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.
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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
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this is a nice study. Have you tried to use radiative kernels
to decompose the ERF (alike what we did in Boucher et al, GRL
2017, https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017GL074647)
? We also observed a compensation effect between the
stratospheric heating and the stratospheric water vapour, but
the latter wasn't so large, and we didn't inject very high.
Best regards,
Olivier
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Hi All
If higher altitudes
give longer life and so lower spray quantities they also give a
slower frequency response, longer phase lag and a greater chance
of being stuck with what you do not want such as reflecting
energy back down during Arctic winters as pointed out by
Jan-Egil Kristjansson.
Advocates for marine cloud brightening are pleased about the short life of spray to give a high frequency response and the ability to stop Arctic cooling in the autumn before, it too, works the wrong way.
Stephen
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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.
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 <gor...@waitrose.com>
Sent: Tuesday, December 17, 2019 9:39 AM
To: Douglas MacMartin <dgm...@cornell.edu>; Andrew Lockley <andrew....@gmail.com>; Govindasamy Bala <bala...@gmail.com>
Cc: geoengineering <geoengi...@googlegroups.com>; arctic...@googlegroups.com
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
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I disagree with you about the “equator” part, I think we *know* that that is not the right answer.
Might depend somewhat on whether your goal is purely to maximize global mean cooling, or to compensate for climate change. Injecting some in each hemisphere but away from the equator gives a better tailoring of climate response to avoid the tropical overcooling and high-latitude undercooling that you get with equatorial injection, and at least in WACCM, for higher injection rates, is *more* effective per unit injection than equatorial injection because you aren’t constantly injecting into region of high aerosol concentrations in the tropical pipe, with associated increased coagulation rates. So better from either perspective in that model. (Oh, and then you also don’t mess with the QBO.) See: Kravitz, B., D.G. MacMartin, S. Tilmes, J.H. Richter, M.J. Mills, W. Cheng, K. Dagon, A.S. Glanville, J.-F. Lamarque, I.R. Simpson, J.J. Tribbia, and F. Vitt, “Comparing surface and stratospheric impacts of geoengineering with different SO2 injection strategies”, J. Geophysical Research A, 124, 2019. doi:10.1029/2019JD030329
So I actually think we do know the answer that you should not inject at the equator.
(Or, rather, if you were only allowed to pick one latitude than the equator would be better than any other single latitude, but if you are willing to simultaneously inject into both hemispheres then you both require less injection and get better climate response.)
(As for altitude, we also know that injecting too high starts decreasing effectiveness, at least for sulfate, since it evaporates…)
From: Haywood, James <J.M.H...@exeter.ac.uk>
Sent: Tuesday, December 17, 2019 9:47 AM
To: Douglas MacMartin <dgm...@cornell.edu>; Andrew Lockley <andrew....@gmail.com>; Govindasamy Bala <bala...@gmail.com>
Cc: geoengineering <geoengi...@googlegroups.com>
Subject: Re: [geo] Climate system response to stratospheric sulfate aerosols: sensitivity to altitude of aerosol layer
Hi All,
Actually I do think we know this. The answer is high up and at the equator.
This maximises the AOD and the global cooling without having the nasties associated with cooling one hemisphere with respect to the other (e.g. Sahelian drought and impacts on North Atlantic hurricanes). Figure 1 clearly shows the AOD being the most significant and the AOD shifting into both hemispheres.
Results look pretty similar with a more sophisticated (GLOMAP) scheme.
Best
Jim
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Agreed!
From: Haywood, James <J.M.H...@exeter.ac.uk>
Sent: Tuesday, December 17, 2019 10:13 AM
To: Douglas MacMartin <dgm...@cornell.edu>; Andrew Lockley <andrew....@gmail.com>; Govindasamy Bala <bala...@gmail.com>
Cc: geoengineering <geoengi...@googlegroups.com>
Subject: Re: [geo] Climate system response to stratospheric sulfate aerosols: sensitivity to altitude of aerosol layer
Hi All,
Agree with much of this, but if you had one point to choose and your aim was solely to cool the planet, it would be close to the equator and high up (a la Pinatubo). Of course, there are tailoring arguments as you mention, and I don't doubt that a more nuanced injection strategy would be more optimal.
Best
Jim
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On Dec 17, 2019, at 1:01 PM, Douglas MacMartin <dgm...@cornell.edu> wrote:
I disagree with you about the “equator” part, I think we *know* that that is not the right answer.Might depend somewhat on whether your goal is purely to maximize global mean cooling, or to compensate for climate change. Injecting some in each hemisphere but away from the equator gives a better tailoring of climate response to avoid the tropical overcooling and high-latitude undercooling that you get with equatorial injection, and at least in WACCM, for higher injection rates, is *more* effective per unit injection than equatorial injection because you aren’t constantly injecting into region of high aerosol concentrations in the tropical pipe, with associated increased coagulation rates. So better from either perspective in that model. (Oh, and then you also don’t mess with the QBO.) See: Kravitz, B., D.G. MacMartin, S. Tilmes, J.H. Richter, M.J. Mills, W. Cheng, K. Dagon, A.S. Glanville, J.-F. Lamarque, I.R. Simpson, J.J. Tribbia, and F. Vitt, “Comparing surface and stratospheric impacts of geoengineering with different SO2 injection strategies”, J. Geophysical Research A, 124, 2019. doi:10.1029/2019JD030329So I actually think we do know the answer that you should not inject at the equator.(Or, rather, if you were only allowed to pick one latitude than the equator would be better than any other single latitude, but if you are willing to simultaneously inject into both hemispheres then you both require less injection and get better climate response.)(As for altitude, we also know that injecting too high starts decreasing effectiveness, at least for sulfate, since it evaporates…)From: Haywood, James <J.M.H...@exeter.ac.uk>
Sent: Tuesday, December 17, 2019 9:47 AM
To: Douglas MacMartin <dgm...@cornell.edu>; Andrew Lockley <andrew....@gmail.com>; Govindasamy Bala <bala...@gmail.com>
Cc: geoengineering <geoengi...@googlegroups.com>
Subject: Re: [geo] Climate system response to stratospheric sulfate aerosols: sensitivity to altitude of aerosol layer
Hi All,Actually I do think we know this. The answer is high up and at the equator.This maximises the AOD and the global cooling without having the nasties associated with cooling one hemisphere with respect to the other (e.g. Sahelian drought and impacts on North Atlantic hurricanes). Figure 1 clearly shows the AOD being the most significant and the AOD shifting into both hemispheres.
<image001.png>
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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
Sent: 17 December 2019 17:24
To: Andrew Lockley; Govindasamy Bala
Cc: geoengineering
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Thanks for these graphs etc. When the world suddenly realises that this is necessary, (and which disaster/flood /fire triggers this) lots of decisions will suddenly have to be made. My guess is that we will have to start with existing aircraft ie lower injection. Too much time will have been lost to go for any form of ideal.
I will work through your graphs in more detail.
Thanks
Joh gorman
From: Simone Tilmes
Sent: 18 December 2019 14:20
To: gor...@waitrose.com
Cc: dgm...@cornell.edu; Andrew Lockley; Govindasamy Bala; geoengineering; arctic...@googlegroups.com
Subject: Re: [geo] Climate system response to stratospheric sulfateaerosols:sensitivity to altitude of aerosol layer
Hi John,
just to your comment, injecting in 16km in the tropics is not in the stratosphere, so that will not be efficient, but injecting outside the tropics would very likely require a much larger injection amount. Using 20km vs 25km altitudes is leading to a 50% increase in injection rate (see a Figure from a paper I am writing, showing SO4 burden per year (top) and SO4 burden vs injection rate, comparing GLENS: injections at 25km in 30N,15N, 30S, 15S, Low: injections at 20km in 30N,15N, 30S, 15S, EQ: injections at the equator, 110L: 25km injections with a model that has a higher vertical resolution),
Cheers, Simone
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