One of the apparently simplest materials, iron, has been touted as being the key to solving one of our biggest and most complex problems - rising levels of atmospheric carbon dioxide, the greenhouse gas that is forcing rising average temperatures across the globe and pushing potentially debilitating climate change on us. Iron lies at the heart of a geo-engineering scheme first discussed in the early 1990s around the time I was starting out as a science journalist.

At the time, there was much ongoing debate about whether it could be used to "seed" the oceans, trigger gigantic algal blooms and stimulate photosynthesis to soak up vast quantities of carbon dioxide, which would then be entrenched as the microbial plant life died and sank to the ocean floor. To my lowly chemist's brain it sounded far-fetched. Surely, the amount of iron needed would be enormous, would require fossil fuel energy to process and transport it and even if it could stimulate algal growth the carbon-rich biomass would not actually be locked away at depth, but enter the food chain and be released once more into the atmosphere on quite a short timescale? But, what did I know?

Well my suspicions have been vindicated it seems by Daniel Harrison of the University of Sydney Institute of Marine Science, New South Wales, Australia, who has sat down and carried out a fully comprehensive analysis of all the various factors involved in the carbon equations. He has taken into account the direct impact of iron seeding in terms of the efficiency of spreading the iron, the impact it will most likely have on new algal growth on the requisite chlorophyll-poor regions of the oceans, the tonnage of carbon dioxide per square kilometer of ocean surface that will be actually absorbed and compared these figures to the hypothetical values that have been suggested by advocates of the approach over the last couple of decades.

"If society wishes to limit the contribution of anthropogenic carbon dioxide to global warming then the need to find economical methods of carbon dioxide sequestration is now urgent," Harrison says. His new calculations take into account not only the carbon dioxide that will be certainly be sequestered permanently to the deep ocean but also subtracts the many losses due to ventilation, nutrient stealing, greenhouse gas production and the carbon dioxide emitted by the burning of fossil fuels to produce the iron salts and to power their transportation and distribution at sea.

What it boils down to is that a single ocean iron fertilization will result in a net sequestration of about 10 tonnes of carbon per square kilometer sequestered for approximately one century and that this will cost almost US$5000 for that same area. "Previous estimates of cost fail to recognize the economic challenge of distributing low concentrations of iron over large areas of the ocean surface and the subsequent loss processes that result in only a small net storage of carbon per square kilometer fertilized," explains Harrison.

Others have addressed the maximum possible contribution by modeling various seeding scenarios and the figure that emerges is a sequestration of about 1 billion tonnes of carbon. However, Harrison points out that those calculations do not take into account the losses he discusses.

Moreover, the true limitation of the approach will be the macro-nutrients present in any given region of the oceans, once they are exhausted no amount of iron seeding will make that region bloom. "Under ideal conditions the cost could be lowered and the efficiency increased but the availability of ideal conditions will be small," Harrison concludes.

"A method for estimating the cost to sequester carbon dioxide by delivering iron to the ocean" in Int. J. of Global Warming, 2013, in press.

David Bradley blogs at and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".