It should be possible to counteract the global warming associated with a doubling of carbon dioxide levels by enhancing the reflectivity of low-lying clouds above the oceans, according to researchers in the US and UK. John Latham of the National Center for Atmospheric Research in Boulder, US, and colleagues say that this can be done using a worldwide fleet of autonomous ships spraying salt water into the air.

Clouds are a key component of the Earth’s climate system. They can both heat the planet by trapping the longer-wavelength radiation given off from the Earth’s surface and cool it by reflecting incoming shorter wavelength radiation back into space. The greater weight of the second mechanism means that, on balance, clouds have a cooling effect.

’Twomey effect’ boosts reflectivity

Latham’s proposal, previously put forward by himself and a number of other scientists, involves increasing the reflectivity, or “albedo”, of clouds lying about 1 km above the ocean’s surface. The idea relies on the “Twomey effect”, which says that increasing the concentration of water droplets within a cloud raises the overall surface area of the droplets and thereby enhances the cloud’s albedo. By spraying fine droplets of sea water into the air, the small particles of salt within each droplet act as new centres of condensation when they reach the clouds above, leading to a greater concentration of water droplets within each cloud.

Latham and co-workers, including wave-energy researcher Stephen Salter of Edinburgh University, claim that such spraying could increase the rate at which clouds reflect solar energy back into space by as much as 3.7 Wm-2. This is the extra power per unit area that scientists say will arrive at the Earth’s surface following a doubling of the concentration of atmospheric carbon dioxide compared to pre-industrial levels — 550 ppm vs 275 ppm (Phil. Trans. R. Soc. A DOI:10.1098/rsta.2008.0137).

New spin on sailing

The 300-tonne unmanned ships used to seed the clouds would be powered by the wind, but would not use conventional sails. Instead they would be fitted with a number of 20 m-high, 2.5 m-diameter cylinders known as “Flettner rotors” that would be made to spin continuously. This spinning would generate a force perpendicular to the wind direction, propelling the ship forward if it is oriented at right angles to the wind (Phil. Trans. R. Soc. A DOI: 10.1098/rsta.2008.0136).

These rotors would be easier to operate remotely than sails and would also serve as the conduits for the upward spray, with the spray consisting of droplets 0.8 µm in diameter generated by passing sea water through micro nozzles. The power for the spray and the cylinder rotation would be provided by oversized propellers operating as turbines.

The immediate effect of seeding clouds in this way would be a local cooling of the sea surface, and as such the technique could be targeted at coral reefs, diminishing polar ice sheets or other vulnerable regions. However, the great thermal heat capacity of the ocean and the currents within it mean that these initial effects would eventually spread across the globe.

Fleet of 1500

Latham and colleagues calculate that, depending on exactly what fraction of low-level maritime clouds are targeted (with some regions, notably the sea off the west coasts of Africa and North and South America, more susceptible to this technique than others), around 1500 ships would be needed altogether to counteract a carbon doubling, at a cost of some £1m to £2m each. This would involve an initial fleet expanding by some 50 ships a year if the scheme is to keep in step with the current rate of increase in atmospheric carbon-dioxide levels.

This cloud-seeding proposal is one of a number of ideas put forward by scientists in recent years to “geoengineer” the Earth in response to climate change rather than, or as well as, deal with the causes of the change. A series of papers on several proposals, including Latham's, have been published in a recent issue of the journal Phil. Trans. R. Soc. A entitled Geoscale engineering to avert dangerous climate change.

Latham maintains that his group’s idea is not pie in the sky and that its feasibility is supported by two of the world’s leading computer climate models, as well as recently obtained experimental cloud data. He points out that, unlike rival techniques, the system could be used to vary the degree of cooling as required and could be switched off instantaneously if needed. However, he adds more research must be done to find out a number of unknowns — such as exactly what fraction of spray droplets will reach the clouds — and to establish that the technique would not create any harmful climatic side effects. More work must also be done on the spray technology, he says.