WoE: Do you find there is a taboo of sorts against studying geoengineering among Earth scientists? It’s fairly unusual to see the topic come up at conferences, so your poster caught our eye.
English: There is more acceptance of studying it in just the last couple of years. I think scientists are facing the reality that countries aren’t doing much to slow the emissions of greenhouse gases. Eventually, we may have to choose between the risks and consequences of climate change and the risks and consequences of climate engineering. The only way to make an educated decision about that is to study it.
WoE: What type of geoengineering are you focusing on?
English: For my PhD, I have been looking at stratospheric aerosols.
WoE: Hold up. What are stratospheric aerosols?
English: They’re the tiny particles that are aloft in the atmosphere about 20 kilometers above the surface of the Earth. One of the leading geoengineering ideas is to inject aerosols into the stratosphere. I decided, after getting help and input from colleagues such as Michael Mills and Brian Toon, to set up a computer model that would analyze exactly how something like that would work.
WoE: And what do stratospheric aerosols have to do with climate?
English: People have suggested we could use a type of a particle for geoengineering that is actually composed of tiny droplets of sulfuric acid. Those are called sulfates. Sulfates reflect sunlight. If you have a layer of these particles up in the stratosphere they reflect part of the incoming solar radiation from the sun back to space. Overall, they have a cooling effect.
WoE: And sulfates can make it all the way up to the stratosphere?
English: Yes, some of the stronger volcanic eruptions can send particles into the stratosphere. They take a couple of years to settle back down to the surface. Very tiny amounts from power plants and other sources can also make it up that far.
WoE: I get that you modeled what might happen if humans decided to inject sulfates into the stratosphere, but what was the precise question you set out to answer?
English: There have been a few other scientists who have looked at geoengineering using stratospheric aerosols, but they didn’t simulate all the processes that can affect the particles. Recently, a team led by Patricia Heckendorn, a researcher based in Zurich, simulated all of these processes in a 2D model and found that the effectiveness of sulfate geoengineering diminished as more sulfate was added. I wanted to use a 3D model that looked at all of the processes, and I wanted to compare our results to Heckendorn’s.
WoE: What processes did you include that others didn’t?
English: For example, our model simulates coagulation, the process by which multiple particles can combine to become one. We also included nucleation – that’s when tiny gas molecules condense on each other to form liquid droplets. Also condensational growth. If you watch, say, water drops grow bigger and bigger on a piece of grass on a foggy morning you’re looking at condensational growth.
WoE: What did you find when you included all of that in your model?
English: What we found was that effective geoengineering required injecting larger masses of sulfuric acid than some have hoped because the particles coagulate and get much bigger than thought. Larger particles fall out of the stratosphere faster to the surface, so they’re not as effective at reflecting light. This matched Heckendorn’s results.
WoE: How much less effective?
English: It depends on how much sulfate we add. The more we add the less effective they become.
WoE: That’s the opposite of what people probably think...
English: It still gets more effective as you add more, but it has a diminishing return. We haven’t done a detailed assessment yet, but the group led by Heckendorn did, and they had a similar result. They found that you would need to inject more than 10 million metric tons of sulfur into the stratosphere per year if you wanted to offset the current forcing from greenhouse gases. People used to think it could be done with about 3 million metric tons.
WoE: Ten million metric tons sure sounds like a lot.
English: It is. Mount Pinatubo released about 10 million metric tons, but that was a one-time shot. Basically, we would need one or two Mount Pinatubo’s every single year.
WoE: Where do we go from here?
English: These results were surprising. If geoengineering is going to work, I think we’re realizing that scientists will need to look at new and creative ways to add particles to the stratosphere in such a way that they don’t grow too big and fall out too quickly.