here's some of the details on the decidedly mixed Montreal Protocol report on SAI (sent 5/17 to the usual lists):
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."
Is
anyone aware of any other risks that could rule out cautiously piloting
SAI (for example in Polar regions per "the Cornell School" led by
Doug)?
Let
me add that my understanding is that one of the problems with basing
everything on modeling and limited lab and atmospheric testing is that
atmospheric systems are so complex that it would be impossible to
definitively (at a 95% confidence level) rule out all potential adverse
unintended consequences. The best way to "test" appears to be thus to
conduct limited pilots that gradually scale up and adjust and modify as
more real time data is obtained. We are after all talking about
mimicking (in a more gradual and deliberative way) the millions of tons
of sulfur aerosol that has been injected into the stratosphere by
volcanoes naturally for as long as we've been on this planet and much
longer! This is not after all "nuclear radiation" but "natural" sulfur
aerosols! This is not to say of course that modeling and other research
should not continue, it's just the idea that purely researching without
testing deployment for another 10 years or so (as David Keith opined in
HPAC discussion) before then potentially going "all in" for deployment
if global conditions become very extreme is a good strategy?
I
of course defer to Doug, Michael MacCracken and others are much more
knowledgeable about all this than myself - but per Mike M's view (I
believe - if you see this Mike please correct me if I'm
misinterpreting!) it is a terrible mistake for policy to determined
based on "scientific criteria" (like less than 5% risk). Rather, policy
should be based on reducing catastrophic risk (even if it's low
possibility) like crossing tipping points that we are much greater risk
of crossing as we warm above 1.5 and 2.0 etc. Doug, I don't know if
you're of the same view, but I certainly agree with everything in your
most recent post on this thread (that I just saw!).
Needless to say, I welcome further evidence and discussion on any of these points!
Best,
Ron