## Practical geoengineering

“Temporary, moderate, and responsive” should be the guidelines of responsible geoengineering, in David Keith’s view. For slowing global warming, and giving humanity time to bring greenhouse gas emissions down to zero (and eventually past zero with carbon capture), he favors the form of “solar radiation management” that reflects sunlight the way volcanoes occasionally do—with sulfate particles in the stratosphere.

The common worry about geoengineering is that because it is so cheap ($1 billion a year) and easy, civilization would become “addicted“ and have to continue it forever, while giving up on the expensive and difficult process of reducing greenhouse gas emissions, thus making the long-term problem far worse. Keith’s solution is to design the geoengineering program as _temporary_ from start to finish. “Temporary“ means shut it down by 2200. (Keith also likes the term “patient” for this approach.)

By “ _moderate_ ” he means there is no attempt to completely offset the warming caused by us, but just cut the rate of climate change in half. That would give the highest benefit at lowest risk—minimal harmful effect on ozone and rainfall patterns, and the fewest unwelcome surprises, while providing enough time (and plenty of incentive) for societies to manage their carbon dioxide mitigation and orderly adaptation. Geoengineering’s leverage is very high—one gram of particles in the stratosphere prevents the warming caused by a ton of carbon dioxide.

“ _Responsive_ ” means careful, gradual, and closely monitored, with the expectation there will be many adjustments along the way, along with the ability to back off entirely if needed. Though climate-change models keep improving, we still do not completely understand how climate works, and that raises the very good question: “How do you engineer a system whose behavior you don’t understand?” Keith’s answer is “feedback. We engineer and control many chaotic systems, such as high-performance aircraft, through precise feedback.” The same goes for governance of geoengineering. It is a complex system that will require sophisticated control by a global set of governing bodies, but we already do that for the far more complex system of global finance.

Keith’s specific program would begin with balloon tests in the lower stratosphere (8 miles up) releasing just 100 grams of sulfuric acid—about the amount of particles in a few minutes of normal jet contrail. “If those studies confirm safety and effectiveness,” Keith said, “then we could begin gradual deployment as early as 2020 with three business jets re-engineered for high altitude. By 2030 you could have about ten aircraft delivering a quarter million tons of sulfur per year at a cost of $700 million.“

The amount of sulfur being released might be up to a million tons by 2070, but that would still be only one-eighth of what went into the stratosphere from the Mt. Pinatubo volcanic eruption in 1991, and _one-fiftieth_ of what enters the lower atmosphere from our current burning of fossil fuels. By then we may have developed more sophisticated particles than sulfate. It could be diamond dust, or alumina, or even something like a nanoscale “photophoretic” particle designed by Keith that would levitate itself above the stratosphere.

This is no quick fix. It is not quick, and it doesn’t try to be a complete fix. It has to be matched with total reduction of greenhouse gas emissions to zero _and_ with effective capture of carbon, because the overload of carbon dioxide already in the atmosphere will stay there for a very long time unless removed. Keith asked, “Is it plausible that we will not figure out how to pull, say, five gigatons of carbon per year out of the air by 2075? I don’t buy it.“

Keith ended by proposing that goal should not be just 350 parts per million (ppm) of carbon dioxide in the atmosphere. (It’s rising past 400 ppm now.) We can shoot for the pre-industrial level of the 1770s. Take carbon dioxide down to 270 ppm.