SAG

[Colin Forrest]

Comment on South Asian Summer Monsoon under Stratospheric Aerosol Intervention 

Dear Michael,  don't worry about these theoretical modelling papers. There are physical rstrictions on the height that aircraft can reach, and the amount of fossil fuel over time used in reaching these heights would leave a legacy of CO2 in the atmosphere at 2100 about one third of current emissions (draft papers attached) .

As the U-2 with a payload of ~2.5 tons is the only viable delivery method for current SAG modelling at these altitudes, deploying 20 Mt of SO2 precursor annually, for example, would require ~ 8 million flights per year, or ~22,000 flights per day. 

 The idea of delivering millions of tons of sulphate precursors over one or several points on the planet would result in excessive coagulation and deposition in that region (see Robock et al 2009) .

 The winds at these levels are light and variable, reverse direction unpredictably and would not distribute aerosol evenly (see examples of winds today 1/09/2024 and one year ago at 70mB and 10mB https://earth.nullschool.net If you access the site the aninimation will show the changes in wind strength and direction more clearly ) . The idea that the aerosol would be uplifted by the Brewer-Dobson circulation or spread evenly zonally from one or several injection points is not borne out by our current limited knowledge of circulation in the stratosphere.

 It would be impossible to monitor which levels of the stratosphere had sufficient concentration and which parts needed more aerosol added. some regions would get a excess aerosol and some areas would be missed due to the variability of the upper level winds.

All aircraft flying in the lower stratosphere, i.e. commercial, military and geoengineering aircraft, would be in a flying through a sulphuric acid atmosphere and this would incur damage to the planes and engines.  

There would be always be a large concentration gradient between the injection points and the areas distant from that point giving uneven cooling. The cooling effect will be less than theoretically modelled due to rapid coagulation and sedimentation in the areas with too much aerosol, and also less than theoretically modelled in the areas with have less than optimum aerosol concentrations.

Injection of SO2 would use up hydroxyl radicals which would otherwise oxidise methane (see Tilmes et al 2018)  It is worrying that although Tilmes et al recognised this in 2018, none of the Stratospheric Aerosol Geoengineering) (SAG) papers since then have discussed this issue.

Regards,  Colin