Almost half of the photosynthesis on Earth is carried out by phytoplankton in the sea. So these tiny cells play a huge part in the global carbon cycle, and in regulating climate by controlling the amount of the greenhouse gas CO₂ in the atmosphere. Phytoplankton are the engine of the 'biological pump' (Fig. 1) that helps maintain a steep gradient of CO₂ between the atmosphere and deep ocean. It has been suggested that we might increase the efficiency of this pump — thereby drawing more CO₂ out of the atmosphere — by artificially supplying nutrients to the surface oceans. This suggestion is highly contentious. Papers by Boyd et al.¹, Abraham et al.² and Watson et al.³ (pages 695, 727 and 730 of this issue) will add fuel to the debate about the desirability of such 'geoengineering' solutions to Earth's ills. The papers describe the results of a fertilization experiment in the Southern Ocean, around Antarctica, and its scientific implications for interpreting past climate change.

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