The analysis of costs and benefits associated with geoengineering was provided by J Eric Bickel and Lee Lane (which below I refer to as BL09), and it can be found here in PDF. Bickel and Lane find that (p. 49):
The direct B/C ratio for stratospheric aerosol injection is on the order of 25 to 1, while the B/C ratio for marine cloud whitening is around 5000 to 1.These numbers are huge -- except I don' t think that they actually believe these numbers. If the benefits of SRM actually exceeded the costs in this manner then the obvious recommendation would be to deploy these technologies immediately. The authors make no such recommendation, and instead offer a cautious recommendation of more research. I agree that more research is worth conducting, but I don't believe the cost-benefit numbers for a second. Obviously support for more research on geoengineering has nothing to do with fanciful cost-benefit numbers.
Here is how I introduce my paper (PDF):
This response paper to BL09 proceeds in two parts. Part I offers a critique of the cost-benefit analysis methodology of BL09, arguing that the analysis is, at best, arbitrary, and more critically, not grounded in a realistic set of assumptions about how the global earth systemIn contrast to SRM, the direct air capture of carbon dioxide from the atmosphere lends itself to more practically useful cost-benefit analyses. My bottom line is that the geoengineering of the earth system as a way to adapt to increasing concentrations of greenhouse gases is a losing proposition.
actually works. I argue that the present understandings of the potential effects of climate engineering are not sufficiently well developed to allow for any meaningful cost-benefit analyses. Nonetheless, I agree with BL09 when they conclude that there is value in further research on climate engineering technologies. My judgment, as apparently was the case as well for the conclusions of BL09, is not based on numbers that result from precise-looking cost-benefit analyses, but rather, on the fact that our understandings are so poor. I further argue that developing informed understandings will require adopting a more scientifically realistic perspective on the role of climate engineering in the global earth system than is reflected in the simplifications presented in BL09. I conclude that the quantitative cost-benefit analysis of BL09 is guilty of precision without accuracy.
Part II of the paper summarizes an analysis of the potential role for air capture technologies to play in the decarbonization of the global economy. BL09 consider air capture only briefly, leaving a more detailed analysis to this response paper. I show that the costs of air capture are comparable to the costs of conventional mitigation, as presented by the Intergovernmental Panel on Climate Change (IPCC) in its 2007 assessment report, as well as the widely cited Stern Review Report by the government of the United Kingdom. Based on this conclusion I argue that air capture deserves to receive a similar close scrutiny as other mitigation policies.
The paper concludes by considering more general criteria for evaluating technological fixes such as technologies of climate engineering. I suggest that stratospheric aerosol injection and marine cloud whitening comprehensively fail these broader criteria whereas air capture does not.
Bjorn, are you listening?