Re: [geo] Re: paper showing that turning down the sun experiments have similar climate results to prescribed stratospheric aerosol experiments
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 kcal...@carnegiescience.edu javascript: http://dge.stanford.edu/labs/caldeiralab https://twitter.com/KenCaldeira Assistant: Dawn Ross dr...@carnegiescience.edu javascript: On Sat, Jul 26, 2014 at 10:57 AM, Nathan Currier natcu...@gmail.com javascript: 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
Re: [geo] Re: paper showing that turning down the sun experiments have similar climate results to prescribed stratospheric aerosol experiments
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 natcurr...@gmail.com 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 kcal...@carnegiescience.edu http://dge.stanford.edu/labs/caldeiralab https://twitter.com/KenCaldeira Assistant: Dawn Ross dr...@carnegiescience.edu On Sat, Jul 26, 2014 at 10:57 AM, Nathan Currier natcu...@gmail.com 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
Re: [geo] Re: paper showing that turning down the sun experiments have similar climate results to prescribed stratospheric aerosol experiments
The attachment gives abstracts of three papers that show the complexity of the mechanisms that result in stratospheric water vapor content, and also the several chemical complexities of stratospheric aerosol chemistry. Models are some way off being able to represent them quantitatively. On 26 July 2014 18:57, Nathan Currier natcurr...@gmail.com 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
