Just about two years ago, Chemistry Nobelist, and atmospheric scientist Paul Crutzen opened a huge can of worms by suggesting that, since the world doesn’t seem to be getting its act together to significantly reduce CO2 emissions, it would be prudent to think about emergency measures in which we engineer ourselves out of the crisis by monkeying directly with the Earth’s solar radiation input instead of dealing with the CO2 content of the atmosphere. The specific proposal was to inject chemicals into the stratosphere that would form sulfate aerosols and hence block sunlight. Crude estimates suggest that the aerosol fix (if it is indeed a fix and doesn’t create more problem than it solves) is more technologically feasible than sci-fi dreams of sunshades at the Lagrange point. Not to say technologically feasible, necessarily, but not so far out as the other schemes. Crutzen’s idea, and related geoengineering proposals, have been discussed here on RealClimate. The subject is once more in the news, thanks to this chipper little op-ed by Ken Caldeira, which appeared in the New York Times this week.
Update: I just noticed that our original RealClimate piece was done before Crutzen’s article was published. You’ll find his article here (subscription not required).
The attraction of the proposal is that we are already conducting an uncontrolled experiment on aerosol-based geoengineering, through the sulfate aerosols injected into the troposphere by dirty coal plants. Along with a lot of nasty health and environmental consequences, this has had some inadvertent benefits in restraining some aspects of global warming. As coal plants get cleaned up in the future some of the cooling aspect of the tropospheric aerosols will be lost. Since aerosols last much longer in the stratosphere than they do in the rainy troposphere, the amount of aerosol-forming substance that would need to be injected into the stratosphere annually is far less than what would be needed to give a similar cooling effect in the troposphere, though so far as the stratospheric aerosol burden goes, it would still be a bit like making the Earth a permanently volcanic planet (think of a Pinatubo or two a year, forever). It might make sense to take a small portion of the aerosol that would have been dumped into the troposphere by retired dirty coal plants, and inject that directly into the stratosphere where it will restore the lost cooling effect while (hopefully) doing less harm than the old stuff dumped into the lower atmosphere. To go farther, though, and count on offsetting the entire unrestrained CO2 production of the coming century with engineered aerosols is fraught with peril.
Scientists just love to think about this kind of stuff, and I’m no different.Harvard is hosting a small workshop on aerosol-based geoengineering, and I have to say I’m looking forward to it. It’s like having a shiny new toy, and the chance that you might actually get to use it to play around with the real Earth and see what happens has a certain fatal attraction to it. Then, too, science thrives on a spirit of free inquiry, and it would be anathema to say that there are some things that just shouldn’t be thought about (though there are certainly some things that, once thought about, shouldn’t be built). But, there’s a real danger of jumping the gun and giving the impression that we already know we have a way out if things get too bad. Ken Caldeira’s Op-Ed is a case in point. “Which is the more environmentally sensitive thing to do: let the Greenland ice sheet collapse, or throw a little sulfate in the stratosphere?” is the way he frames the issue. To be sure, Ken only gets 400 words to make his case (which seems to be that the folks who work on this sort of stuff ought to get some more money), but those 400 words leave little room to explain the vast array of problems that need to be resolved before we can even begin to think of this as an out. Caldeira’s Op-Ed makes it seem like a slam-dunk, needing maybe only a diversion of 1% of climate research funds in order to do the trick.
Here are a few of the problems that need to be worked out: There’s the issue of the effect of the aerosols on stratospheric chemistry (think how unanticipated the chemistry of the Ozone Hole was), and the question of just where the aerosols would go once injected. There’s the question of the effect of the aerosols as cloud-condensation nuclei if they work their way into the tropical upper troposphere — an increase in high cirrus clouds could well lead to warming. Then, there’s the full range of possible effects on the atmospheric circulation. Held and collaborators (PNAS 2006) have implicated the joint effect of aerosols and greenhouse gases in the trend towards Sahel drought, and generally there are issues in what inhomogeneous aerosol forcing might do to things like the North Atlantic Oscillation. Also, a planet with a dim Sun and high CO2 is not the same thermodynamically as a planet with brighter Sun and lower CO2, because the reduced sunlight at the surface is not able to sustain as much evaporation, which has consequences for global rainfall. In a recent essay in Le Monde, Edouard Bard has pointed out additional problems with geoengineering.
In my mind, the most serious peril of sulfate geoengineering is one that stems from a problem that is not at all in dispute: the fact that the lifetime of CO2 in the atmosphere is centuries to millennia, whereas the lifetime of aerosols in the stratosphere is at best a few years. That means committing the future generations to continue the aerosol injection basically every year more or less forever. We’re banking a lot on confidence in future stability and prosperity of the world here. A patrician in the glory days of the Roman Empire might well have expected the Pax Romana to go on forever, but really nobody expects a Dark Age.
One also has to wonder whether the international treaties and organizations needed to agree on and execute a geoengineering scheme are significantly easier to realize than the agreements needed to decarbonize the energy future, which would offer safer and more durable climate protection. And once you open the Pandora’s box of geoengineered climate, what do you do if nations disagree about what kind of climate they want, or if some poor nation objects to suffering drought in order to cancel heat waves in Chicago? Great fodder for science fiction novels about climate wars, but I’d prefer not to have to think about it happening for real.
The problem is that geoengineering a sunshade is being sold as insurance long before anybody has any idea whether it would work and what the unintended consequences would be. It’s not really insurance. It’s more like building a lifeboat, but a lifeboat based on a design that has never been used before which has to work more or less perfectly the first time the panicked passengers are loaded into it. The problem is that by the time we know enough to have any confidence at all in this lifeboat, CO2 may have risen to the point where the lifeboat becomes not just a backup, but a necessity. Would diverting 1% of the world’s climate research funds into this problem clarify the issues in time? I doubt it. Would devoting 10% a year to the problem be worth it? I doubt that, too, in comparison to more pressing research needs.
Now, can we please get back to the serious business of trying to figure out how to economically reduce global CO2 emissions?