Hi Nathan, The changes to amount of water vapor in the lower stratosphere, like in the troposphere, is likely controlled by the stratospheric temperature change. i.e. follows C-C relationship. Therefore, the source mechanism for H2O in the low stratosphere may not be that important. I do agree that a full climate-chemistry investigation is still lacking for aerosol geoengineering!
On Sun, Jul 27, 2014 at 8:40 PM, Nathan Currier <[email protected]> wrote: > > > Hi, Ken – > > Thanks much for your response. Of course I see how you meant this, and I > don’t really disagree as long as it’s clearly stated that, given the > successful achievement of the same radiative forcing impacts with the two > approaches, then there’s not much difference between the SRM and turning > down the sun. What I was saying – and, as I said, it was more question than > comment – grows out of a general sense that we should all still be looking > for any possible ways that one might not successfully achieve the forcing > impacts currently assumed, for some reason or another. > > So, while you write that the surface temperature is not “super sensitive > to changes in the stratosphere,” I was referencing one of the possible ways > in which the surface apparently can be quite sensitive to such changes: for > me, at least, when I first heard of the Solomon et al paper a few years > back, what first popped out at me was how, with so little H2O up in the > stratosphere, changes in it could have such huge impacts on surface > temperature, comparable with changes in CO2 forcing, I think, over the same > period. Then later I began wondering whether stratospheric SRM techniques > could impact this stratospheric hydrology, as opposed to the much more > frequently discussed surface hydrology SRM impacts. > > So, that’s why I brought up the methane issue, since there clearly is an > interaction at the surface between sulfur chemistry and methane oxidation, > and methane is very clearly an important source of H2O to the stratosphere. > Luckily, the methane doesn’t seem to be getting oxidized in the right place > to have such a big impact, but could *that* somehow or another get > shifted if you add a bunch of sulfur to the stratosphere? Again, it was > really a question, where I was asking: have such possibilities been > explored? > > Cheers, > > > Nathan > > > > > > On Saturday, July 26, 2014 3:01:47 PM UTC-4, kcaldeira wrote: > >> Nathan, >> >> Your questions are interesting but your initial statement is not true >> when it comes to first order effects on surface temperature and hydrology. >> [".. the similarity ("turning down the sun" and the SRM) depends on the >> ability of the models to reproduce, among other things, the stratospheric >> chemistry correctly. "] >> >> Surface temperature and hydrology is not super sensitive to changes in >> the stratosphere. Key is getting the effective radiative forcing right both >> in terms of total amount and latitudinal distribution. >> >> To repeat what I said in the previous email: >> >> *Of course, what tool you use depends on your goals in using a tool. The >> attached paper shows results indicating that for many purposes, "turning >> down the sun" is a clear and efficient way of simulating many aspects of >> solar geoengineering.* >> >> *If you are looking at effects on the stratosphere or looking at effects >> of diffuse radiation, then you would need to simulate aerosols, but if you >> are just trying to get an idea of temperature and hydrological changes, >> then it seems that "turning down the sun" does a pretty good job.* >> >> 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 >> >> Assistant: Dawn Ross <[email protected]> >> >> >> >> On Sat, Jul 26, 2014 at 10:57 AM, Nathan Currier <[email protected]> >> wrote: >> >>> This is more question than comment, but it seem to go without saying >>> that the similarity ("turning down the sun" and the SRM) depends on the >>> ability of the models to reproduce, among other things, the stratospheric >>> chemistry correctly. >>> >>> Solomon et al, 2010* (see below) concerned the hugely under-represented >>> impacts of stratospheric water vapor on surfacing warming, and suggested >>> increased stratospheric water vapor might have accounted for 30% of global >>> warming in the 90s. They also noted: >>> >>> Current global climate models simulate lower-stratospheric temperature >>> trends poorly, and even up-to-date stratospheric chemistry-climate models >>> do not consistently reproduce tropical tropopause minimum temperatures or >>> recently observed changes in stratospheric water vapor. >>> >>> >>> >>> Their paper only dealt with changes in one of the two ways that the >>> stratosphere gets its water vapor, but I’ve often wondered about the >>> potential impacts of sulfur SRM on the other one – that is, stratospheric >>> methane. Solomon et al, while suggesting that methane’s role is relatively >>> weak in H2O creation near the tropopause (where it apparently counts most >>> for surface warming), noted: >>> >>> Estimates of the forcing due to (stratospheric) methane oxidation have >>> varied widely among different studies, perhaps because of different shapes >>> of the water profile in the region of greatest sensitivity. >>> >>> >>> One of my questions has been whether prolonged use of stratospheric >>> sulfur SRM, unlike the sudden pulse of a volcano, could potentially give >>> rise to analogous “competitive” reactions to those seen in the troposphere >>> involving sulfur chemistry and methane hydroxylation (i.e., Shindell et al >>> 2007, 2009, 2012, etc), one of the factors that have driven our assumed >>> increase in the indirect forcing effects of methane over the last half >>> decade. >>> >>> I wonder, if additions of sulfur did actually lead, at the decadal scale >>> of the methane lifetime, to increases in stratospheric H2O, what could its >>> maximum impact be? What percentage of all stratospheric H2O comes from >>> methane, in the first place? Does current SRM modeling account for such >>> possible interactions? >>> >>> Best, >>> >>> Nathan >>> >>> >>> >>> *Solomon et al 2010: >>> >>> >>> >>> abstract >>> http://www.sciencemag.org/content/327/5970/1219.abstract >>> >>> full paper >>> http://www.climate.unibe.ch/~plattner/papers/solomon10sci.pdf >>> >>> discussions of, from NOAA & RealClimate: >>> http://www.noaanews.noaa.gov/stories2010/20100128_watervapor.html >>> >>> http://www.realclimate.org/index.php/archives/2010/01/ >>> the-wisdom-of-solomon/ >>> >>> >>> >>> On Friday, July 25, 2014 11:39:52 AM UTC-4, kcaldeira wrote: >>> >>>> Folks, >>>> >>>> Andrew wrote something the other day about "turning down the sun" >>>> experiments NOT being a good analogue for stratospheric aerosol >>>> geoengineering. >>>> >>>> Of course, what tool you use depends on your goals in using a tool. The >>>> attached paper shows results indicating that for many purposes, "turning >>>> down the sun" is a clear and efficient way of simulating many aspects of >>>> solar geoengineering. >>>> >>>> If you are looking at effects on the stratosphere or looking at effects >>>> of diffuse radiation, then you would need to simulate aerosols, but if you >>>> are just trying to get an idea of temperature and hydrological changes, >>>> then it seems that "turning down the sun" does a pretty good job. >>>> >>>> Enjoy, >>>> >>>> Ken >>>> >>>> >>>> http://link.springer.com/article/10.1007/s00382-014-2240-3 >>>> >>>> >>>> >>>> *Modeling of solar radiation management: a comparisonof simulations >>>> using reduced solar constant and stratospheric sulphate aerosols* >>>> >>>> Sirisha Kalidindi · Govindasamy Bala · >>>> Angshuman Modak · Ken Caldeira >>>> >>>> >>>> Abstract The climatic effects of Solar Radiation Management >>>> (SRM) geoengineering have been often modeled >>>> by simply reducing the solar constant. This is most likely >>>> valid only for space sunshades and not for atmosphere and >>>> surface based SRM methods. In this study, a global climate >>>> model is used to evaluate the differences in the climate >>>> response to SRM by uniform solar constant reduction and >>>> stratospheric aerosols. Our analysis shows that when global >>>> mean warming from a doubling of CO2 is nearly cancelled >>>> by both these methods, they are similar when important >>>> surface and tropospheric climate variables are considered. >>>> However, a difference of 1 K in the global mean stratospheric >>>> (61–9.8 hPa) temperature is simulated between the >>>> two SRM methods. Further, while the global mean surface >>>> diffuse radiation increases by ~23 % and direct radiation >>>> decreases by about 9 % in the case of sulphate aerosol >>>> SRM method, both direct and diffuse radiation decrease by >>>> similar fractional amounts (~1.0 %) when solar constant >>>> is reduced. When CO2 fertilization effects from elevated >>>> CO2 concentration levels are removed, the contribution >>>> from shaded leaves to gross primary productivity (GPP) >>>> increases by 1.8 % in aerosol SRM because of increased >>>> diffuse light. However, this increase is almost offset by >>>> a 15.2 % decline in sunlit contribution due to reduced >>>> direct light. Overall both the SRM simulations show similar >>>> decrease in GPP (~8 %) and net primary productivity >>>> (~3 %). Based on our results we conclude that the climate >>>> states produced by a reduction in solar constant and addition >>>> of aerosols into the stratosphere can be considered >>>> almost similar except for two important aspects: stratospheric >>>> temperature change and the consequent implications >>>> for the dynamics and the chemistry of the stratosphere >>>> and the partitioning of direct versus diffuse radiation reaching >>>> the surface. Further, the likely dependence of global >>>> hydrological cycle response on aerosol particle size and the >>>> latitudinal and height distribution of aerosols is discussed. >>>> _______________ >>>> 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 >>>> >>>> Assistant: Dawn Ross <[email protected]> >>>> >>>> -- >>> 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. > -- Best wishes, ------------------------------------------------------------------- G. 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