This discussion began with Alan Robock's list of 20 arguments against geoengineering, so it's fitting to end with some comments on his most recent, more subtle contributions to this debate. Quite correctly, he asserts that his "values do not affect the conclusions of [his] science." This is the case for all good scientists. But he also points out that values may influence "what research [we] do or not do in the first place."
For many scientists involved in environmental research--and in research on global warming in particular--our concern for humanity's well-being at least partly motivates our quest to understand a complex problem so that we may respond to it effectively and efficiently. But there's another step, which, to avoid possible misinterpretation, I pose as a question: Within our fields of study, is it possible that values might influence the specific experiments we perform?
Alan claims that I have made "several uncalled for accusations" about his motives. This is manifestly wrong. In this roundtable I have not said anything about anyone's motives. I did, however, point out that there's some inconsistency between Alan's model study, in which he ignores mitigation, and his strong support for mitigation. I have urged from the start that geoengineering should only be considered as a supplement to mitigation. In that context, I believe that studies such as those by Alan, Philip Rasch, and Simone Tilmes that consider geoengineering alone are in danger of being misrepresented or misinterpreted. They greatly overstate the possible climate and stratospheric ozone side effects that might arise in a joint mitigation-geoengineering scenario.
It's undeniable that in a joint mitigation-geoengineering framework, these studies are extreme. They drive the climate-atmospheric chemistry system with a stratospheric aerosol loading far greater than would be necessary if we consider geoengineering and mitigation together--i.e., if we see geoengineering solely as a way to gain time to develop and implement the carbon-neutral technologies that are required to avoid dangerous changes in the climate, sea level, and ocean chemistry. This is what I consider to be a realistic option for the future, perhaps the only realistic option. Contrary to Alan's claim, I don't define "realistic" as a scenario that "keep(s) the climate at the present level."
As Ken Caldeira writes, "Prudence demands that we consider what we might do if cuts in carbon dioxide emissions prove too little or too late to avoid unacceptable climate damage."
If mitigation fails, either because we've underestimated the sensitivity of the climate system and/or because we've underestimated the technological and/or political challenges of reducing greenhouse-gas emissions, then we'll probably have to resort to some form of geoengineering. Clearly, this will then be a case where both mitigation and geoengineering are operating in tandem to avoid dangerous environmental change. This is what I see as a realistic scenario. We still have much to learn.