In some places, though, iron is the limiting nutrient. Adding iron to such places should cause a bloom of planktonic algae, thus sucking carbon dioxide out of the atmosphere.

The empty house, in a middle-class corner of southern California, is two storeys high and boasts a three-car garage. Roses bloom around a kidney-shaped swimming pool, which is green with algae.

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.