RE: [geo] Climate system response to stratospheric sulfate aerosols:sensitivity to altitude of aerosol layer

[john gorman]

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
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  
Sent: Tuesday, December 17, 2019 4:36 AM
To: Govindasamy Bala 
Cc: geoengineering ; Douglas MacMartin 

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,  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  wrote:
Is 26k less good than 25? 

On Tue, 17 Dec 2019, 08:37 Govindasamy Bala,  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  wrote:
So what's your judgement on the ideal injection altitude?

Andrew 

On Mon, 16 Dec 2019, 10:36 Govindasamy Bala,  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  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,  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  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  On 
Behalf Of Govindasamy Bala
Sent: Saturday, December 14, 2019 9:38 PM
To: Andrew Lockley 
Cc: 

RE: [geo] Climate system response to stratospheric sulfate aerosols:sensitivity to altitude of aerosol layer

[Douglas MacMartin]

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 
Sent: Tuesday, December 17, 2019 9:39 AM
To: Douglas MacMartin ; Andrew Lockley 
; Govindasamy Bala 
Cc: geoengineering ; 
arcticmeth...@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


From: Douglas MacMartin
Sent: 17 December 2019 17:24
To: Andrew Lockley; Govindasamy 
Bala
Cc: geoengineering
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 mailto:andrew.lock...@gmail.com>>
Sent: Tuesday, December 17, 2019 4:36 AM
To: Govindasamy Bala mailto:bala@gmail.com>>
Cc: geoengineering 
mailto:geoengineering@googlegroups.com>>; 
Douglas MacMartin mailto:dgm...@cornell.edu>>
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, 
mailto:bala@gmail.com>> 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 
mailto:andrew.lock...@gmail.com>> wrote:
Is 26k less good than 25?

On Tue, 17 Dec 2019, 08:37 Govindasamy Bala, 
mailto:bala@gmail.com>> 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 
mailto:andrew.lock...@gmail.com>> wrote:
So what's your judgement on the ideal injection altitude?

Andrew

On Mon, 16 Dec 2019, 10:36 Govindasamy Bala, 
mailto:bala@gmail.com>> 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