Building such aircraft has never been done, and would require (i) design, (ii) test and certify, and (iii) build (e.g., for 20t payload need ~25 aircraft). The metric here refers to the start of a deployment intended to reduce climate impacts, rather than to cool by 0.5C. The uncertainty here focuses therefore on the timing to get to the first handful of aircraft rather than the build-out rate for a growing fleet (to avoid questions around scenario, and because build rate beyond initial fleet is not likely limiting factor for deployment magnitudes). While not directly associated with climate impacts, if this development is not possible in a timely manner high-latitude/low-altitude strategies might be used instead, which would change the pattern of expected climate impacts.
Metric
Time to develop, test, certify, and deliver a fleet capable of delivering ~1Mt/yr payload to 20km is over 10 years from first large-scale funding
Uncertainty
Smith (2024) suggests that timescale could be longer, but assumes certification for commercial airspace. Given that this constraint might not apply, conversationally multiple people think this would be low risk, but since there isn't more than paper studies (Bingaman et al shows a design, on paper), we categorize as medium.
Decision relevance
Degree of impact (and uncertainty) depend on how much slower than 10 yrs. Less consequential if initial deployment can be started with existing aircraft at lower altitude and higher latitude.