☀ A Quick Q&A on solar geoengineering with … Wake Smith and David Keith

'We have a lot more climate change in our future than most people realize. So we will need additional tools by which to manage that.'

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Q&A

☀ A Quick Q&A on solar geoengineering with … Wake Smith and David Keith

While governments and companies grapple with ways to reduce carbon emissions, there’s growing interest in geoengineering as a way of mitigating rising temperatures. One possible method: stratospheric aerosol injection, releasing particles into the stratosphere to mimic volcanic cooling effects by reflecting solar radiation away from Earth.

I email asked Wake Smith and David Keith a few questions about what it might look like to implement this technology. They co-wrote the recent MIT Technology Review piece, “Solar geoengineering could start soon if it starts small,” in which they discuss the potential for a country or group of countries to launch small-scale deployment of SAI within five years. They argue that such a deployment would still have noticeable effects on the stratosphere's composition and could provide “as much cooling as sulfur pollution from international shipping did before the recent cleanup of shipping fuels.”

But Keith and Smith caution that a subscale deployment risks political backlash. As such, they oppose near-term SAI deployment and support a moratorium until the science is thoroughly assessed and a governance framework is established. Nevertheless, they argue that policymakers may need to address the challenges and implications of solar geoengineering sooner than anticipated, given the plausibility of subscale deployments.

Smith is a Lecturer in Yale College and a Senior Fellow at the Mossavar-Rahmani Center for Business and Government. He formerly acted as Senior Industry Partner at New State Capital Partners, President and Chairman of Pemco World Air Services, and COO of Atlas Air Worldwide Holdings.

Keith is Professor of Applied Physics at the Harvard School of Engineering and Applied Sciences and Professor of Public Policy at the Harvard Kennedy School. He is the founder of Carbon Engineering, a carbon-capture firm, and has been named one of TIME magazine’s Heroes of the Environment. He also led the development of Harvard’s Solar Geoengineering Research Program.

1/ Can you briefly explain the process of stratospheric aerosol injection and the scientific theory behind it?

Smith: SAI seeks to replicate what the very largest of volcanic eruptions do. Most volcanoes outgas smoke, ash, and sulfur dioxide into the lowest layer of the atmosphere — the troposphere — where they hang around for a few days and descend via gravitational settling or with the next rain storm. But the very largest volcanoes — the couple-of-times-a-century monsters such as Mount Pinatubo in the Philippines in 1991 — are so powerful that they blast their gaseous contents all the way through the troposphere and into the next layer of the atmosphere, the stratosphere. If sulfur dioxide is deposited into the stratosphere, it endures there for 12–18 months before it settles back down to the earth. After a few weeks, the sulfur dioxide (SO2) will naturally evolve to sulfuric acid (H2SO4), in which state it is reflective of sunlight. If just one or two percent of the sunlight streaming into the earth’s climate system is reflected back into space, that would be sufficient to offset the amount of warming that our greenhouse gas emissions is causing. For lots of reasons, the earth already reflects about 30 percent of the sunlight that streams towards it back out to space, because it bounces off of reflective surfaces such as clouds, snow, ice, sand — more or less anything white-ish. So if we could just increase our reflectivity from ~30 percent to ~32 percent, we could (in theory) counteract global warming and cool down the earth. We might do that by putting millions of tonnes of SO2 (or other species of sulfur) onto aircraft, flying them way up into the stratosphere, and releasing that SO2, where it might mimic the effect of large volcanoes and artificially cool the planet. Mount Pinatubo reduced global average surface temperatures on the order of 0.5°C for a year after the eruption, so we know that this can cool the earth quite substantially. Lots of caveats and complications, but that’s the quick theory.

2/ How do you envision international airspace cooperation to make this happen?

Smith: This could be managed in the existing air space management regime. This might require 500 or 1000 aircraft, among the tens of thousands that already operate. The planes would just file a flight plan and obey air traffic control rules as they fly up to altitudes about twice as high as most planes fly, but from an ATC standpoint, easy peasy.

Keith: From my perspective, it would make sense to start with a slow ramp up, incrementally increasing the amount of cooling, and the number of aircraft starting from zero with a peak sometime late this century. Under an incremental [solar radiation management] scenario, one would have no reason to have more than 100 aircraft until after 2050, perhaps well after that.