Thoughts on 50 km SAI paper

[Ron Baiman]

Dear Colleagues,

Some impressions (from a non-expert) on the 50 km SAI paper: 

“ This bit from the intro seemed to me to be the main takeaway: “ . To minimize the Antarctic ozone loss, it is essential that some sulfate aerosols from the intervention remain at high altitudes in the polar stratosphere. By doing so, high-altitude sulfate aerosols reduce NOx levels, slowing NOx-driven ozone loss and allowing ozone to accumulate in the middle stratosphere, which can offset the ozone loss caused by reactive halogen species in the lower stratosphere. In addition, sulfate aerosol concentrates in the mid-high latitudes rather than accumulating in the tropical lower stratosphere. This significantly reduces the tropical stratospheric heating when compared to lower-altitude SAI approaches. In addition, higher AOD in mid-high latitudes with SAI50 produces more surface cooling compared with SAI25 and consequently helps to preserve the sea ice at both Poles.”

It seems a key issue is how much additional GHG warming is caused by warming lower stratosphere leading to more stratospheric water vapor - don’t know if they try to quantify this in the paper. Also of course the difficulty of injecting at such a high altitude. 

One intriquing positive effect of lower altitude injection appears to be polar jets strengthening though this might be offset by mid-latitude jets weakening?: “ Furthermore, the tropical lower stratospheric warming caused by SAI strengthens the polar jets (Ferraro et al., 2015; Tilmes et al., 2009; Visioni et al., 2020). It also weakens the subtropical jets (Ferraro et al., 2015; Tilmes et al., 2018a), subsequently shifting weather patterns including the precipitation belts, dry zones and storm tracks.”.

Again don’t know if these impacts are quantified.

In summary the paper seems an interesting and potentially important modeling exercise but may not have much  near- term practicality.

Best,

Ron 

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