If it were to become apparent that dangerous changes in global climate were inevitable, despite greenhouse gas controls, active methods to cool the Earth on an emergency basis might be desirable. ... It seems feasible that it could be developed and deployed in ≈25 years at a cost of a few trillion dollars, <0.5% of world gross domestic product (GDP) over that time.
The "few trillion dollars" part is a problem. Not because it represents a particularly high cost scheme for neutralizing the warming part of AGW, but because it's so all-or-nothing. If you invest "only" $10 billion in this scheme the marginal abatement effects are going to be pretty close to zero, because so much of the cost is front-loaded into enabling technologies that will eventually deliver direct benefits.
I refer the interested reader to the classic The Political Economy of Very Large Space Projects:
I am not particularly convinced that this requires a few trillion dollars. Is there some hard evidence that sunshades are impossible to deploy with less than $10 billion?
The "few trillion dollars" part is a problem. Not because it represents a particularly high cost scheme for neutralizing the warming part of AGW, but because it's so all-or-nothing. If you invest "only" $10 billion in this scheme the marginal abatement effects are going to be pretty close to zero, because so much of the cost is front-loaded into enabling technologies that will eventually deliver direct benefits.
I refer the interested reader to the classic The Political Economy of Very Large Space Projects:
http://www.jetpress.org/volume4/space.htm
With minor adjustments, it could be generalized as The Political Economy of Very Large Projects.