In The Field

AGU: A new way to cool the earth

On Thursday and Friday there will be some sessions on “geoengineering” — intervening in the climate deliberately in an attempt to counteract greenhouse warming. One of the presentations, previewed at a press conference today, was an idea for a way of cooling the earth I hadn’t come across before: stripping off some of the high cloud.

David Mitchell, of the Desert Research Institute in Reno, points out that cirrus clouds warm the earth by letting light come in but detaining longwave radiation on its way out. (Clouds in the lower atmosphere are cooling, because they reflect back sunlight more than they trap heat). To get rid of them he suggests, a little counterintuitively, adding particles that will encourage water vapour to freeze into ice crystals.

You’d think that would make more clouds, and you would be sort of right. But the processes by which ice grows around these nuclei will outcompete the processes that make up the ice particles normally found in cirrus clouds; instead of getting lots of little ice particles in a cloud you’ll get a few bigger particles — big enough to fall down to the lower atmosphere.

The nuclei would make some clouds where you wouldn’t otherwise get them, because they will allow particles to grow in places where there is not enough water vapour for cirrus clouds to form normally. But the net effect, according to Mitchell’s models, is to dry the upper troposphere out, decrease the amount of cirrus and cool things down.

How well it would work in practice is unknown. It would almost certainly have some effects beyond just letting more outgoing radiation to leave the planet unmolested, and as yet no one knows what those might be. Mitchell wants to do some more model work to address some of those questions, and then — perhaps, maybe — some small-scale experiments. Unlike the much more widely discussed idea of putting aerosols into the stratosphere, the cirrus-busting technique could be tried, and indeed continued, at a regional level, if that was desired; it could also be stopped in days if it was suddenly not wanted.

So if you think geoengineering is worth doing research on, then it looks like this idea should be added to the list for a preliminary look-see. If you don’t, it’s another thing to worry about.

I’ll try and blog a bit more about geoengineering tomorrow.


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