Hi Doug, I agree we have some level of control over the aerosol distribution, though precisely how fine this control might be remains to be seen - that depends on engineering capabilities and atmospheric transport processes.
I don't think we've particularly misled people here (reviewers' comments helped iron out some poor choices of wording on this, actually). The closest we come to making a comment against the use of solar dimming experiments is: "Therefore, in terms of stratospheric dynamics, solar dimming experiments cannot be considered analogs for stratospheric aerosol geoengineering. Because the stratospheric polar vortices and the tropospheric eddy-driven jets are intimately coupled, this also means the tropospheric midlatitude dynamical response is different in solar dimming and stratospheric aerosol geoengineering experiments." That's pretty uncontroversial in my opinion. Incidentally this stratospheric response would occur similarly even with a uniform aerosol distribution, because the stratospheric heating gradient arises mainly from differences in solar and infrared heating, not the concentration of the aerosol itself. The primary goal of this study was to discuss the physical processes determining the stratospheric response to stratospheric aerosol geoengineering and the subsequent impact on the troposphere. The solar dimming case is in some sense an illustration of what happens when the stratospheric circulation response is minimal. We showed that the radiative influence of the aerosols on the stratosphere impacts stratospheric dynamics and hence the midlatitude jets. Of course the conclusion leading from that is that it's important to correctly characterise this radiative influence in our climate model simulations. I'm not suggesting this has been correctly characterised in this study - the limitations have already been discussed here. I'm suggesting that the presence of aerosol has a dynamical impact which will not be represented by solar dimming in any configuration (because aerosol induces local heating in parts of the stratosphere), so it would be worth bearing in mind when designing and interpreting geoengineering experiments. One way we might reduce these effects would be to use a different, less-absorbing aerosol (such as titania, discussed in the paper). Thanks Angus On Tuesday, 6 January 2015 16:06:37 UTC, Doug MacMartin wrote: > > Hi Angus, > > > > Re your first point, there isn’t such a thing as “the forcing > distribution… resulting from aerosol injection”. Anyone implementing > geoengineering gets to choose the latitude and altitude and amount and > seasonal timing of injection, and those choices will change the > distribution. Turning down the sun isn’t something we’re ever likely to > do, so again, that is just a modeling hack to easily simulate other > approaches, and intercompare models using consistent assumptions. > > > > So if your goal is to understand whether applying some particular spatial > distribution of radiative forcing simply by appropriately turning down the > sun is sufficient or not, then it makes no sense to compare it with > aerosols that have a different spatial distribution. If your goal was > simply to compare space mirrors with one specific choice of aerosol > forcing, that seems a bit odd to me, as it would be hard to know what > differences you have control over and what differences you don’t. The only > reason many of us are turning down the sun in climate models is because it > simplifies a step. I confess I haven’t read your paper yet, but at least > Andrew was led to infer that your paper was relevant in deciding whether > solar dimming was an acceptable modeling approximation, so if the latter > was your goal, at least one person was misled about the paper’s conclusions. > > > > doug > > > > *From:* [email protected] <javascript:> [mailto: > [email protected] <javascript:>] *On Behalf Of *Angus Ferraro > *Sent:* Tuesday, January 06, 2015 1:34 AM > *To:* [email protected] <javascript:> > *Cc:* [email protected] <javascript:>; [email protected] <javascript:> > *Subject:* Re: [geo] Stratospheric dynamics and midlatitude jets under > geoengineering with space mirrors, and sulfate and titania aerosols - > Ferraro - JGR Atmos - Wiley > > > > Dear Prof. Bala, > > Thank you for your comments. > > I agree that in this paper we, to some extent, compare 'oranges and > apples'. However, this is because, to some extent, solar reduction and > aerosol injection *are *oranges and apples. We have a good idea of the > forcing distribution from solar reduction, but there is more uncertainty in > that resulting from aerosol injection. Here, we chose an aerosol > distribution that is supported by the literature (see Section 2 of the > paper). We then scaled it up so it counterbalanced a quadrupling of CO2. As > Alan Robock pointed out, this is unrealistic because the aerosol layer > shape and size distribution would change at such large concentrations. > Indeed, it is unclear whether such a large forcing could even be achieved > with sulphate aerosol (titania may be another matter). > > We chose a large forcing to better discern a signal in a noisy > environment. As it happens, we could probably have done 2xCO2 instead, > given the magnitude of the response simulated. *Provided* these responses > scale linearly with forcing, this is useful. If they do not, they are still > a qualitatively valid way of looking at mechanisms, and this is the purpose > of the paper (as we stated in the text). > > We do not suggest in this paper that solar dimming experiments are > completely invalid, nor that they shouldn't be used. They are useful for > the reasons Ken highlighted earlier in this conversation - one of which is > that one does not need to make assumptions about particle sizes and > distributions. What this paper demonstrates is that the stratospheric > response to aerosols can be large, and that it matters for midlatitude > surface climate because of the coupling between the stratospheric polar > vortex and the midlatitude jets. The 'correct' stratospheric aerosol > spatial and size distribution is uncertain, and consequently so are aspects > of the tropospheric response. As Andrew has expressed, the danger is that > we take the advantage of solar dimming experiments (that we do not need to > make assumptions about the aerosol layer) too far, and that we lose sight > of the fact that there are important sources of uncertainty derived from > the behaviour of the aerosol in the stratosphere. > > Thanks > > Angus > > > On Tuesday, 6 January 2015 03:30:03 UTC, bala wrote: > > This paper by Ferraro et al, JGR (2015) has the same problems as Ferraro > et al ERL (2014). Strangely, this new JGR study does not cite the previous > ERL paper (Maybe because the Ph.D is cited?) > > 1. This paper compares apples and oranges as before in the ERL paper: > Solar constant reduction is uniform but aerosols are heavily concentrated > in the tropics. Therefore, the forcing distribution is very different in > the SRM simulations and hence the methodology is severely flawed. In fact, > this is the major reason for this work and the ERL paper to produce > different climates in the tropics. In our paper sent by Ken in this thread, > we use uniform reduction in solar radiation in both SRM simulations and > produce very similar tropospheric climates. > > 2. The tropical stratosphere warms by about 15 deg C in the aerosol cases > in this paper. No other study shows this huge warming. Most studies show a > warming of about 1-3 deg C. I am not sure why such a strong heating is > produced in this model. I had a conversation with Alan Robock about this. > He just told me there is something wrong with the model in Ferraro's study. > In any case, the uncertainty in stratospheric heating produced by aerosol > SRM and its implications to stratospheric climate may be be worth exploring > more. > > > > On Tue, Jan 6, 2015 at 7:51 AM, Ken Caldeira <[email protected]> > wrote: > > Andrew, > > > > Poorly supported sweeping statements are rarely useful. Tools that are > good for one purpose and may not be good for other purposes. > > > > The utility of a "solar dimming" approach for simulating solar > geoengineering depends on what your purpose is. If you are concerned with > dynamics of the stratosphere and upper troposphere, then solar dimming will > not suffice. > > > > However, dimming sunlight produces a surface climate that is broadly > similar to that obtained with a uniform aerosol layer, so if your concern > is broad features about climate near Earth's surface, solar dimming may > suffice. See attached paper: > > > > · Kalidindi, S., G. Bala, A. Modak, and K. Caldeira, 2014: Modeling > of solar radiation management: a comparison of simulations using reduced > solar constant and stratospheric sulphate aerosols. > <http://dge.stanford.edu/labs/caldeiralab/Caldeira_research/Kalidindi_RadMgmnt.html> > *Clim Dyn*, 1–17, doi:10.1007/s00382-014-2240-3. > > > > Below are a few figures illustrating the similarity. Note that there is > random variability between simulations even if they have the same forcing. > > > > Solar dimming experiments are useful because they easy to implement and > there is a minimum of additional assumptions (i.e., the only assumption is > a single scalar, and no assumptions need to be made about particle sizes, > distributions, etc). When simplicity, clarity, and ease of implementation > is a virtue (as with the G1 simulations of GeoMIP) solar dimming can be a > very useful model configuration. > > > > Best, > > > > Ken > > > > > > > > > > > > > > > > > > > _______________ > Ken Caldeira > > Carnegie Institution for Science > > Dept of Global Ecology > > 260 Panama Street, Stanford, CA 94305 USA > > +1 650 704 7212 [email protected] > http://dge.stanford.edu/labs/caldeiralab > > https://twitter.com/KenCaldeira > > > > My assistant is Dawn Ross <[email protected]>, with access to > incoming emails. > > > > > > > > On Mon, Jan 5, 2015 at 5:36 PM, Andrew Lockley <[email protected]> > wrote: > > Poster's note : another study showing solar dimming is a risky > approximation to make in SRM models > > http://onlinelibrary.wiley.com/doi/10.1002/2014JD022734/abstract > > Stratospheric dynamics and midlatitude jets under geoengineering with > space mirrors, and sulfate and titania aerosols > > A. J. Ferraro, A. J. Charlton-Perez and E. J. Highwood > DOI: 10.1002/2014JD022734 > > Journal of Geophysical Research: Atmospheres > > Abstract > > The impact on the dynamics of the stratosphere of three approaches to > geoengineering by Solar Radiation Management is investigated using > idealized simulations of a global climate model. The approaches are > geoengineering with sulfate aerosols, titania aerosols and reduction in > total solar irradiance (representing mirrors placed in space). If it were > possible to use stratospheric aerosols to counterbalance the surface > warming produced by a quadrupling of atmospheric carbon dioxide > concentrations, tropical lower stratospheric radiative heating would drive > a thermal-wind response which would intensify the stratospheric polar > vortices. In the Northern Hemisphere this intensification results in strong > dynamical cooling of the polar stratosphere. Northern Hemisphere > stratospheric sudden warming events become rare (1 or 2 in 65 years for > sulfate and titania respectively). The intensification of the polar > vortices results in a poleward shift of the tropospheric midlatitude jets > in winter. The aerosol radiative heating enhances the tropical upwelling in > the lower stratosphere, influencing the strength of the Brewer-Dobson > Circulation. In contrast, solar dimming does not produce heating of the > tropical lower stratosphere so there is little intensification of the polar > vortex and no enhanced tropical upwelling. The dynamical response to > titania aerosol is qualitatively similar to the response to sulfate. > > -- > 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 [email protected]. > To post to this group, send email to [email protected]. > Visit this group at http://groups.google.com/group/geoengineering. > For more options, visit https://groups.google.com/d/optout. > > > > -- > 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 [email protected]. > To post to this group, send email to [email protected]. > Visit this group at http://groups.google.com/group/geoengineering. > For more options, visit https://groups.google.com/d/optout. > > > > > -- > > 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: [email protected]; [email protected] > Web:http://caos.iisc.ernet.in/faculty/gbala/gbala.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 [email protected] <javascript:>. > To post to this group, send email to [email protected] > <javascript:>. > Visit this group at http://groups.google.com/group/geoengineering. > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "geoengineering" group. 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