RE: [geo] Solar geoengineering as part of an overall strategy for meeting the 1.5°C Paris target

[Douglas MacMartin]

Hi Stephen,

The first number I found when I re-googled this was 13%, Table 1 of 
https://journals.ametsoc.org/doi/full/10.1175/2011JCLI3972.1.  But regardless, 
since the statistics are not uniform across the ocean, the patchiness doesn’t 
average out, and I think it is fair to say that the radiative forcing from MCB 
will be more spatially heterogeneous than that from SAI, and hence there is 
more potential for the response to also be.  If the goal is to keep the climate 
as close as possible to the climate one would have had with the same global 
mean temperature but lower CO2 (certainly a plausible goal, not the only 
plausible goal), then not obvious without actually conducting research which 
method leads to better compensation; presumably a combination of MCB and SAI 
would do better than either alone (obviously).  I think we would both agree 
that it would be nice to have research so that we can start answering questions 
like this instead of just guessing.  

Agree completely that there are many more advantages and disadvantages than 
could possibly be listed in a table (we were considerably over-length as it 
was, so I wound up having to condense the table, only listing one key advantage 
or disadvantage relative to stratospheric sulfate). We were trying to keep this 
high-level and not get in to a lengthy discussion of pros and cons that is, 
unfortunately, almost entirely hypothetical at this point.  

Two major advantages of MCB that you didn’t list are:

1)  More likely to be societally acceptable (this is related to your first 
point, but different in that the perceived risk might be much more important 
than any actual risk), and

2)  More testable; as an engineer I would view that as a pretty major plus. 
 (That is, for MCB you can do a full-scale radiative forcing over a small area, 
whereas for SAI the time constants involved mean that any test that is 
“full-scale” in radiative forcing is also global, and hence not acceptable 
pre-deployment; this has implications for research strategy (paper currently 
being drafted so don’t expect to see it soon).)

Insofar as both techniques would lead to less of a temperature drop in the 
event of a volcanic eruption than would have happened absent any deployment, I 
don’t really think that’s a big advantage compared to the open question about 
how well either technique can compensate for climate change.  I also doubt that 
the fossil fuel usage to bring material to the stratosphere is a significant 
factor in choosing between them (assuming we ever are in a position to choose 
between them).  

 

doug

From: geoengineering@googlegroups.com [mailto:geoengineering@googlegroups.com] 
On Behalf Of Stephen Salter
Sent: Tuesday, April 03, 2018 9:46 AM
To: geoengineering@googlegroups.com
Subject: Re: [geo] Solar geoengineering as part of an overall strategy for 
meeting the 1.5°C Paris target

 

 

Hi All

In the recent Phil. Trans. Roy. Soc. special issue on Solar Geoengineering to 
meet Paris the Paris target  at
 
http://rsta.royalsocietypublishing.org/content/376/2119/20160454  table 1 
lists, as a disadvantage relative to stratospheric sulphur,  that marine 
stratospheric clouds cover 10% of the Earth which means that marine cloud 
brightening is ‘patchy’.  No reference to this number is given and it 
contradicts the 18% ‘low not high clouds’ mentioned by Charlson and Lovelock in 
their 1987 paper about the CLAW hypothesis and the effects of dimethyl-sulphide 
from phytoplankton.   There is also the Jones Hayward Boucher paper of 2009 
which concluded that marine cloud brightening over the best 3% of the oceans 
would offset about half the thermal damage since preindustrial times. 

However as clouds move the patchiness is smoothed out.  Furthermore the life of 
condensation nuclei will be approximately half the mean time between rain 
showers so clear skies mean longer nuclei lifetimes.  The Twomey effect is 
logarithmic so it is better to have half the dose over double the area and 
spraying under clear skies gives nuclei a chance to spread.  I argue that some 
short-term patchiness is much less serious if the patches are our patches.  
Finally the recent paper by Ahlm et al.  at doi:10.5194/aco-2107-484 suggests 
that marine cloud brightening works much better than I would have expected with 
no clouds.

The Royal Society table did not have any space for the disadvantages of 
stratospheric sulphur relative to marine cloud brightening and I am reluctant 
to knock any technology about which I am not an expert.  However if I was 
forced to suggest entries for the contents of a disadvantage table they would 
be as follows:

Nasty acid everywhere compared with medicinally beneficial salt mainly over sea.

Very little control over the areas affected.

Very long shut-down times in the event of a volcanic eruption leading to 
over-cooling.

Reflection of outgoing 

Re: [geo] Solar geoengineering as part of an overall strategy for meeting the 1.5°C Paris target

[Stephen Salter]

Hi All

In the recent Phil. Trans. Roy. Soc. special issue on Solar 
Geoengineering to meet Paris the Paris target at 
http://rsta.royalsocietypublishing.org/content/376/2119/20160454table 1 
lists, as a disadvantage relative to stratospheric sulphur, that marine 
stratospheric clouds cover 10% of the Earth which means that marine 
cloud brightening is ‘patchy’.No reference to this number is given and 
it contradicts the 18% ‘low not high clouds’ mentioned by Charlson and 
Lovelock in their 1987 paper about the CLAW hypothesis and the effects 
of dimethyl-sulphide from phytoplankton.There is also the Jones Hayward 
Boucher paper of 2009 which concluded that marine cloud brightening over 
the best 3% of the oceans would offset about half the thermal damage 
since preindustrial times.


However as clouds move the patchiness is smoothed out. Furthermore the 
life of condensation nuclei will be approximately half the mean time 
between rain showers so clear skies mean longer nuclei lifetimes.The 
Twomey effect is logarithmic so it is better to have half the dose over 
double the area and spraying under clear skies gives nuclei a chance to 
spread. I argue that some short-term patchiness is much less serious if 
the patches are */our/* patches. Finally the recent paper by Ahlm et 
al.at doi:10.5194/aco-2107-484 suggests that marine cloud brightening 
works much better than I would have expected with no clouds.


The Royal Society table did not have any space for the disadvantages of 
stratospheric sulphur relative to marine cloud brightening and I am 
reluctant to knock any technology about which I am not an expert.However 
if I was forced to suggest entries for the contents of a disadvantage 
table they would be as follows:


Nasty acid everywhere compared with medicinally beneficial salt mainly 
over sea.


Very little control over the areas affected.

Very long shut-down times in the event of a volcanic eruption leading to 
over-cooling.


Reflection of outgoing infra-red during polar winters leading to warming.

Use of fossil fuel from aircraft release rather than the use of energy 
from the wind.


Even though recent climate modelling has not yet used monodisperse spray 
with sizes of both liquid drops and dry salt residues both in the 
Greenfield gap and has not varied spray patterns with seasons or surface 
temperature anomalies, the majority of ensemble results show that as 
well as cooling there is a trend for dry places to become wetter and wet 
ones to become drier.We can hope that we can learn how to improve on 
this trend.


I suggest that the choice of time and place to do marine cloud 
brightening will confer advantages similar to the use of an accelerator, 
steering and brakes on road vehicles.







On 02-Apr-18 9:16 PM, Andrew Lockley wrote:


https://keith.seas.harvard.edu/publications/solar-geoengineering-part-overall-strategy-meeting-15%C2%B0c-paris-target 




  Solar geoengineering as part of an overall strategy for meeting the
  1.5°C Paris target


  Citation:

Douglas G. MacMartin, Katharine L. Ricke, and David W. Keith. 
4/2/2018. “Solar geoengineering as part of an overall strategy for 
meeting the 1.5°C Paris target 
.” 
Philosophical Transactions of the Royal Society, 376, 2119.
Download Citation 


Download
	macmartin_ricke_keith_ptrs.pdf 
 
	1.03 MB



  Abstract:

Solar geoengineering refers to deliberately reducing net radiative 
forcing by reflecting some sunlight back to space, in order to reduce 
anthropogenic climate changes; a possible such approach would be 
adding aerosols to the stratosphere. If future mitigation proves 
insufficient to limit the rise in global mean temperature to less than 
1.5°C above preindustrial, it is plausible that some additional and 
limited deployment of solar geoengineering could reduce climate 
damages. That is, these approaches could eventually be considered as 
part of an overall strategy to manage the risks of climate change, 
combining emissions reduction, net-negative emissions technologies and 
solar geoengineering to meet climate goals. We first provide a 
physical science review of current research, research trends and some 
of the key gaps in knowledge that would need to be addressed to 
support informed decisions. Next, since few climate model simulations 
have considered these limited-deployment scenarios, we synthesize 
prior results to assess the projected response if solar geoengineering 
were used to limit global mean temperature to 1.5°C above 
preindustrial in an overshoot scenario that would otherwise peak near 
3°C. While there are some important differences, the resulting climate 
is closer in many