"Wait, don't do it!"
That was my first reaction after reading about Paul Crutzen's semi-crazy idea to ameliorate anthropogenic global warming by filling the stratosphere with sulfur. In case you've missed the story, there's a wired article that covers the main points [LINK]. It all stems from an editorial Crutzen published in Climatic Change, [LINK] . The idea is that putting sulfur into the stratosphere (about 20 km above you, say) would reflect sunlight, reducing the amount of energy reaching Earth's surface. That would cool the globe, no doubt, but there are problems.
We know it will work. Volcanoes do this same thing, more or less. We also know it would be temporary, because the sulfur would only float around the stratosphere for a few years before being used up in chemical reactions and slowly deposited back into the troposphere and back to the surface. Crutzen covers all this in the paper, which is mostly a quick back-of-the-envelop calculation mixed with some previous results. Crutzen, it should be pointed out, is not actually in favor of the idea; the media doesn't really seem to be mentioning that so much. In the paper he is extremely hesitant, saying essentially that if we keep pumping CO2 into the atmosphere we may start to experience catostrophic warming (~5 degrees C), which would necessitate rapid action to reduce the global temperature. To that end, he proposes a community wide, multidisciplinary effort to test this geo-engineering scenario. He thinks we need to model the effects, but also consider possible ecological consequences.
So what are the problems with reducing the sunlight getting to the surface? Well, one that is pointed out by the Wired article is that it will directly impact plants and photosynthesis. This might be especially pronounced in the tropics, where plants have evolved to expect a lot of sunlight. Changing the amount of light reaching the surface might give some plants a benefit and others a disadvantage, which could potentially throw the natural balance out of whack. Land-use issues aside, we don't have any idea really what the distribution of plant species in the tropics means for the global carbon cycle, not to mention the hydrological cycle. A second potential problem is that the additional sulfur in the stratosphere might change the stratospheric heating rates, which would change the temperature distribution, which would alter the large-scale temperature gradients, and might impact the Brewer-Dobson circulation. This would have unknown effects on the general circulation of the midlatitudes, possibly altering large-scale weather patterns (think El Nino or North Atlantic Oscillation). A third issue, also mentioned by Crutzen, is that cooling the surface won't save the ocean. As CO2 increases, it will continue to be taken up by the ocean. Unfortunately, that increases the acidity of the upper ocean, where lots of little creatures grow. Many of those little creatures grow calcium carbonate shells, but they can't do it in acidic conditions. That means they die. Not only do those organisms play an important role in the carbon cycle (and other biogeochemical cycles), but they are also the foundation of the entire marine food chain. If they die, then large species suffer, and larger ones suffer even more, and even humans who like to eat seafood will suffer.
So those are my first three potential problems with this plan. However, I'll take Crutzen's side. He basically says that our policy makers have their heads in their behinds, partly because they don't have good solutions and partly because they are not forward thinking, and so there is not going to be a reduction in greenhouse gas concentrations any time soon. Since we know we will face global warming, we need to figure out what to do if the warming starts to get out of control. This sulfur parasol effect is one possibility, and it should be investigated. Along the way, we will continue to learn important things about the climate system, even if the sulfur parasol turns out to be an untenable solution.
1. BioEd Online: Should we flood the air with sulphur? [LINK]
2. Crutzen, Paul J., 2006: Albedo Enhancement by Stratospheric Sulfur Injections: A contribution to resolve a policy dilemma? Climatic Change doi: 10.1007/s10584-006-9101-y [possible LINK]
3. Geo-engineering in vogue, on RealClimate [LINK]