There was an interesting article in the NY Times this week on possible geo-engineering solutions to the global warming problem. The story revolves around a paper that Paul Crutzen (Nobel Prize winner for chemistry related to the CFC/ozone depletion link) has written about deliberately adding sulphate aerosols in the stratosphere to increase the albedo and cool the planet – analogous to the natural effects of volcanoes. The paper is being published in Climatic Change, but unusually, with a suite of commentary articles by other scientists. This is because geo-engineering solutions do not have a good pedigree and, regardless of their merit or true potential, are often seized upon by people who for various reasons do not want to reduce greenhouse gas emissions. However, these ideas keep popping up naturally since significant emission cuts continue to be seen as difficult to achieve, and so should be considered fairly. After all, if there was a cheaper way to deal with the CO 2 problem, or even a way to buy time, shouldn’t we take it?

First a little history [Update: See Spencer Weart’s essay on the history of climate modification ideas]. Geo-engineering ideas first reached the public in the 60s when there was still a lot of enthusiasm for technical fixes of the world’s problems. One example was suggested by the Soviets who wanted to melt the Arctic (either using soot or nuclear devices) in order to warm up their frozen North. More recently, there was a proposal to dam the Straits of Gibraltar in order to prevent more saline Mediterranean Sea water (because of the Aswan Dam) from affecting the North Atlantic conveyor circulation (no, it didn’t make sense to us either). With such a pedigree, geo-engineering is generally seen as fringe entertainment at best, although some of the new ideas concerning atmospheric carbon dioxide sequestration are being looked into seriously.

Edward Teller is the scientist most associated publicly with the idea of creating a stratospheric shield to prevent excessive global warming, though he built on an idea from Freeman Dyson (who has subsequently become a bit of global warming contrarian)*. However, as Teller’s collaborator Stanislaw Ulam once said after discussing some new ideas with him: “Edward is full of enthusiasm about these possibilities; this is perhaps an indication they will not work”. And given Teller’s estrangement from the scientific community in his later years, it was not likely that the concept would be taken very seriously, and indeed it hasn’t been.

*Which in turn built on a idea from Budyko…(see comment below).

But now Paul Crutzen has stepped into the fray. He has a much more solid reputation amongst climate scientists than Teller, and thus his ideas will be taken more seriously. I haven’t seen the new paper yet (it’s out in August) but there are a number of questions that need to be addressed before any geo-engineering proposal combatting global warming should be thought of as anything more than an interesting idea. First, the idea has to actually work, second, the side effects need to be minimal, and third, it has to be able to keep up with an increasing forcing from ever higher greenhouse gas levels, and fourth, it has to be cheaper than the simply reducing emissions at source. These are formidable hurdles.

Would it work? In most of the cases under discussion the target is the global mean temperature, and so something that balances the global radiative forcing of greenhouse gas increases is likely to ‘work’. However, having no global mean forcing is not the same as having no climate change. A world with higher GHGs and more stratospheric aerosols is not the same as a world with neither.

Thus there will be side effects. For the stratospheric sulphate idea, these fall into two classes – changes to the physical climate as a function of the changes in heating profiles in solar and longwave radiation, and chemical and ecological effects from the addition of so much sulphur to the system. Physically, one could expect a slight decrease in surface evaporation (a ‘dimming’ effect) and related changes to precipitation, a warming of the tropopause and lower stratosphere (and changes in static stability), increased Eurasian ‘winter warming’ effects (related to shifts in the wind patterns as are seen in the aftermath of volcanoes). Chemically, there will be an increase in ozone depletion (due to increases in heterogeneous surface chemistry in the stratosphere), increases in acid rain, possibly an increase in high cirrus cloud cover due to indirect effects of the sulphates on cloud lifetime. Light characteristics (the ratio of diffuse to direct sunlight) will change, and the biosphere may react to that. Dealing with the legal liability for these predictable consequences would promise to be a lively area of class action litigation…. On the positive side, sunsets will probably be more colorful.

Could it keep up? GHGs (particularly CO 2 ) are accumulating in the atmosphere and so even with constant present-day emissions, the problem will continue to get worse. Any sulphates put in the stratosphere will only last a couple of years or so and need to be constantly updated to maintain concentrations. Therefore the need for the stratospheric sulphates will continue to increase much faster than any growth of CO 2 emissions. This ever-increasing demand, coupled with the impossibility of stopping once this path is embarked upon is possibly the biggest concern.

How expensive would it be? I will leave the detailed costing to others, but stemming from the last point, the cost will continue to rise indefinitely into the future unless this proposal is coupled with an concomitant effort to reduce CO 2 emissions (and concentrations) such that the need for the sulphates will diminish in time.

Crutzen’s paper may well address these issues comprehensively (and I look forward to seeing it) but, in my opinion, the proposals are unlikely to gain much traction. Maybe an analogy is useful to see why. Think of the climate as a small boat on a rather choppy ocean. Under normal circumstances the boat will rock to and fro, and there is a finite risk that the boat could be overturned by a rogue wave. But now one of the passengers has decided to stand up and is deliberately rocking the boat ever more violently. Someone suggests that this is likely to increase the chances of the boat capsizing. Another passenger then proposes that with his knowledge of chaotic dynamics he can counterbalance the first passenger and indeed, counter the natural rocking caused by the waves. But to do so he needs a huge array of sensors and enormous computational resources to be ready to react efficiently but still wouldn’t be able to guarantee absolute stability, and indeed, since the system is untested it might make things worse.

So is the answer to a known and increasing human influence on climate an ever more elaborate system to control the climate? Or should the person rocking the boat just sit down?