The voyage, a joint venture by India’s National Institute of Oceanography and the Alfred Wegener Institute for Polar and Marine Research in Germany, was controversial from the start. Some environmental groups claimed it was akin to pollution, and thus illegal.

Those researchers, led by Wajih Naqvi and Victor Smetacek, created a bloom of phytoplankton by fertilising an area of 300 square kilometres with six tonnes of iron sulphate, which dissolves in water. In two weeks the bloom’s mass doubled. But it also proved to be extremely tasty for small crustaceans called copepods, which gobbled the phytoplankton up so quickly that even with further iron fertilisation the bloom stopped growing. As a result, only a small amount of CO2 was dispatched to the ocean floor.

The problem lay with the species of phytoplankton in the bloom. In previous experiments the blooms had consisted of a group of algae known as diatoms. As diatoms have shells made of silica they are protected from copepods and so are more likely to die without being eaten and thus take take their carbon to the ocean floor. But in the area where the researchers were working natural blooms had already depleted much of the silicic acid, which the diatoms use for shellmaking. The result was that the beneficiaries of the iron were instead groups of algae such as Phaeocystis, which are among the most heavily grazed by copepods.

Since silicic-acid levels are naturally low across about two-thirds of the Southern Ocean, the expedition’s results suggest that iron-fertilisation would remove less CO2 from the atmosphere than optimists had hoped.