Winston Churchill once famously said, "Democracy is the worst form of government except all the others that have been tried." Climate engineering may indeed be a bad idea, but so far, better ideas to mitigate global warming show little traction.
We can all agree that eliminating carbon emissions is the right thing to do and that everyone in the world should set aside narrow short-tem self interest and instead work together to provide a better environment for future generations. And when Alan Robock provides 20 reasons why geoengineering may be a bad idea, we can all agree--it is a bad idea. However, what we appear to be achieving in the meantime may be much worse. For all the recent talk about reducing carbon dioxide emissions, the concentration of atmospheric carbon dioxide is growing more rapidly than supposedly pessimistic scenarios predicted even a few years ago.
Preliminary climate model simulations show that in a high-carbon-dioxide world, Earth's climate would be more similar to that of several centuries ago with climate engineering than without it. It won't work perfectly, but imperfection isn't an argument against improvement. The question is whether, in the face of rising greenhouse gas concentrations, climate engineering will improve environmental conditions or merely make things worse. This is an open research question that needs to be vigorously pursued, but an examination of a few of the criticisms on Alan's list demonstrates that so far, none are nonnegotiable:
Continued ocean acidification. Emissions reduction and climate engineering are two levers of action that can be employed jointly or separately. Ocean acidification is a consequence of excess atmospheric carbon dioxide getting dissolved into the ocean, not climate engineering. Climate engineering cannot reverse every adverse consequence of carbon dioxide emissions, but no thoughtful person ever claimed it would.
Ozone depletion. The Mount Pinatubo eruption lofted more than enough aerosols into the atmosphere to compensate for a doubling of atmospheric carbon dioxide, yet ozone concentrations fell by only 3 percent. And it's believed that this small reduction was caused by chlorine from human-made chlorofluorocarbons, which are now banned by the Montreal Protocol. So while the threat to the ozone layer is worth studying in greater detail, it's expected to diminish with time. Furthermore, schemes have been proposed that might preferentially scatter ultraviolet radiation, compensating for any minor reduction in the protection that the ozone provides us from ultraviolet light.
Effects on plants. Alan is correct that we need to study possible effects of climate engineering on plant growth. After the Mount Pinatubo eruption, vegetation everywhere grew more vigorously, taking up more carbon from the atmosphere. This is because diffuse sunlight is able to reach down to enhance photosynthesis in the lower leaves of forest trees, which are normally shaded by the upper canopy in direct sunlight. In general, plant growth responds almost linearly to changes in the amount of sunlight--a 2-percent reduction in sunlight might be expected to produce 2 percent less photosynthesis. But people growing crops in greenhouses often elevate the carbon dioxide level to fertilize their plants, and this effect is typically larger than 2 percent. Therefore, it's possible that a high-carbon-dioxide world with slightly reduced but more scattered sunlight would have higher crop yields than today's world. In computer simulations, vegetation grew more vigorously in an engineered high-carbon-dioxide world than it did in the natural low-carbon-dioxide world. Of course, we can expect these changes to affect natural ecosystems in unforeseen ways, and so should certainly be the subject of intense study.
More acid deposition. The amount of sulfur used in a climate engineering system would be a small percentage of today's emissions from power plants. So if current sulfur emissions regulations were tightened by a few percent when such a system was deployed, there would be no increase in overall sulfur emissions. Furthermore, there's nothing magical about sulfur--other compounds such as silica or calcium carbonate could be used to scatter incoming sunlight, although perhaps at somewhat greater economic cost.
I'll leave Alan's other points for future missives, but the take-home message is, preliminary climate model simulations indicate that climate engineering may mitigate some but not all of the effects of rising greenhouse gas concentrations. While we might prefer near-universal cooperation in carbon dioxide emissions reduction, it's clearly time to plan what we will do if those emissions reductions don't come quick enough or are not deep enough to prevent a climate crisis. The question isn't whether we need to plan for such an eventuality, but what form that planning should take.