The latest expedition to the Southern Ocean to test the theory that fertilising the ocean combat climate change has concluded that the process sucks negligible amounts of CO2 from the atmosphere.
The theory goes that sprinkling iron into areas of the ocean lacking in the metal will stimulate the growth of algae, which will absorb CO2 from the air as they grow and then carry some of this greenhouse gas to the depths of the ocean when they die.
The German-Indian Lohafex project is the latest expedition to test out the theory in practice. The team found that dumping six tonnes of iron into the ocean did indeed boost algal growth - but that within two weeks the algae were being eaten by a voracious band of tiny crustaceans called copepods, drastically cutting the amount of carbon captured.
These results are just the latest to show that a process that at first glance appears relatively simple is actually far from it. Previous studies - including most recently an investigation into the effects of natural iron fertilisation, where the metal is washed from land into the ocean - have produced conflicting data as to how much extra carbon might be captured.
The Lohafex team suggest that the area of ocean they tested may have been deficient in silicic acid as well as iron, which prevented a bloom of silica-shelled diatoms from forming. Previous fertilisation experiments that did show carbon capture found that diatoms were the blooming species. ‘Since the silicic acid content in the northern half of the Southern Ocean is low, iron fertilisation in this vast region will not result in removal of significant amounts of CO2 from the atmosphere,’ the German researchers conclude.