Marine cloud seeding is one of the best documented geoengineering proposals. It is centred around the idea that some forms of clouds tend to have a net cooling effect on the Earth’s climate, by increasing albedo or reflectivity.

Our regular visitors also know the infrared absorption of clouds (on average) could outweigh increased reflectivity and thus cloud increases could actually speed up warming.

New research by Kari Alterskjaer from the University of Oslo, presented during last week’s General Assembly of the European Geosciences Union, using cloud observations and a general circulation model, shows the effects of the cloud seeding geoengineering technique itself may also be uncertain.

Droplet size determines effect

The idea, first proposed by John Latham in 1990, suggests a fine spray of sea water, blown continuously into the air from unmanned ships, could make existing clouds whiter. Alterskjaer warns this depends on the size of the salt aerosols (sea water droplets) and that (if too small) these could compete with existing condensation nuclei – possibly decreasing cloud cover and creating a net warming effect.

…and feasibility

Bigger droplets on the other hand could well be too heavy – and may fall down to the sea before reaching the cloud base. Her study suggests the balance is so delicate that 70 times as much salt would be needed to create the cloud effects as the proponents have suggested.

We could not find a link to the study, so we can not yet show Alterskjaer’s calculations.

In one of his original publications (Atmospheric Science Letters, 2002, PDF) on albedo enhancement by marine clouds John Latham acknowledges the technique could – under specific circumstances – actually lead to warming – and states much depends on design of the spray system – that has reached greater detail with more recent publications.

Favoured bits of ocean

Alterskjaer’s research confirms earlier findings that the geoengineering technique would likely be most effective immediately west of Africa and west of North and South America. That reminds us of El Niño control, indeed through geoengineering – and indeed highly theoretical.

© Rolf Schuttenhelm | www.bitsofscience.org