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Leaf albedo engineering

Cross posted from Heliophage

Lets brighten this up...

I wrote a little piece for Nature today today about a paper by Andy Ridgwell at Bristol and some of his colleagues on changing the albedo of crops. The gist as published:

Manipulating the waxiness of crops through traditional breeding techniques or genetic modification should raise their albedo by about 20%, from 0.2 to 0.24. On the basis of climate modelling they calculate that the planet would cool by a modest 0.11 ºC. “It’s very small on the global average,” says Ridgwell. But “what is more important is the summertime effect in specific regions”. The mid-latitudes of North America and Eurasia could cool by as much as 1 °C in June, July and August, according to the models. Ridgwell and his colleagues report their results in Current Biology.

The models also show pronounced cooling in the North Atlantic Ocean and the Barents Sea in the wintertime — which might have a positive effect on sea ice — but a drying out of the soil in some parts of the subtropics. Ridgwell points out that climate models do not predict future precipitation well on a regional basis and treats the latter results more as evidence that there might be effects far from the fields being changed than as a clear indication that there would be damaging consequences.

There are some interesting details and implications to this “bio-geoengineering” scheme. Though you might think that reflecting more light off the surfaces of leaves means less photosynthesis, according to the paper the evidence in the literature suggests not. This may be because more reflective leaves stay cooler and more efficient; another possibility is that the light is reflected mostly from leaves in direct sunlight (which are not constrained by a lack of light) and some of what is reflected ends up with leaves that are in shadow (which are constrained by lack of light). More detailed studies, of course, may show that in fact photosynthesis does go down.

Making the plants more reflective, if it proved a good idea at all, might well necessitate genetic engineering, which in some places is distrusted. That engineering might be more acceptable in energy crops than it is in food crops. It might make sense, if people are going to engineer energy crops for other purposes, to make them a little lighter too, all other things being equal.

Another point is that this is very small beer as geoengineering goes. A similar but more dramatic proposal along similar lines by Robert Hawmey (pdf) has a radiative forcing of about 0.6 Wm-2, which is smallish by the standards of the CO2 forcing; I would guess if they expressed it in the same way the forcing in the Ridgwell et al scheme would be a good bit less than that. But it might still have some marginal utility. This is a trend I suspect we will be seeing more and more of in geoengineering studies over the next few years, a shift away from totalising projects such as sunshades for the whole earth and layers of aersosol all through the stratosphere towards smaller regional and semi regional ideas.

Talking about this trend Tim Lenton has suggested that we may be moving towards a discussion of geoengineering that has some similarities to Socolow’s “wedge” approach to decarbonization: breaking the big problem down into smaller lumps that feasible technologies could bite off and chew; as I report in the Nature piece, Tim and some colleagues are looking at setting up a unit to compare geoengineering schemes and their potential payoffs on this basis. I’m not sure this is necessarily a good development. Every geoengineering scheme has strange knock-ons and side effects around the edges, and it seems reasonable to suspect that the more such schemes you have, the more chance there is for one of the side effects to be unexpectedly serious — or for two of them to interact with each other catastrophically. But that said, the fact that it is probably a lot easier to find little forcings than big ones suggests that the portfolio approach may be in the ascendant for a while.

Image from flickr user ecstacist under a creative commons licence


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