Sydney - One way to curb greenhouse gases that speed climate change is to seed the ocean with iron dust to create plankton blooms that suck in carbon dioxide and then sequester it on the ocean floor.The science is not in question: After Mount Pinatubo in the Philippines erupted in 1991 and spread 40 000 tons of iron dust over the ocean there was a measurable decline in carbon dioxide in the atmosphere.What is in doubt is the economics: Can this form of geo-engineering ever be made cost-effective? Sydney University engineer Daniel Harrison thinks not.His study, published in the International Journal of Global Warming, shows that supplying iron to iron-deficient areas of the ocean does not store carbon long enough to be a commercially attractive contributor to climate management.Weighing on the economics is the difficulty of making sure you are seeding in the right place at the right time."It may be very difficult to identify you were fertilising under the ideal conditions until afterwards," Harrison said. "And every time you fertilised under less than ideal conditions you push the overall cost up towards the very high average cost I calculated for the Southern Ocean."His study of geo-engineering in the stretch of water to Australia's south found that spreading iron sulphate only when conditions were absolutely ideal would mean the overall contribution to cleaning up the planet would be very small.Low efficiencyThe study used average results and calculated the price at $400 per ton of carbon dioxide sequestered from the atmosphere for 100 years or more.Currently, carbon permits being traded in Australia are set at $24 per ton.The cost of ocean iron fertilisation (OIF) is high because the efficiency of impregnating oceans is low: Around 98% of the carbon dioxide initially sequestered is back in the atmosphere within 100 years."If the ocean is to play a greater role in storing carbon, we'll need to develop more effective and economical technologies that are competitive with abatement opportunities on land," he said.Harrison's findings accord with those of others working in the field.Ken Busseler of the Woods Hole Oceanography Institution in the United States has been quoted as saying that "we're nowhere near the point of recommending ocean iron fertilisation as a geo-engineering tool".Wilfried Rickels of the Kiel Institute for the World Economy in Germany is a bit more sanguine, saying in a paper that "no scientific or economic criterion has been identified so far that would suggest that OIF should be rejected, a priori, as an adjunct climate protection measure".