Credit: JOHN MACNEILL

The reality of anthropogenic-induced climate change is no longer in question. Nevertheless, the amount of carbon dioxide vented into the atmosphere each year as a result of human activities shows no sign of levelling off (let alone decreasing). As a result, geoengineering solutions are beginning to look more attractive, at least as a last resort. Proposals come in many flavours of expense, impact and economic viability, with ocean fertilization, the injection of sulphate aerosols into the upper atmosphere and carbon sequestration being some of the more prominent ideas.

Writing in a special issue on “Geoscale engineering to avert dangerous climate change”, John Latham and colleagues propose a different way of cooling the Earth, which involves seeding low-level maritime clouds with seawater particles (Phil. Trans. R. Soc. A doi:10.1098/rsta.2008.0137; 2008). The scheme aims to exploit the Twomey effect, in which an increase in the number of cloud condensation nuclei — seawater particles in this case — leads to more, but smaller, cloud droplets. The resulting brighter clouds reflect more sunlight back into space, leading to the desired cooling effect.

In a companion paper, Stephen Latham and colleagues suggest a relatively low-cost engineering solution to the challenge of injecting the seawater particles into the marine boundary layer. They propose that wind-driven vessels (see image), based on a 1920s design forgotten during the turmoil of the 1929 economic depression, could spray seawater into the atmosphere (Phil. Trans. R. Soc. A doi:10.1098/rsta.2008.0136; 2008).

The beauty of the proposal is its relative simplicity. The resources required are just seawater and wind, the vessels should not need much maintenance, and the effect of cloud albedo increases could be quite large compared with the effort needed to generate the spray.

As with all geoengineering proposals, more research needs to go into both the efficiency of the scheme at an industrial scale and the possible (unexpected) side effects. The injection of seawater into the atmosphere could be stopped immediately should undesirable effects develop, such as substantial reductions in precipitation over populated land areas. But, as the authors concede, the response of the Earth system could linger much longer.