|Review of "post-review revised" version of manuscript "A Model Intercomparison of Stratospheric|
Solar Geoengineering by Accumulation-Mode Sulfate Aerosols" by Debra Weisenstein et al.
I was one of the 3 reviewers who reviewed the submitted version of this manuscript in August 2021,
the analysis presenting results from a model intercomparison comparing interactive
stratospheric aerosol simulations within co-ordinated multi-model experiments to explore the
global dispersion and radiative forcing that would result from a continuous source of sulphur dioxide
or accumulation mode sulphate aerosol particles with two different emissions scenarios:
one emitting only at 30N and 30S, the other as a constant source between 30S and 30N.
As I identified in my original review, the intercomparison (across 3 different models), represents
a potentially very interesting contribution to understand the predictions from the models, each
having differing sophistication in their aerosol modules, and in the vertical and horizontal
resolution of the GCM's advection.
My original review explained the analysis would be publishable in a revised form, my review found
the aim and design of the model experiments to be poorly described, with the Introduction and
interpretation needing to include some discussion also of the tropical stratospheric reservoir,
in relation to expected differences between the two scenarios.
I also noted several places where the manuscript had unscientific language or vague statement,
and a few places where the wording was poor, or over-simplifying the changes to the stratospheric
aerosol layer that would occur in this hypothesised large-scale injection of precursor gas or
idealised particle for solar radiation management.
I made a list of 20 specific revisions that were needed, before the manuscript could potentially
then proceed to publication, with also a request that the authors change the title, and querying
the terminology "Solar Geoengineering by Accumulation-Mode Sulfate Aerosols", in being inconsistent
with what the authors state that particular model experiment is representing.
With some of the comments relatively fundamental to the narrative of the manuscript, I found
that major revisions were needed, but with the Figures much of the results section is in good shape,
these were relatively minor revisions.
The authors have replied positively to each of the specific comments I made, with the instances
of imprecise wording in the manuscript now remedied, with also most of the instances where
particle size changes were over-simplified now also much improved.
One of my main comments was to advise the authors to change the term "geoengineering by
accumulation-mode sulfate aerosols" to simply "geoengineering by sulfate aerosols.
This suggestion was on the basis that that terminology also somehow suggests a relative ease
of achieving that particular desired size, or that the particles will remain at a particular
"target size" in their months to years lifetime in the stratosphere.
As I explained in my review, the vertical and meridional variations in stratospheric aerosol
particle size seen in the years after the Pinatubo eruption, and other major eruptions, and
from my own experience from analysing a range of major eruption scenario experiments within
interactive stratospheric aerosol models suggest the range of variation in particle size
would likely be broader than many readers would infer from that "geoengineering by
accumula,ion-mode sulfate aerosols" terminology. Somehow that terminology suggests, to me
at least, is that it is a relatively simple issue to achieve the required particle size.
In their reply to this comment, the authors explain that their preference to retain that
same terminology -- and of course this is their manuscript -- and given the improvements
in the wording the authors have made, despite my continued opposition to that term (it
still communicated an over-simplified situation), it is OK for that terminology to remain
in place within this particular article.
Although the manuscript is much improved, there are still some places where
the wording requires minor changes, to then be fine to proceed to publication.
These remaining few minor revisions are listed below, with 2 of the 6 required changes being
more substantial than the others, and requiring some explanation here.
Both of these two remaining "substantive points" relate to the way the text in the revised
manuscript refers to the specific type/class of models whose results are analysed in this
The first of the 2 is to ask the authors to change the name they have used for the type/class
of models used in the analysis, specifically within the new title the authors have added in
the revised manuscript, where the models are referred to as "aerosol-climate models".
Whilst I accept that the models can generally be referred to within that broad class of
models that include the radiative effects of aerosol in their predictions of the earth's
climate, the functionality being applied for this analysis, requires a particular capability
for "interactive stratospheric aerosol". As the authors will be aware, there is currently
ongoing a model intercomparison project/iniative "ISA-MIP" (Timmreck et al., 2018) which
has designed model experiments to specifically inter-compare these interactive stratospheric
aerosol models, including more background (volcanically quiescent) conditions, and
experiments for major eruptions, and to hindcast predictions through the series of more moderate
stratospheric-injecting eruptions that have occurred so far in the 21st century.
For that MIP, the terminology the community agreed for these models was "interactive
stratospheric aerosol models" (the ISA within ISA-MIP) and whereas these models are
certainly aerosol-climate models, the interactive stratospheric aerosol capability is key
here in terms of being able to predict the onward variations in particle size, and
associated residence time, from the initial "emission size distribution" the particles
initially as they mixed into the ambient air around the aircraft.
And that 1st of the 6 changes is then requesting to change the title from "An aerosol-climate
model intercomparison" instead to "An interactive stratospheric aerosol model intercomparison".
It is notable to me that each of the 3 interactive stratospheric aerosol models compared
here also having the particular sophistication to represent microphysical processes within their
interactive predictions of the stratopheric aerosol layer's variations. And then I am advising
here to also add the word "microphysics" before "intercomparison" -- this then being an
"interactive stratospheric aerosol microphysics intercomparison. I leave it up to the authors
however to choose either of those -- each an improvement on the much less specific
"aerosol-climate model" terminology, which is too broad to adequately communicate the particular
type of experiments the manuscript analyses results from.
The other of the 2 remaining substantive change also relates to the type of model, where I think
the authors current description of the "modus operandi" of the models needs changing.
Specifically the wording:
"As input, they would take the particle size distribution from aircraft plume model studies
but could represent any hypothetical input of particles".
This reference to an "input size distribution" under-plays
the value of the interactive stratospheric aeorsol models, and could lead some readers to
mis-understand the aerosol-climate models to simply be enacting a "prescribed but globally varying"
size distribution from some other experiments with an interactive stratospheric aerosol model.
This is not the case, with the interactive models doing much more than simply representing a
particular "input size distribution".
Each of the 3 interactive stratospheric aerosol microphysics models used in the study predict how
the initially localised plume of geoengineering aerosol would progress to the "response" of a
global enhancement to the stratospheric aerosol layer, and how that would evolve in the months
of years of the continued injection/emission.
The global spatial variation of the particle size, and its temporal variability across different
seasons and years, with the internal variations in the stratospheric circulation and its dynamical
states that occur through the simulations. The relationship between a particular engineered
particle size at emission, and the eventual variation in size one would see at a global scale
(across the tropics, midlatitudes and high latitudes of each hemisphere) is far from certain.
In addition to the transport variations, the coagulation of the particles, and the subsequent
removal and vertical distribution from gravitational settling, will likely cause substantial
variations in the progressions in particle sizes that would occur as the continuing (or
intermittent) plume(s) are dispersed globally over months and years, would introduce subsequent
variations that differ from the initial "emission size distribution" introduced from the
emission location (e.g. aircraft or tethered pipe).
This issue communicates a bit more about the basis of my objection to my the
"geoengineering by accumulation-mode sulphate" terminology, in some readers inferring an
over-simplified situation in how the dispersed particles would "end up" within the "enhanced
state" of a geoengineered stratospheric aerosol layer.
Whilst I am content to concede that to the authors choice of terminology, when referring to
the models being used in the analysis, the text needs to better communicate what the interactive
stratospheric aerosol models actually represent. In particular the reference to an
"input size distribution", seems to be an over-simplfication.
The other 4 changes are more minor, and self-explanatory, and I then list below the 6 minor revisions
which I'm advising are required before the manuscript can then proceed to publication in ACP.
Remaining minor revisions
1) Manuscript title: Page 1, line 1 -- please change "An aerosol-climate model intercomparison"
to either "An interactive stratospheric aerosol model intercomparison" or preferably (from my perspective)
"An interactive stratospheric aerosol microphysics model intercomparison". I note that the authors
refer to "interactive stratospheric aerosol-climate models" (e.g. page 1, lines 20-21), and a
potential variant of that 2nd suggested alternative could be to change the words "aerosol microphysics"
2) Abstract: Page 1, lines 12-14 -- this 1st line of the Abstract still seems poorly worded to me.
I don't understand what the authors are trying to communicate here. The wording currently states:
"Studies of stratospheric solar geoengineering have tended to focus on sulfuric acid aerosols, and
almost all such climate model experiments assume that SO2 is injected to increase the sulfuric acid
aerosol burden of the stratosphere."
I think this sentence should be replaced with a more meaningful comment on previous work, in relation
to the difference between imposing a particular particle size, and simulating the size distribution
interactively. This paper is the 1st ever inter-comparison of interactive stratospheric aerosol
simulations of the geoengineering-enhanced stratospheric aerosol layer, and it is this functionality
that I'd suggest this functionality the first sentence of the Abstract focuses on communicating.
A specific suggestion would be to replace that sentence with:
"Previous model comparisons of stratospheric solar geoengineering have mostly tended to focus
on climate model experiments assuming a particular prescribed particle size for geoengineered sulfate
The 1st part of the 2nd sentence of the Abstract should also be improved -- as it is similarly poorly
worded at present -- my specific recommendation is to re-word that 1st part to instead begin "By contrast,
a key finding from interactive modeling studies is that the radiative forcing of a geoengineered
stratospheric aerosol layer would increase sub-linearly....."
3) Abstract: Page 1, lines 28-30 -- The "We use" wording here is colloqial and needs to be changed,
with also the "injection patterns" and "belt" somehow (to me at least) not sufficiently communicating
the scientific issue being explored. The experiment that injects at 30N and 30S is presumably a specific
"deployment scenario", aimed to force only the mid-latitudes, whereas the constant emissions from 30S-30N
is a more theoretical scenario, perhaps idealsed to achieving the longest residence time for the
emitted particles. Suggest to re-word:
"We use two different injection patterns"
"The models carried out two different "geoengineering-enhancement scenarios" or similar more scientific
terminology than "patterns".
I think there is an error here also where you state "injecting in a belt along the equator between
30S and 30N" --- perhaps it's simply a case of deleting "along the equator", but I'd recommend also
changing "in a belt" to "uniformly" or "with a uniform emission rate" or similar.
4) Abstract: Page 1, lines 30-32 -- This last sentence of the Abstract also needs to be improved,
as it's not clear what is meant by "opposite impacts" -- and the term "radiative efficacy" seems
somehow ill-defined (or not yet introduced). Perhaps a simple re-wording of "opposite impacts
on radiative efficacy" to "strongly differing radiative forcing efficacy" -- or just
"strongly differing radiative forcing".
5) Introduction: Page 3, lines 27-30 -- The current wording "Detailed modelling of this complex process
for a full range of stratospheric physical, chemical and microphysical conditions awaits further studies"
should be improved to better communicate the scientific issue here (rather than the technical aspects
of the processes involved). I mean the question of how the particle size progresses, over a timescale of
months to years, from the initial size at the plume-scale, to the global-scale variations in particle
size in the "dispersed state" of the geoengineering-enhanced stratospheric aerosol layer.
Suggest instead "A priority for future modeling studies could potentially be to establish how the
initial "engineered particle size" at the plume-scale progresses to the global-scale variations in
particle size in the "dispersed state" of the geoengineering-enhanced stratoshperic aerosol layer."
The follow-on setence should also be re-worded. The current text refers to "the GeoMIP models"
but, further to my general comments above, I think the authors mean the interactive stratospheric aerosol
models -- i.e. "the ISA-MIP models". Also change "can be used to simulate" to "have the functionality to
explore how the stratospheric aerosol layer responds with the global dispersion of the geoengineering
particles." Or something like this.
6) Introduction: Page 3, lines 30-33 -- The 1st of the 2 setnences here refers to ozone, temperature
and circulation -- but the impacts on ozone depends strongly on the stratospheric chemistry scheme,
which is outside the scope of this article. I suggest to narrow the scope of this sentence to instead
to focus on the aerosol changes. A specific suggestion is to change:
"These GCMs can effectively simulate changes in global aerosol
burden, radiative forcing, ozone, and stratospheric temperature and circulation
"Those ISA-MIP models with microphysical aerosol schemes can also address the key issue of how the
particle size distribution progressees, this being a key determinant of subsequent global aerosol
burden and radiative forcing."
As in my general/overarching comments above, the 2nd sentence here also needs changing, with
this issue of an "input size distribution" needing to be better explained.
The wording says:
"As input, they would take the particle size distribution from aircraft plume model studies
but could represent any hypothetical input of particles".
I think this reference to an "input size distribution" could be mis-interpreted by some readers
unfamiliar with the types of model involved. The value of the interactive stratospheric aerosol
models is more than simply that they can represent an input size from a plume-scale model.
It's this issue of how the emitted particles subsequently transform in a "globally dispersed
state", as they become part of a geoengineering-enhanced stratospheric aerosol layer.
With the re-wordings in points 1 to 5, it may be that this is then sufficiently explained,
with then this final sentence explaining of the potential for future work to involve a combination
of plume-scale models and the global interactive stratospheric aerosol GCMs.
A specific suggestion for re-wording could be:
"There is the potential for future interactive stratospheric aerosol model experiemnts to
link directly with plume-scale model experiments, and seek to realistically represent
potential alternative deployment scenarios".
Timmreck, C., Mann, G.W., Aquila, V., Hommel. R., Lee, L. A., Schmidt, A. et al. (2018):
"The Interactive Stratospheric Aerosol Model Intercomparison
Project (ISA-MIP): motivation and experimental design"
Geosci. Model Dev., 11, 2581–2608.