A Case for Climate Engineering by David Keith

Author:David Keith [Keith, David]
Language: eng
Format: epub, pdf
ISBN: 9780262317795
Publisher: MIT Press
Published: 2013-07-15T05:00:00+00:00

Technology and cost to reach the stratosphere

How hard would it be to move one million tons of sulfur per year to the stratosphere? What would it cost? Popular writing is filled with the whizbang aspects of geoengineering hardware. Stories in media outlets ranging from Popular Science to the BBC debate the merits of naval guns, giant hoses suspended by balloon, dirigibles, and airplanes.

The excessive focus on deployment hardware arises from a healthy instinct: readers want a physical picture of what might be done, and journalists find it hard to paint a simple graphic picture of stratospheric chemistry or climate response. In effect, however, the whizbang treatment distracts audiences from the hard questions of risk and trade-off that are largely decoupled from the particular choice of hardware. That said, I describe some of the likely deployment technologies here because I want to make clear that the technology is not the primary issue. Deployment is neither hard nor expensive.

First, we must understand how high one needs to get material for stratospheric geoengineering, since altitude plays a big role in determining the cost and difficulty of delivery technologies. While the stratosphere sometimes extends down below 10 kilometers (32,000 feet), where it is easily reached by passenger jets, air in this part of the stratosphere is rapidly mixed back into the lower atmosphere. Aerosol injected here would have a lifetime measured in months rather than years. This might be useful for the kinds of short-term tests described in Phase 2, but would not likely be sensible for large-scale aerosol geoengineering.32

The top of the stratosphere is about 50 kilometers (164,000 feet), far from the reach of practical aircraft, but one need not go nearly so high. The natural flow of air in the stratosphere rises upwards from tropical latitudes and sinks downwards over the poles, so, if one can inject aerosol a bit above the bottom of the stratosphere in the tropics, it would be carried upwards and then toward the poles, resulting in a long lifetime and a relatively even distribution of aerosol throughout the stratosphere. Computer simulations confirm this intuition and suggest that injection in the tropics at altitudes a bit over 20 kilometers (65,000 feet) would be adequate.

One could in principle use existing aircraft such as jet fighters, but modern business jets are more efficient and much cheaper. A stock Gulfstream G650, a top-of-the-line business jet, cruises at altitudes up to 50,000 feet. If a G650 were retrofitted with a low-bypass military engine such as the Pratt & Whitney F100, it could lift a payload of 13 tons to 60,000 feet, an altitude that would likely be adequate for the minimal deployment described in Phase 3. A fleet of just twenty aircraft acquired within a few years at a cost of $1.5 billion should enable sufficient radiative forcing to produce large-scale climatic effects that are just barely detectable.33

The requisite deployment technology does not exist as ready-to-go hardware today, but it could be supplied by any number of vendors using what the aerospace industry calls commercial off-the-shelf technology.

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