Further to the post and subsequent discussion on Sunshades, which grew out of this article on geoengineering, I thought I’d point to the new paper by Damon Matthews and Ken Caldeira in PNAS (Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0700419104). It’s an interesting paper that has some fascinating insights into the links between climate and the carbon cycle, and I think contains some pretty bad news for would-be geoengineers.
The paper uses a University of Victoria inermediate complexity GCM along with a land cover and carbon cycle model (the Hadley Centre MOSES2 and TRIFFID — which is pretty much the best acronym in the business) to track climate from 1900 to 2100, using historical data up to the end of the twentieth century and the IPCC A2 carbon-dioxide emissions scenario from then on. Left to itself this gives a temperature increase of 3.5 centigrade over the 200 years. They then compared this baseline to alternative scenarios in which geoengineering strategies were turned on and off at various times. The geoengineering effect — think of it as a layer of sulphates in the stratosphere, though the model wasn’t that specific — was calibrated to reduce the incoming sunlight in such a way as to counteract the radiative forcing of the carbon dioxide at any given time.
They found that the geoengineering could reduce the change in temperature in the model to something pretty negligible, though with some latitude-dependent effects; in the geoengineered world the poles warm a little compared to 1900 while the tropics cool a little. It also appeared that you could get back to 1900 temperatures even if you started the geoengineering well into the twentyfirst century, as long as you did enough of it.
Various reports of this work have highlighted a fairly obvious subsequent finding: if you stop the geoengineering while having done nothing about carbon emissions you can get some truly horrendously quick warming; your protection vanishes almost instantaneously and the potential warming you have stored up by allowing carbon-dioxide levels to rise suddenly all appears at once. Though it’s nice to have some figures on this, it hardly comes as a surprise. Stephen Schneider has been going on about the fact that once you start you can’t stop for decades, and in Tom Wigley’s Science article (Science 314 pp. 452 – 454 (2006) DOI: 10.1126/science.1131728) last year, which explored the possibility of using a brief period of geoengineering to buy time in which to develop and field the technology needed for radical emissions reduction, there was a nasty looking blip in the warming rate at the point where the geoengineering was turned off. But it’s still a sobering thought. While geoengineering through something like sulphate in the stratosphere is “reversible”, in that if it starts having nasty effects you can just turn it off and the sulphate will fall out in a few years, that doesn’t just leave you with the status quo ante — it leaves you facing a far faster rate of warming that ypu have ever seen, and the adaptation challenges that go along with that.
There’s an extra wrinkle in this paper, too; in the geoengineered world, you get increased carbon-dioxide uptake by the biosphere through the carbon-dioxide fertilisation effect on plants, but no offsetting increase in the carbon dioxide given off by soil respiration, which is taken to be temperature dependent. Turn the geoengineering off and the resultant warming drives up soil respiration in a positive feedback, releasing yet more carbon dioxide and pushing temperatures yet higher. It’s a good example of the links between climate and the carbon cycle and the ways they can mess you up. Not as good an example, though, as that offered by the precipitation outlook, which seems to me the most startling result here.
In the scenario with A2 emissions taken out to 2100 and no geoengineering there is a significant decline in precipitation with respect to 1900 in a lot of places: the Sahel and central Africa, Europe, India, southeast Asia, China, Australia, the east of both north and south America. This sounds pretty bad. However, the geoengineered scenario is yet worse. The reduced precipitation areas spread to cover basically all of the Americas south of Canada, all of Africa south of the Sahara, all of Eurasia except the middle east, Pakistan and Kazhakstan. There are no increases in precipitation anywhere.
There’s a fairly straightforward geophysiological reason for this. With no increase in temperature (and a slight cooling in the tropics) there’s no increase in evaporation in the geoengineered world’s oceans, so there’s no increase in water vapour from that source. And because there is higher carbon dioxide, there’s reduced evapotranspiration on the continents. A great deal of the water that gets out of the soil and back into the sky does so through the stomata, pores in leaves that let carbon dioxide in and water out. In a higher carbon-dioxide world, plants can keep their stomata more tightly closed and get by with fewer of the things in the first place. Closed down stomata mean less water getting out into the sky. Drier skies, weaker rains.
The effect of this decreased stomatal conductance has already been identified through an increase in the amount of water running off the continents (see Gedney et al in Nature). In a world where temperature and carbon dioxide levels became decoupled it could be really signficant.
This seems to me as strong a scientific argument against this sort of geoengineering as has yet been seen. Obviously more studies with better models could provide a more sophisticated understanding of the issues. But the fact that the model shows a major aspect of climate actually being made worse by geoengineering for reasons that, on inspection, make a fair bit of sense seems pretty damning, though it doesn’t of itself offer arguments against regional geoengineering schemes such as putting a regional cap on the arctic in order to save sea ice, as discussed in my feature.
The article also shows again the importance of the various interactions between the carbon cycle and the climate, a subject that would make a fine theme for a science book, in anyone was thinking of writing one…
Bonus fact, courtesy of a back of the envelope caclulation by Ian Woodward: if you counted all the earth’s stomata you’d come up with something quite close to Avogadro’s number.