Ken Caldeira suggests slowing warming with sulfate particles. Greg Mankiw worries about unintended consequences. I may be misremembering, but . . . weren't sulfates the things from coal plants that caused acid rain?
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Sulfate particles do indeed cause acid rain, but the effect is local and we could choose sulfate-injection sites accordingly. Moreover, acid rain is something we've dealt with before (yea unto the billions of years, since large volcanoes do exactly this -- an eruption of Mount Tambora caused 1815 to be known as "The Year Without a Summer"), and atmospheric sulfates settle out in a year or two anyway. If we don't like the consequences, we can stop them very quickly.
To note: this idea was originally proposed by Paul Crutzen, who won a Nobel Prize for his work in atmospheric chemistry. In addition to being fractionally as expensive as curbing the use of fossil fuels, the effects are immediate (contrast with the hundreds of years it will take for accumulated carbon dioxide gas to dissolve into the ocean).
This is as close to a free lunch as things come, and I've been puzzled that it hasn't caught on.
It would appear to depend on the mode you use for the sulfate. Sulfate is the counterion of sulfuric acid. If the sulfate is added as either the full acid or the half acid (sulfuric acid has two acidic protons), then, yes, you are adding acid directly to the atmosphere. If it is added as the full salt, then the you are adding a weak base to the atmosphere. I will have to do some searching for the proposed reagents.
You could add Sulfate as bi-sulfate which is HSO4, which tends to dissociate in solution but is a rather weaker acid than Sulfuric Acid (H2SO4. Although, most Sulfate salts are soluble in water so it might be possible to add something like Calcium sulfate and not get acids...although I'm not sure Ca 2+ floating around is really that much better because you might just end up with weakly basic water supplies farther down the cycle.
Here's a point of skepticism: where do we get enough of these sulfate particles to inject them into the stratosphere at an equivalent rate of a "five gallon bucket's worth every second" for, evidently, "several months"?
Seems like what the author really needs to be proposing is that we go find a couple active volcanoes that are overdue for a Big One, robotically drill a couple shafts down into the cone...and if that doesn't do the trick, drop a few thousand pounds of dynamite down the hole with a lit fuse.
And after bizarrities like that, the only question remaining is whether or not the roadrunner successfully gets away.
Interesting that this should be proposed by someone named Caldeira, considering what a caldera is.
Making that much sulfate is that much of a problem. Around 165 million tonnes of sulfuric acid are produced a year globally (165 is the 2001 figure) and there are only around 31,557,600 seconds in a year. Doing it would probably raise the price of fertilizer and a few other things which sulfuric acid is also use to produce a bit, but there's plenty of production capacity. It could easily be reacted with any number alkali earth-based minerals to get MSO4 compounds.
I'm not an expert on the chemistry involved in acid rain, but I don't think this would contribute to that specifically, since metal sulfates are stable at the temperature involved. The simplified version of why is below.
The exhaust stream from the combustion of hydrocarbons with sulfur in them will contain CO2, CO, SO3 (SO3 reacts with water to form sulfuric acid (H2O + SO3 -> H2SO4)), H2O, and H2SO4. All of these compounds are fairly volatile and will exit in the exhaust.
Most of these fuels will also contain metals, which will form solid oxides and hydroxides (which can react with H2SO4 and SO3 to form sulfates), and sulfates, but since they're solids a relatively a small amount makes it out in the exhaust as particulate aerosols. Thus, any combustion process producing metal sulfate particulate aerosols will likely also likely be producing good amounts of H2SO4 that won't have anything to react with until it comes down as acid rain, but you can produce metal sulfate aerosols without adding much H2SO4 to the the atmosphere.
Ooops - I was wrong about that part. The primary combustion product that matters for acid rain is SO2, which reacts with water to form a sulfite (H2SO3), which is then oxidized to the sulfate.
PLease correct me if I am wrong. But another problem with this solution is that the sulfate particles fall to the earth, while CO2 stays in the aptmoshphere.
Adding more pollution to pollution is not the answer. In fact, it will only make the entire problem worse as it will make people think it is okay to continue using fossil fuels in such an inefficient manner.
"weren't sulfates the things from coal plants that caused acid rain?"
Yes, they're also the things that are thought to have disguised the effects of rising CO2 concentrations upon temperature from the 40s to the 70s. Thus producing the falling temps in that time period.
So we know that it works: question is, are we prepared to pay the cost?
How about a word from the pro-global-warming faction?
We have a political movement that gains strength by emphasizing the negative aspects of global warming. But are we really sure that warming is, on balance, a bad thing? I haven't seen much of an attempt to make that case; what little analysis has been done has had trouble rising above its political context. Actually, given the history of the last millennium and the last ice age, one would think the presumption would be in favor of warming.
It makes sense to focus on the harms from global warming when we're debating how much humanity should add to the warming trend. But before we start intervening on the other side, we need a much clearer consideration of the costs and benefits.
The main reason that stratospheric SO2 won't cause [much] acid rain is that it'll stay there for two years, while smockstack or tailpipe SO2 stays for two weeks.
To maintain a certain sun reflective stock requires 2% as much turnover when injected into the stratosphere as when emitted close to the ground, and the acid rain is just that -- the turnover.
-dk