University of East Anglia, Norwich, UK

A biogeochemist is keen to find out whether oceanic plankton can help to keep our planet cool.

For years I have been fascinated by the idea that oceanic plankton can play a significant part in controlling climate. This concept is, of course, at the heart of gaian ideas of the Earth as a self-regulating system, proposed by James Lovelock.

It was given expression through the CLAW hypothesis (published two decades ago by R. Charlson, J. Lovelock, M. Andreae & S. Warren), which supposes that the gas dimethyl sulphide produced by marine plankton influences cloud formation and hence albedo and climate.

However, direct evidence for a link between plankton and clouds has been slow to emerge. A recent paper (N. Meskhidze & A. Nenes Science 314, 1419–1423; 2006) shows a tantalizing seasonal and spatial association between sea-surface chlorophyll (an indicator of biological activity) and atmospheric properties for a six-year period over a substantial area of the Southern Ocean.

Over high-chlorophyll areas, the number of cloud droplets doubled whereas the droplets' size decreased by 30% compared with other regions, leading to an atmospheric cooling comparable to that over highly polluted regions.

Meskhidze and Nenes attribute these changes to plankton emitting the gas isoprene. I am sceptical whether the sea-to-air flux of this compound is sufficient to produce the observed effects, but finding out what does give rise to the apparent association will keep me and other scientists involved in projects such as the Surface Ocean – Lower Atmosphere Study (http://www.solas-int.org) busy for many years.

It is vital to understand what is happening in order to be able to predict how future changes in biological activity in the oceans may mitigate or enhance climate change.