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 <dgm...@cornell.edu> 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:* geoengineering@googlegroups.com <geoengineering@googlegroups.com> *On > Behalf Of *Govindasamy Bala > *Sent:* Saturday, December 14, 2019 9:38 PM > *To:* Andrew Lockley <andrew.lock...@gmail.com> > *Cc:* geoengineering <geoengineering@googlegroups.com> > *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 <andrew.lock...@gmail.com> > 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 geoengineering+unsubscr...@googlegroups.com. > 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: gb...@iisc.ac.in; bala....@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 geoengineering+unsubscr...@googlegroups.com. > 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: gb...@iisc.ac.in; bala....@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 geoengineering+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/geoengineering/CAD7fhVnfGtvCzuWyf4j7v7jvpKUmMgvN_mvMYOCnDnr4R4AKKQ%40mail.gmail.com.
