We review the capabilities and costs of various lofting methods intended to deliver sulfates into the lower stratosphere. We lay out a future solar geoengineering deployment scenario of halving the increase in anthropogenic radiative forcing beginning 15 years hence, by deploying material to altitudes as high as ∼20 km. After surveying an exhaustive list of potential deployment techniques, we settle upon an aircraft-based delivery system. Unlike the one prior comprehensive study on the topic (McClellan et al 2012 Environ. Res. Lett. 7 034019), we conclude that no existing aircraft design-even with extensive modifications-can reasonably fulfill this mission. However, we also conclude that developing a new, purpose-built high-altitude tanker with substantial payload capabilities would neither be technologically difficult nor prohibitively expensive. We calculate early-year costs of ∼$1500 ton −1 of material deployed, resulting in average costs of ∼$2.25 billion yr −1 over the first 15 years of deployment. We further calculate the number of flights at ∼4000 in year one, linearly increasing by ∼4000 yr −1 . We conclude by arguing that, while cheap, such an aircraft-based program would unlikely be a secret, given the need for thousands of flights annually by airliner-sized aircraft operating from an international array of bases.

This paper presents the estimated direct costs of a stratospheric aerosol injection (SAI) program through the end of this century. It displays a range of future solar geoengineering deployment scenarios that are intended to reduce anthropogenically-caused radiative forcing beginning in 2035. The scenarios reviewed herein include three commonly modeled representative concentration pathways (4.5, 6.0, and 8.5) and three possible radiative forcing targets (halving future warming, halting warming, and reversing temperatures to 2020 levels). The program relies on three successive generations of newly designed high-altitude tanker aircraft to deliver aerosols to an altitude of ∼20 km. Sulfates are assumed to be the aerosol used in conjunction with the first generation tanker, supplanted by an as-yet-determined ‘Aerosol 2’ with the later generation aircraft. The aggregate cost over the remainder of the 21st century and the annual cost in 2100 both vary by an order of magnitude between the cheapest and the most expensive scenarios. However, the cost-per-ton of deployed aerosol varies little among scenarios and the cost-per-degree-of-warming-avoided is similarly consistent. Relative to other climate interventions and solutions, SAI remains inexpensive, but at about $18 billion yr−1 per degree Celsius of warming avoided (in 2020 USD), a solar geoengineering program with substantial climate impact would lie well beyond the financial reach of individuals, small states, or other non-state potential rogue actors and would instead be the exclusive domain of large national economies or coalitions including at least one such economy.

What are the benefits and drawbacks, and for whom?