Robock did an interesting paper which suggests further complex effects, especially with large injections http://www.agu.org/pubs/crossref/2009/2008JD011652.shtml (surprisingly cited by only 2, according to google). <http://www.agu.org/pubs/crossref/2009/2008JD011652.shtml>I particularly like this paper as it shows that massive cooling is available, which has big implications for timing of the start date of geoengineering, and also for the potential to tackle warming from methane emissions. I wonder if the methods discussed in Robock's chemistry-lead model could be tied into the paper which Ken sent? From an initial reading, they seem to be focussing on different aspects of the effects.
Whilst I'm 'on the phone', I wanted to mention an idea I had for sulfur distribution, which appears to be new. If you were to create hose capable of reaching the stratosphere, you wouldn't need a lifting balloon if the hose transported a mix of H2 and H2S. You'd simply need to use a valve at the top which allowed enough gas mix to flow out, whilst maintaining enough pressure in the hose to keep it inflated. Ideally, you'd put a burner on to to flare off the hydrogen. Whether there's enough oxygen at that height to sustain a flame would have to be calculated. It might be best to have a series of small burners, rather than one large one. Robock abstract below: It has been suggested that the Toba volcanic eruption, approximately 74 ka B.P., was responsible for the extended cooling period and ice sheet advance immediately following it, but previous climate model simulations, using 100 times the amount of aerosols produced by the 1991 Mount Pinatubo eruption, have been unable to produce such a prolonged climate response. Here we conduct six additional climate model simulations with two different climate models, the National Center for Atmospheric Research Community Climate System Model 3.0 (CCSM3.0) and National Aeronautics and Space Administration Goddard Institute for Space Studies ModelE, in two different versions, to investigate additional mechanisms that may have enhanced and extended the forcing and response from such a large supervolcanic eruption. With CCSM3.0 we include a dynamic vegetation model to explicitly calculate the feedback of vegetation death on surface fluxes in response to the large initial reduction in transmitted light, precipitation, and temperature. With ModelE we explicitly calculate the effects of an eruption on stratospheric water vapor and model stratospheric chemistry feedbacks that might delay the conversion of SO2 into sulfate aerosols and prolong the lifetime and radiative forcing of the stratospheric aerosol cloud. To span the uncertainty in the amount of stratospheric injection of SO2, with CCSM3.0 we used 100 times the Pinatubo injection, and with ModelE we used 33, 100, 300, and 900 times the Pinatubo injection without interactive chemistry, and 300 times Pinatubo with interactive chemistry. Starting from a roughly present-day seasonal cycle of insolation, CO2 concentration, and vegetation, or with 6 ka B.P. conditions for CCSM3.0, none of the runs initiates glaciation. The CCSM3.0 run produced a maximum global cooling of 10 K and ModelE runs produced 8–17 K of cooling within the first years of the simulation, depending on the injection, but in all cases, the climate recovers over a few decades. Nevertheless, the “volcanic winter” following a supervolcano eruption of the size of Toba today would have devastating consequences for humanity and global ecosystems. These simulations support the theory that the Toba eruption indeed may have contributed to a genetic bottleneck. A 2010/1/15 Alvia Gaskill <[email protected]> > The paper presents results for a number of scenarios and their expected > impacts on radiative forcing and ozone depletion, but doesn't show them for > perhaps the most important set of conditions evaluated, namely, the > latitudinal and vertical variation of precursor release. From page 5: > > > "The temporal and spatial distribution of S injection has > > a strong influence on the particle size distribution. In our > > experiments the S is distributed over a zonal band of 9 > *.*5◦ > > wide and 1.2 km high. To test the dependence of particle > > growth on the spatial distribution we performed three more > > geoengineering calculations with the source of 5 MT > */*a S: in > > the first experiment the SO > 2 is injected at the equator as well > > but spread vertically from 20 to 25 km, the second spread > > emissions from 30 > ◦S to 30◦N, with emissions at 20 km only > > and the third spread emissions in the same latitudinal range > > as the second but additionally spread emissions vertical from > > 20 to 25 km. The first produced a burden of 4.5 Mt S, the > > second a burden of 4.3 Mt S, the third a burden of 4.8 Mt S. > > Thus the spreading of emissions is one potential method to > > improve the efficiency of geoengineering injections, though the > > continuous nature of the emission still leads to particles larger > > than expected based on volcanic analogues." > > What would the expected reduction in shortwave forcing at the surface be > from these scenarios? Would it be significantly more than 0.4W/m2 per Mt S, > i.e. better than the monthly or annual injections at the same altitude and > location as the other scenarios modeled? Also, not studied except for the > 24Km case, would be the residence time at even higher altitudes, up to 30Km > (~100,000 ft) which is achievable with stratospheric balloons. > > I had been using the relationship of 6Mt S offsets ~4.5W/m2 (0.75W/m2 per > MtS) for 12 months in my estimates (see attached comments which seem to have > stood the test of time and perhaps inspired others, and Table 5 > specifically) based on the estimate in Crutzen's 2006 paper which was itself > based on a twelve month period from December 1991 to November 1992. The > Heckendorn et al. paper finds only about half that offset, 0.36W/m2 per MtS > at 5Mt dropping to 0.27 at 10Mt. The Rasch and Crutzen paper did find a > diminishing efficiency, but only at much higher burdens due to their > assumption of the formation of smaller aerosol at the outset and the fact > that the sulfur burden increased over time as lifetimes of more than one > year were assumed. > > Note also that these models in this paper assume some degree of coagulation > that might not actually be experienced, although a strong case is made that > at least with the injections that simulate a continuous or periodic volcanic > eruption, conditions different than those from Pinatubo might occur. > > It should be noted that the Pinatubo aerosol cloud eventually collapsed > into two fairly shallow layers and this effect might be expected to occur > with man-made injections as well even with injections at much higher > altitudes and throughout a vertical range of 20-30Km, (~6 miles) although > with continuous replenishment and variation of locations for release, this > might not occur at all. > > Thus, Ken's comment on first reading is disheartening, but a careful review > of the paper raises as many questions as it answers. > > > > > > ----- Original Message ----- > *From:* Ken Caldeira <[email protected]> > *To:* geoengineering <[email protected]> > *Sent:* Friday, January 15, 2010 2:27 AM > *Subject:* [geo] Potentially important paper: Maybe not so easy to > arbitrarily scale up continuous sulfate forcing > > Folks, > > The attached paper may have flown under the radar a bit, but it is worth > taking note of. > > In the scenarios they considered in their model, which included injections > of up to 10 MtS/yr, they did not achieve more than a 2 W/m2 reduction in > sunlight reaching Earth's surface. > > If they are correct, increased cleverness will be required to offset > planetary mean warming with stratospheric aerosols. > > Best, > > Ken > > > ---------------------- > > > P Heckendorn1, DWeisenstein2, S Fueglistaler3, B P Luo1, E Rozanov1,4, M > Schraner1, L W Thomason5 and T Peter1 > The impact of geoengineering aerosols on stratospheric temperature and > ozone > Environ. Res. Lett. 4 (2009) 045108 (12pp) doi:10.1088/1748-9326/4/4/045108 > > Here is some text from the concluding discussion: > > *The 2D AER aerosol model was used to simulate geoengineering > scenarios where global albedo is reduced with the > help of stratospheric sulfuric acid aerosol enhancements. The > AER model resolves all relevant microphysical processes in > the formation of sulfuric acid aerosols after injection of SO2 > into the stratosphere. In a model–observation comparison by > SPARC (2006), the AER model ranked among the best models > for the description of the stratospheric aerosol formation > after volcanic eruptions. > > ... > > We performed hypothetical geoengineering experiments > where S in the form of SO2 is continuously emitted at the > equator at 20 km altitude. The continuous emission of S > leads to aerosol size distributions with larger mode radii than > observed and modelled after volcanic eruptions. Continuous > coagulation of the freshly nucleated geoengineering particles > with the already grown background particles is identified > to be the most important process for the formation of > large particles, and condensation further adds to the aerosol > growth. The geoengineering particles grow to such sizes > that they experience substantial gravitational settling and > have accordingly shorter residence times in the stratosphere. > > ... > > If the S would be deposited only twice per year instead of > continuously, the mode radius of the aerosol size distribution > would be smaller and hence less S would be lost. However, > the injection of such large amounts of S in a short time would > be unfavourable from a technical point of view. Spreading the > emissions over a larger area is another strategy to reduce the > particle size and improve the efficiency of geoengineering by > S injection. Furthermore, use of particles other than sulfate to > reduce the global SWsurface flux could perhaps avoid some of > the drawbacks and side-effects presented here, provided these > particles could escape coagulation, but additional unknowns > would then need to be investigated. > * > ___________________________________________________ > Ken Caldeira > > Carnegie Institution Dept of Global Ecology > 260 Panama Street, Stanford, CA 94305 USA > > [email protected] > http://dge.stanford.edu/DGE/CIWDGE/labs/caldeiralab > +1 650 704 7212; fax: +1 650 462 5968 > > > ------------------------------ > > -- > You received this message because you are subscribed to the Google Groups > "geoengineering" group. > To post to this group, send email to [email protected]. > To unsubscribe from this group, send email to > [email protected]<geoengineering%[email protected]> > . > For more options, visit this group at > http://groups.google.com/group/geoengineering?hl=en. > > > -- > You received this message because you are subscribed to the Google Groups > "geoengineering" group. > To post to this group, send email to [email protected]. > To unsubscribe from this group, send email to > [email protected]<geoengineering%[email protected]> > . > For more options, visit this group at > http://groups.google.com/group/geoengineering?hl=en. > >--
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