iron fertilization CO2 sink is relatively modest at 1.0 to 3.7 GtCO2/yr9 and that the total ocean sequestration capacity until the end of the century is 85 to 315 GtCO2, assuming continuous iron fertilization of the entire iron-limited Southern Ocean, Equatorial Pacific, and subpolar North Pacific (Aumont and Bopp, 2006; Zahariev et al., 2008).

ISA Team: These calculations could not have taken into account:

  • increased surface pH from phytoplankton growth,
  • the ocean cooling induced by an ISA program (or other MCB method) and consequent O2 absorption,
  • vertical mixing of nutrients that takes place in a healthy ocean,
  • the role of predator food chains in moving organic carbon to the ocean bottom unoxidized,
  • the high capacity rate of the ocean crust to chemically sequester all forms of carbon.


Cost too high

NRC: Early cost estimates for ocean iron fertilization were quite low (<$10/tCO2), reflecting the large leverage of the amount of iron added per [unit of] organic carbon fixed via photosynthesis (e.g., Ritschard, 1992). However, more recent studies factor in new information, suggesting lower biological efficiency leading to [carbon export and sequestration and] leakage of CO2 back to the atmosphere (Markels et al., 2011). For example, one estimate of the cost of ocean iron fertilization is approximately $450/tCO2 (Harrison, 2013). Improved cost estimates would also require information on technological issues (e.g. iron spreading and approaches to limit scavenging), the efficiency of atmospheric CO2 uptake, and verification and monitoring requirements.

ISA Team: At $1/tCO2e removed, the cost of an ISA program is an order of magnitude below the lowest of the above estimates, and would additionally include depletion of powerful greenhouse gases and tropospheric cooling.


Downstream nutrient supply

NRC: Studies have identified a number of possible drawbacks to iron fertilization as a CDR method (Buesseler et al., 2008; Strong et al., 2009; Williamson et al., 2012). In particular, the ecological impacts on the marine food web and fisheries due to continuous, extensive iron fertilization may be substantial but are poorly characterized. It is also likely that iron fertilization will have downstream effects on nutrient supply, and thus productivity and food web dynamics, in other ocean regions.

ISA Team: At ~10g/km2/day, the initially proposed ISA-delivered iron concentration is far from extensive. (10g/km2/day is based on quasi-permanent ISA application to the 20m km2 Southern Ocean, and 75,000t iron/yr applied to each pole.) As such it is not expected to have any appreciably deleterious effect on downstream nutrient supply. If future ISA programs with significantly increased concentrations are proposed then careful measurement of sensitive ocean areas would be required. Damage to deep ocean ecosystems can be minimised by continuously monitoring the redox milieu below ISA plumes, and emission paused if problems emerge. Once ISA emission ceases, particle residence time in the air is the order of up to three weeks.


Ecological drawbacks

NRC: An intended consequence of ocean iron fertilization involves shifting plankton community composition toward larger cells that will lead to enhanced downward-sinking flux; the long-term impact of this shift on higher trophic levels, including fish, seabirds, and marine mammals, is not well known but may be addressable in part by studying analogous regions with substantial natural iron fertilization.

ISA Team: We do not intend to induce any particular plankton community composition, but rather to allow a biodiverse plethora of all ocean life to appear and evolve naturally, ideally protected from limitation by industrial-scale fishing activity. This is the surest and most efficient way for carbon to be transported from surface waters to the ocean crust and sediments for permanent sequestration. Studies on the influence of fertilising dust on ocean ecosystems during the glacial age have not revealed any severe disturbances to ocean life.


Toxic blooms

NRC: Iron addition often stimulates the growth of Pseudonitzschia diatom species, some of which are associated with toxin-producing harmful algal blooms (Moore et al., 2008). In the case of a specific iron addition experiment in the subpolar North Pacific Ocean, the iron-stimulated Pseudonitzschia diatoms were shown to produce domoic acid, a neurotoxin that has the potential to harm fish, marine mammals, and humans (Trick et al., 2010).

ISA Team: The planned ISA-field trial(s) monitoring program will include investigations on toxin formations. However, we do not believe such toxic blooms would develop from ISA plume fertilization, with its proposed very low iron concentration. N.B. ISA emission would be paused during heavy rainfall over a plume source.


Atmospheric N2O release

NRC: A number of scientific studies have raised concerns about how ocean iron fertilization may potentially also alter ocean biogeochemistry. Changes in the air-sea fluxes of climate-active trace gases such as dimethylsulfide, methane, and nitrous oxide (N2O) could in principle either partially cancel out or amplify the benefits from enhanced ocean CO2 uptake (Diaz and Rosenberg, 2008). A substantial component of ocean N2O production is thought to arise from microbially driven nitrification of ammonia and organic nitrogen released from sinking particles in the upper ocean. Nitrification is expected to increase due to iron fertilization, and because N2O is a much more powerful greenhouse gas than CO2, the effect could be to greatly diminish the climate impact of iron fertilization (Barker et al., 2007; Jin and Gruber, 2003).

ISA Team:  N2O is produced in a side reaction both from nitrate reduction, and the opposite reaction, ammonium oxidation. However, that side reaction is minimised in the presence of increased oxygen concentration. Since the ISA method both cools the ocean and activates vertical currents, the resulting enhanced oxygen transport into the ocean should minimize N2O generation.

This is further confirmed from glacial age data, which indicates that N2O values regularly dropped during the dusty cold periods. We suggest the cause was an elevated oxygen content in the deep ocean from:

  • activated vertical ocean water movement,
  • abundant life in the oceans,
  • strong overturning circulation activated from cooling to the ocean and ice-capped surfaces in polar regions, likely induced by Iron Salt Aerosol cooling processes resulting from wind-blown dust.

As such, we would expect the same effects from an artificial ISA application to polar regions.

The only anoxic environment in a healthy, predator-rich ocean should be the organic carbon-containing sediments at the ocean bottom, where a large mass of faecal pellets arrives having had too little time to become substantially oxidized. (Anoxic sediments like this also exist below oxic surfaces such as tidal flats, mangrove forests and moors. These natural anaerobic sediment milieus contain microbial consortia that consume the N2O produced by the anoxic conditions.) (Sun et al. 2017).

Danger of increased N2O being generated by an ISA program exists only from a stratified, sick ocean resulting from too few predators. The increased fraction of organic litter raining slowly down might deplete oxygen levels sufficiently for the above-mentioned ammonium and nitrate reactions to take place above the ocean bottom.

This highlights the fact that ISA will work most efficiently in a healthy environment. The most efficient carbon transport from ocean surface to ocean bottom will result if whales, seals, birds and fish – the whole biological environment within the ISA plume affected region – is protected.


Ozone layer harm

NRC: There is also the potential for the release of methyl halides to the atmosphere from phytoplankton, [and also continental plants] that might lead to possible depletion of stratospheric ozone (Wright, 2003).

ISA Team: Iron Salt Aerosol adds a protection to the stratospheric ozone layer by depleting methyl-halides such as CH3Cl. In the day-time airborne ISA desorbs (emits) atomic chlorine (Wittmer & Zetzsch, 2016) – a powerful oxidant that acts about 10 times faster than the OH radical oxidant that exists naturally in dust-free air (Tsai W-T, 2017; Burkholder et al., 2013).  It is this oxidation function that depletes methyl-halides.


Reduced subsurface dissolved oxygen levels

NRC: Increased export of organic carbon to the subsurface ocean would also likely reduce local subsurface dissolved oxygen levels, exacerbating the declines in subsurface oxygen already expected under a warmer climate. A resulting expansion of low-oxygen, hypoxic regions of the coastal or open ocean would potentially have significant biological ramifications (Keeling et al., 2010). Iron fertilization on a large scale could potentially also have downstream effects by reducing the nutrient supply to low-latitude ecosystems.

ISA Team: As well as releasing oxygen into surface water, Phytoplankton also emit dimethyl-sulphide that acts to form cloud condensation nuclei (CCN). These seed the creation of stratocumulus clouds that cool the ocean surface, further helping to increase the level of oxygen in surface waters by increasing its solubility.

See also answers to:

  • Downstream nutrient supply
  • N2O “cooler waters absorb more oxygen”


Poor ocean acidification mitigation

NRC: Although ocean iron fertilization would act to remove CO2 from the surface ocean and transport it to depth, the effects on partially mitigating ocean acidification in surface waters due to rising atmospheric CO2 levels would be minimal at best and would somewhat increase the rate of acidification of subsurface waters (Cao and Caldeira, 2010).

ISA Team: The effect on pH of iron fertilization-induced assimilation of hydrogen carbonate by phytoplankton is more than minimal. Iron fertilization significantly alkalizes the surface water.


NRC: In addition to these concerns over the effectiveness and environmental impacts of OIF projects, there are significant ethical and legal concerns as well. These are discussed further in Chapter 4.

ISA Team: Agreed. Control of a major ISA program would be need to remain robustly independent of influences from the fossil fuel industry, that has seriously delayed climate mitigation efforts by funding denial campaigns, risking most of Earth’s ecosystems.


Future research

NRC: Looking forward, the committee highlights several important future research directions:

  • Understanding the effectiveness of iron inputs on stimulating biological organic carbon production and increasing carbon export;
  • Determining the fate of the sinking organic carbon and iron in the subsurface ocean as a result of deliberate ocean iron fertilization;
  • Assessing potential downstream effects that may limit biological productivity or change other aspects of biogeochemistry in other regions;
  • Detection and accounting of net changes in subsurface ocean carbon sequestration and the effective lifetime of the carbon sequestration; and
  • Understanding the ecological and biogeochemical consequences of extended and large-scale iron fertilization.

ISA Team: Agreed. If we want a safe, effective, affordable solution to climate change then research to confirm our claims would be helpful.



NRC: In summary, current limitations of ocean iron fertilization as a viable CDR method include the limited knowledge regarding the method’s effectiveness in regard to carbon capture, concerns regarding the environmental impacts and cost of largescale and sustained OIF, and the associated ethical and legal issues. Although about a dozen ocean iron fertilization field experiments have been conducted, their purpose was fundamental scientific research primarily related to the basic controls on ocean biology and biogeochemistry. Many unresolved issues remain regarding scalability, efficacy, verification, and environmental impacts. Given these limitations and unknowns, the committee concludes that the risks and costs currently outweigh the benefits. The committee considers this an immature CDR technology with high technical and environmental risk.

ISA Team: Since Iron Salt Aerosol cooling effects occurred naturally during the glacial epochs, and our artificially enhanced ISA proposal is a mimic of this benign natural method, we do not see how our particular proposal can represent a “high technical and environmental risk“.

Further, we see the main risk as delaying an ISA program and instead waiting for ocean circulation patterns to become further degraded. Either way, there is no technical reason for delaying:

  • Fisheries protection, to strengthen the biological carbon pump,
  • Forestry protection, in order to safeguard the flow of alkalinity to the oceans from the enhanced rock weathering tree roots provide.



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