Effect on the ozone layer will mostly be a function of the aerosol concentration (or rather, surface area), so scales similarly to the amount of reflected sunlight. So injecting at lower altitude means (i) we need to put more in to get the same concentration, and (ii) we need a higher concentration to get the same cooling because we have to compensate for the higher stratospheric water vapour. So if your goal is to maximize the ratio of cooling to ozone loss, we should probably still be injecting at higher altitude.
(More complicated than that due to transport, so it is true that you could inject at a low enough altitude that the aerosols never make it to high latitudes (and don’t need to go that low; 16km is way too low to do anything useful in the tropics since it’s below the tropopause) then there’s less impact on ozone, and, if you inject enough, you could still get some tropical cooling, but then you screw up meridional temperature gradients. And need a lot of injection.) More broad answer is that yes, this is a more complicated optimization of different metrics, but for altitude they’re mostly going to point in the same direction. (Also, IMHO, ozone loss has been way over-emphasized in terms of downsides… a big problem if you did a lot of cooling today, but probably not a big problem if you do a moderate amount of cooling in 20 years.) doug From: john gorman <[email protected]> Sent: Tuesday, December 17, 2019 9:39 AM To: Douglas MacMartin <[email protected]>; Andrew Lockley <[email protected]>; Govindasamy Bala <[email protected]> Cc: geoengineering <[email protected]>; [email protected] Subject: RE: [geo] Climate system response to stratospheric sulfate aerosols:sensitivity to altitude of aerosol layer I am surprised that this conversation has not mentioned the negative effect on the ozone layer. This would seem to be a reason for injecting at lower altitude eg 16 km or 55,000feet. I see the fairly minor increase in quantity needed at lower altitude to be a reason for injecting at lower altitude considering the massive extra difficulty in injecting at the higher altitudes. John gorman From: Douglas MacMartin<mailto:[email protected]> Sent: 17 December 2019 17:24 To: Andrew Lockley<mailto:[email protected]>; Govindasamy Bala<mailto:[email protected]> Cc: geoengineering<mailto:[email protected]> Subject: RE: [geo] Climate system response to stratospheric sulfate aerosols:sensitivity to altitude of aerosol layer I think it is clear that we don’t know that yet. If you want my guess, it would be the same as Bala’s, that once you’re far enough from the tropopause there’s not that much benefit to going higher. The answer will also depend on the latitude of injection. One of a long list of questions that, if there were any appreciable funding, would not be fundamentally hard to answer. From: Andrew Lockley <[email protected]<mailto:[email protected]>> Sent: Tuesday, December 17, 2019 4:36 AM To: Govindasamy Bala <[email protected]<mailto:[email protected]>> Cc: geoengineering <[email protected]<mailto:[email protected]>>; Douglas MacMartin <[email protected]<mailto:[email protected]>> Subject: Re: [geo] Climate system response to stratospheric sulfate aerosols: sensitivity to altitude of aerosol layer Considering all effects, what's your view on the ideal height? Andrew On Tue, 17 Dec 2019, 08:47 Govindasamy Bala, <[email protected]<mailto:[email protected]>> wrote: 26 km is probably not going to add any more benefit compared 25 km if you consider the effect identified in our paper but it is better when sedimentation effect is considered. More experiments with the NCAR WACCM model would be good to precisely nail this down. On Tue, Dec 17, 2019 at 2:10 PM Andrew Lockley <[email protected]<mailto:[email protected]>> wrote: Is 26k less good than 25? On Tue, 17 Dec 2019, 08:37 Govindasamy Bala, <[email protected]<mailto:[email protected]>> wrote: Andrew, Sedimentation effect works in the same direction as the effect we identified in our study. Therefore, higher the altitude of injection, the better. My judgement: 25 km would be good. On Mon, Dec 16, 2019 at 8:54 PM Andrew Lockley <[email protected]<mailto:[email protected]>> wrote: So what's your judgement on the ideal injection altitude? Andrew On Mon, 16 Dec 2019, 10:36 Govindasamy Bala, <[email protected]<mailto:[email protected]>> wrote: Andrew, Many modeling groups (e.g. Tilmes and others) have already performed simulations that inject aerosols at different heights and thus have included the sedimentation effects and many many other effects. These studies simulate the NET effects and hence hard to interpret and quantify the individual effects. The strength of our ESD paper is that it changes only one variable and identifies its individual contribution to the total problem. What we have learnt during the course is that there are too many variables in the aerosol SRM problem (transport, location of injection, aerosol-cloud interaction, aerosol-radiation interaction, aerosol micro physics and the resulting size distribution of the aerosols, etc.) and the resulting uncertainties could be too large. This is of course known to many of us for a long time...... On Mon, Dec 16, 2019 at 3:41 PM Andrew Lockley <[email protected]<mailto:[email protected]>> wrote: If I understand from the email below , you used aerosols with no fall speed. Are experiments planned to simulate aerosol descent? Andrew On Mon, 16 Dec 2019, 05:43 Govindasamy Bala, <[email protected]<mailto:[email protected]>> wrote: Andrews, We did not do experiments with aerosols above 22 km. It is likely that the cooling effect will be larger when aerosols are at 25 km. Beyond that it is likely that the additional cooling benefits disappear. We need more experiments to confirm this. The sensitivity to height in our paper arises mainly because of the increases in stratospheric water vapor (which partly offsets the cooling efficiency of the aerosols) that is associated with the stratospheric heating by the aerosols. This increase in stratospheric water vapor is largest when the aerosols (and the heating) is close to the tropopause. In our paper, we have isolated the effect of just one factor. As Doug has pointed out, the sedimentation effect would also lead to more cooling if aerosols are injected at higher altitudes... Best, Bala On Sun, Dec 15, 2019 at 9:05 PM Douglas MacMartin <[email protected]<mailto:[email protected]>> wrote: This is a great study to understand the effectiveness per unit mass *in the stratosphere*. Also keep in mind that there’s an additional factor, that at lower altitudes it takes higher injection rates to achieve the same burden in the stratosphere (i.e., lower lifetime at lower injected altitude). If the only thing you cared about was cost, then since there are existing studies demonstrating that you can design an aircraft to get to ~20-21km, we roughly know that it could be done, but higher altitude injection means less total sulfur injected and hence smaller side effects, and should be better understood both on the modeling and implementation cost as the trade may well be worth it. doug From: [email protected]<mailto:[email protected]> <[email protected]<mailto:[email protected]>> On Behalf Of Govindasamy Bala Sent: Saturday, December 14, 2019 9:38 PM To: Andrew Lockley <[email protected]<mailto:[email protected]>> Cc: geoengineering <[email protected]<mailto:[email protected]>> Subject: Re: [geo] Climate system response to stratospheric sulfate aerosols: sensitivity to altitude of aerosol layer Dear Andrew, Thanks for the posting. The heights studied were 16, 19 and 22 km, height that are relevant to solar radiation modification problem.. The final paragraph in the paper is worth reading to get more quantitative information from this modeling study. "To summarize, for the same mass, the efficiency (defined as changes in surface temperature per Tg S) of volcanic aerosol is less when it is prescribed at lower altitudes in the stratosphere (Fig. 9). For example, in our simulations, there is a surface cooling of 0.44K for each teragram of sulfur placed in the stratosphere at about 16 km altitude (100 hPa). There is an additional surface cooling of 0.15K per Tg S when the prescribed altitude is increased from about 16 km to about 22 km (37 hPa)." On Sat, Dec 14, 2019 at 12:55 AM Andrew Lockley <[email protected]<mailto:[email protected]>> wrote: Poster's note : this has significant implications for the engineering of delivery systems. I can't do the pressure altitude conversion in my head, but it's a lot higher than what's generally been planned for. We're gonna need a bigger boat. https://www.earth-syst-dynam.net/10/885/2019/ Climate system response to stratospheric sulfate aerosols: sensitivity to altitude of aerosol layer Krishna-Pillai Sukumara-Pillai Krishnamohan et al. Received: 01 May 2019 – Discussion started: 23 May 2019 – Revised: 24 Oct 2019 – Accepted: 08 Nov 2019 – Published: 13 Dec 2019 Abstract top<https://www.earth-syst-dynam.net/10/885/2019/#top> Reduction of surface temperatures of the planet by injecting sulfate aerosols in the stratosphere has been suggested as an option to reduce the amount of human-induced climate warming. Several previous studies have shown that for a specified amount of injection, aerosols injected at a higher altitude in the stratosphere would produce more cooling because aerosol sedimentation would take longer. In this study, we isolate and assess the sensitivity of stratospheric aerosol radiative forcing and the resulting climate change to the altitude of the aerosol layer. We study this by prescribing a specified amount of sulfate aerosols, of a size typical of what is produced by volcanoes, distributed uniformly at different levels in the stratosphere. We find that stratospheric sulfate aerosols are more effective in cooling climate when they reside higher in the stratosphere. We explain this sensitivity in terms of effective radiative forcing: volcanic aerosols heat the stratospheric layers where they reside, altering stratospheric water vapor content, tropospheric stability, and clouds, and consequently the effective radiative forcing. We show that the magnitude of the effective radiative forcing is larger when aerosols are prescribed at higher altitudes and the differences in radiative forcing due to fast adjustment processes can account for a substantial part of the dependence of the amount of cooling on aerosol altitude. These altitude effects would be additional to dependences on aerosol microphysics, transport, and sedimentation, which are outside the scope of this study. The cooling effectiveness of stratospheric sulfate aerosols likely increases with the altitude of the aerosol layer both because aerosols higher in the stratosphere have larger effective radiative forcing and because they have higher stratospheric residence time; these two effects are likely to be of comparable importance. -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]<mailto:[email protected]>. To view this discussion on the web visit https://groups.google.com/d/msgid/geoengineering/CAJ3C-04wbNfg0E3q_8GtwXay88n_2r%2BhzYfVfrNPjq9SpJd9pg%40mail.gmail.com<https://groups.google.com/d/msgid/geoengineering/CAJ3C-04wbNfg0E3q_8GtwXay88n_2r%2BhzYfVfrNPjq9SpJd9pg%40mail.gmail.com?utm_medium=email&utm_source=footer>. -- With Best Wishes, ------------------------------------------------------------------- G. Bala Professor Center for Atmospheric and Oceanic Sciences Indian Institute of Science Bangalore - 560 012 India Tel: +91 80 2293 3428; +91 80 2293 2505 Fax: +91 80 2360 0865; +91 80 2293 3425 Email: [email protected]<mailto:[email protected]>; bala.gov<http://bala.gov>@gmail.com<http://gmail.com> Web:http://dccc.iisc.ac.in/dr_govindasamy_bala_profile.html ------------------------------------------------------------------- -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]<mailto:[email protected]>. To view this discussion on the web visit https://groups.google.com/d/msgid/geoengineering/CAD7fhV%3Dc5Q4XVod8rAide3VNOmN1uyPbp6B6TCRKij474F_Meg%40mail.gmail.com<https://groups.google.com/d/msgid/geoengineering/CAD7fhV%3Dc5Q4XVod8rAide3VNOmN1uyPbp6B6TCRKij474F_Meg%40mail.gmail.com?utm_medium=email&utm_source=footer>. -- With Best Wishes, ------------------------------------------------------------------- G. Bala Professor Center for Atmospheric and Oceanic Sciences Indian Institute of Science Bangalore - 560 012 India Tel: +91 80 2293 3428; +91 80 2293 2505 Fax: +91 80 2360 0865; +91 80 2293 3425 Email: [email protected]<mailto:[email protected]>; bala.gov<http://bala.gov>@gmail.com<http://gmail.com> Web:http://dccc.iisc.ac.in/dr_govindasamy_bala_profile.html ------------------------------------------------------------------- -- With Best Wishes, ------------------------------------------------------------------- G. Bala Professor Center for Atmospheric and Oceanic Sciences Indian Institute of Science Bangalore - 560 012 India Tel: +91 80 2293 3428; +91 80 2293 2505 Fax: +91 80 2360 0865; +91 80 2293 3425 Email: [email protected]<mailto:[email protected]>; bala.gov<http://bala.gov>@gmail.com<http://gmail.com> Web:http://dccc.iisc.ac.in/dr_govindasamy_bala_profile.html ------------------------------------------------------------------- -- With Best Wishes, ------------------------------------------------------------------- G. Bala Professor Center for Atmospheric and Oceanic Sciences Indian Institute of Science Bangalore - 560 012 India Tel: +91 80 2293 3428; +91 80 2293 2505 Fax: +91 80 2360 0865; +91 80 2293 3425 Email: [email protected]<mailto:[email protected]>; bala.gov<http://bala.gov>@gmail.com<http://gmail.com> Web:http://dccc.iisc.ac.in/dr_govindasamy_bala_profile.html ------------------------------------------------------------------- -- With Best Wishes, ------------------------------------------------------------------- G. Bala Professor Center for Atmospheric and Oceanic Sciences Indian Institute of Science Bangalore - 560 012 India Tel: +91 80 2293 3428; +91 80 2293 2505 Fax: +91 80 2360 0865; +91 80 2293 3425 Email: [email protected]<mailto:[email protected]>; bala.gov<http://bala.gov>@gmail.com<http://gmail.com> Web:http://dccc.iisc.ac.in/dr_govindasamy_bala_profile.html ------------------------------------------------------------------- -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]<mailto:[email protected]>. 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