A Small Step Forward for Geoengineering

Published: September 20th, 2010

Michael D. Lemonick

Nobody wants to be a defeatist, but considering the rate at which nations are reducing emissions of heat-trapping greenhouse gases — which is to say very slowly, if at all — it’s not unreasonable to begin thinking of what else the world might do to avoid major climate disruptions.

One idea that’s gotten increasing attention lately is geoengineering, a grab-bag of technologies that include doping the ocean with iron to fertilize carbon-eating algae. Most geoengineering schemes, however, involve cooling the planet by reflecting sunlight back into space. There are lots of ways to do it, ranging from the low-tech (painting everyone’s roof white) to high tech (lofting outward-facing mirrors into space).

A diagram of various planetary-scale geoengineering proposals.
Credit: Lawrence Livermore National Laboratory.

And then there’s the medium-tech approach: mimic the action of volcanoes by spewing clouds of sulfate particles into the stratosphere. When volcanoes do this, it can cause a pretty significant, albeit temporary, cooling. Keep pumping the stuff up there, the theory goes, and you could offset the warming from increasing greenhouse gases for as long as you wanted.

It sounds so simple! But it’s not. It turns out that blocking sunlight to cool the Earth also affects the hydrologic cycle of precipitation and evaporation, which ecosystems and civilizations depend on for survival. My Climate Central colleague Phil Duffy co-authored a paper addressing that very topic a couple of years ago — and now, George Ban-Weiss and his colleague Ken Calderia, both at the Carnegie Intstitution for Science's Department of Global Ecology in Palo Alto, California have moved the ball forward with a new study in the journal Environmental Research Letters.

“It’s been established,” says Ban-Weiss, “that if you have a uniform distribution of particles, you’re going to under-cool the poles and over-cool the Equator.” Naturally, then, you should just put more particles over the poles — but not so fast!

“A uniform distribution,” continues Ban-Weiss, “is best if you want to avoid disrupting the hydrologic cycle.”

In short, you can’t win. You are, as Phil Duffy says, “in tradeoff city.” But by creating what engineers call an optimization model, Ban-Weiss and Calderia have found a way to make the best of an imperfect situation by adjusting the particle distribution to maximize temperature control, while minimizing water-cycle disruption. Specifically, they say, you can reduce the temperature increase you’d otherwise get by doubling CO2 by 94 percent, and tamp down the interference this would cause in the water cycle by 74 percent.

Now for the caveats (and take a deep breath, because there are plenty). First of all, emphasizes Ban-Weiss, “this is an idealized study. It isn’t a realistic model.” It isn’t clear, for example, how you could make sure the aerosols go where you want them, and stay there. The model also doesn't take into account the fact that aerosols interact with the climate system in all sorts of ways — helping to seed cloud formation (or inhibit it under some circumstances), reacting with tropospheric ozone, and more. As Calderia put it in a Carnegie press release: “…this is just one model and it does not include all processes that are important in reality. Our results are illustrative and do not provide a sound basis for making policy decisions."

Blocking out sunlight, moreover, would do nothing to prevent ocean acidification, caused by seawater absorbing extra CO2 from the air, which could wreak havoc with sea life in all sorts of ways. And then there are the “unknown unknowns” — the unanticipated side effects that come so often with new technologies. Such concerns led to a conference last spring in California to explore the controls that might be placed on geoengineering experiments to avoid unpleasant surprises. Another conference, on the ethics of using sunlight-blocking technologies, is happening next month in Montana.

In short, while the new study represents a step forward, there’s still plenty to do before we start shooting sulfates into the stratosphere.