2022 Montreal Protocol report's mixed assessment of SAI Impacts on Ozone Layer

[Ron Baiman]

Dear Colleagues,

Below are key points the 2022 Montreal Protocol report's ( https://ozone.unep.org/system/files/documents/Scientific-Assessment-of-Ozone-Depletion-2022-Executive-Summary.pdf ) mixed assessment of SAI impact on ozone layer.

The report focused on a spring injection of SAI in Antarctica (where the Ozone hole is largest) and found in model simulations, after 20 years of SAI sufficient to reduce global cooling by 0.5 C, loss of ozone in Antarctica in October similar to losses in the 1990s that if continued would delay ozone hole recovery by 25 to 50 years, but less loss if SAI is started later, and for larger applications enhancement of Ozone in the winter in NH mid-latitudes.

Details from the ES Chap. 5, p. 21-22 report below:

"Additional ozone depletion due to SAI is simulated in
spring over Antarctica, with magnitudes dependent on
the injection rate and timing. Simulations of strong SAI
show an increase in total column ozone (TCO) in mid-lat-
itudes (40–60°N) in the winter Northern Hemisphere.
º For October over Antarctica, SAI simulations that achieve
a global mean surface cooling of 0.5 °C in the first 20
years, show a reduction of TCO of around 58 ± 20 DU,
assuming 2020–2040 halogen conditions. This reduc-
tion brings TCO values close to the observed minimum in
the 1990s. Less ozone loss would be expected for a later
SAI start date, when halogen concentrations are project-
ed to be lower.
º Beyond the first 20 years, the continued application of
strong SAI, to offset almost 5 °C of warming by 2100, re-
duces Antarctic ozone in October by similar amounts (55
± 20 DU) throughout the 21st century despite declining
abundances of ozone-depleting substances (ODS). In
this case, ozone hole recovery from ODSs is delayed by
between 25 and 50 years. A peakshaving scenario po-
tentially leads to less ozone depletion.
º Under stronger SAI scenarios, ozone is significantly
enhanced in NH mid-latitudes in winter owing to strato-
spheric heating from injected sulfur, which leads to in-
creased equator to poleward transport of ozone.
º Ozone loss within the Arctic polar vortex has not yet
been robustly quantified for SAI."

Best,

Ron