One consequence of the whole Superfreakonomics fiasco, which has been thoroughly reported elsewhere in the blogosphere, is that I've changed my mind about geoengineering ideas. Up until now, I was of the opinion that the various proposals to check climate change by doing various things to the atmosphere or the oceans were no substitute for reducing CO2 emissions, but they were worth at least studying in order to have an emergency reserve option. And in fact, I always liked the stratospheric sulphur one because it didn't involve massive space structures and it was, at least theoretically, reversible - the stuff rains out within weeks to months, so it's possible to switch the thing off.
I also preferred it because one version - very differently from the daft 18-mile hose with helium balloons and sharks with lasers etc - involved simply changing the specification for Jet-A1 aviation fuel in order to let the refinery leave more sulphur in it. Rather than all that incredibly complicated and expensive fantasy engineering - what James Nicoll would call our viewgraph future - this could be done cheaply.
But the Superfreaks have permitted me, at least, to think this through further. The problems with any climate-engineering, rather than emissions-engineering, approach are just too bad.
The mechanism of action for the sulphate plan is basically that it creates more nuclei for water droplets to coalesce around, and therefore creates high-altitude clouds that reflect heat back out to space. Unfortunately, the nuclei are particles of various sulphate compounds, and when they dissolve in water with sunlight, you get sulphuric acid and hydrogen ions; acid rain. And the plan implies doing this globally, so rather than just damaging forests in northern Europe like we used to, we'd be acidifying the sea at the same time as we'd be acidifying it anyway by asking it to take up a whole lot of CO2.
Then there are the consequences in terms of meteorology rather than climatology, about which the best that can be said is "we have no idea" and the worst that can be said is "there is a nontrivial chance of losing the monsoon, gaining nastier hurricanes, or maybe both". In fact, these are worryingly like the consequences of acute climate change themselves, which tends to make you wonder what the point is.
Like the really bad climate change scenarios, these all carry a lot of political risk as well; Ken Caldeira, who was misquoted in the book and who came up with the sulphate plan in the first place, remarked back in the 1980s that one solution to climate change would be a nuclear war, which looked if anything more likely at the time. (For the inevitable hard of thinking troll, his point was that a nuclear war would both fill the upper atmosphere with cloud-seeding dusts of various kinds, and effectively stop humanity emitting lots of CO2, by destroying industrial civilisation - not that this was a desirable option.)
Unfortunately, anything that risks the Indians running out of rain, the Chinese out of drinking water, or the Americans out of coastal cities, is by definition a threat of the same class as a medium-sized ballistic missile attack, and you can bet that the powers in question would draw the conclusions that follow from that.
And also, there's a serious class break for all climate-engineering plans; what happens if it works for an extended period of time, but for some reason we have to stop? If the underlying problem isn't addressed, as soon as the sulphates rained out, the world would heat up until it hit radiative equilibrium right then, which is as good as any definition of the end of the world. This is the flip side of the advantage of reversibility.
There are, of course, alternative approaches; I think of them as emissions-engineering rather than climate-engineering. They essentially aim to absorb the CO2 rather than change other parts of the system's response, and are as such significantly less difficult, because they have a more direct feedback loop. Unfortunately, one of the most promising - feeding the ocean plankton - has been subjected to a large-scale trial and doesn't seem to work, at least not reliably.
I can still imagine a scenario where these could come in handy; but I'm increasingly convinced that nobody who floats them is being serious.