Clouds, climate change, and emergence

On tuesday the Telluride Science Research Center hosted a nice public lecture, "Clouds in a bowl of soup," by Graham Feingold, an atmospheric scientist.

He emphasised how the atmosphere is a complex system that exhibits emergent phenomena, particularly pattern formation and synchronisation. He discussed how one sees these phenomena in other systems, such as soup (Rayleigh-Benard convection cells) and fire flies.
Aside: in a similar vein there is a nice Physics Today, Quick Study, The universe in a cup of coffee by John Wettlaufer.

Emergent behaviour results from simple rules. The four rules for clouds are

1. Drops form on aerosols (suspended particles) and grow by vapour diffusion.

2. Drop coalescence generates rain. (Aerosols can influence rain).

3. Drops fall and evaporate.

4. Continuity of air flow.

Some of the work he described is in this paper, which includes the figure below.

Some key physics relevant to climate change is that clouds generally reflect sunlight and have a cooling effect. The big question is whether global warming then increases or decreases cloud formation. Is the feedback positive or negative? It seems people aren't really sure. Intuitively, you might think that the increased water in the atmosphere means more clouds but (as is often the case) it turns out to be more complicated than that.