Alan Robock suggests that I must respond to the "totality" of his argument--that it's not enough to pick off his 20 theses one-by-one, as if the "totality" of his position is somehow more than the sum of its parts.
For example, he repeats the canard that “geoengineering won't stop ocean acidification." The list of things that climate engineering won't do is endless. Reducing non-carbon greenhouse gases and black carbon soot also won't stop ocean acidification, but that shouldn't stop us from reducing them. Carbon emissions and climate engineering are two different things. Let's fault carbon emissions for what they do and fault climate engineering for what it may do.
In my earlier post, I wrote: "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." Alan says that I'm wrong. But where exactly? Do we expect climate engineering at this level to destroy on average more than 3 percent of the ozone? Did chlorine not play an important role? Does the threat of ozone destruction increase with time?
Alan also asks us to agree "that if geoengineering is ever implemented, it must be geoengineering plus mitigation, and never geoengineering instead of mitigation." But what if we fail to enact effective mitigation? And what if there was substantial death and suffering as a consequence, and we thought the suffering could be alleviated, at least in part, by climate engineering? Shouldn't we then pursue it without mitigation?
While I'm in emotional agreement with Alan's statement, the rational side of me doesn't think it makes much sense. Some things sound nice, but may not be particularly reasonable when fully considered intellectually. Isn't the point that we wouldn't want to deploy climate engineering systems unless the benefits clearly outweigh the risks, where those risks include effects on both the physical climate system and sociopolitical systems?
Obviously, we'd like to avoid creating a world in which climate engineering is a necessity. Unfortunately, we may be heading down this path. Therefore, we have no choice but to carefully explore climate engineering's potential benefits and risks.
These benefits and risks can be safely explored using computer models and laboratory experiments. (If a decision is made that we need to proceed further, at some point these experiments would have to move outdoors.) These experiments should evaluate both intended and unintended environmental consequences of climate engineering--but we also need to investigate how we might go about constructing such systems, as it's the concrete proposals that will drive the environmental research.
Any real scientific research program will steer clear of value judgments and focus instead on the physical science and technology of climate engineering. Scientists have values, but science is about facts.