Thank you for this good news about Pinatubo and the Arctic. This is not contradictory with the figures I had got: -38 % in the geoengineering efficiency at a particular point and date, -30 % for a larger range of both, which is clearly lower than a -100 % reduction in efficiency.
However, I was suggesting particular points of concern, which could worsen these figures, e.g. if the scattering was anisotropic, with a larger part of the light diffracted only some degrees around its incident direction. So, my conclusion is that trying to mimic the Pinatubo requires either to be sure that we use the same type of aerosols, with the same particle size distribution or mean size (diffraction scattering is very sensitive to the size/wavelength ratio), or that precise optics calculations guarantee a similar behavior. Denis Bonnelle. De : [email protected] [mailto:[email protected]] De la part de Alvia Gaskill Envoyé : vendredi 15 mai 2009 17:47 À : [email protected]; [email protected]; [email protected]; Professor Tom Wigley Cc : [email protected]; [email protected]; Sam Carana; Davies, John; Peter Wadhams Objet : [geo] Geometry, Arctic and Aerosols I started a different thread on this as my email service gagged on the size of the 60+ responses to the original. Since Ken hid the membership lists for both groups, I no longer know who is or isn't a member, so I will continue to copy those on the original posting. I am also having some trouble following this argument, but note that the Pinatubo aerosol DID reduce forcing over Greenland and the Arctic. The sea ice in 1992 was the thickest of any of the last 20, I believe. So it wasn't cancelled out by forward scattering of tangential rays. Therefore, there must be some kind of error in the calculations or assumptions. There is also some confusion, I believe in what we are calling "stratospheric aerosols" over the Arctic. The Pinatubo aerosols were in the Overworld stratosphere, the stratosphere above the altitude of the tropical tropopause (>53,000 ft.) Benford's proposal along with others that seek to have the aerosol descend and be gone before the Arctic winter involves "tropospheric aerosols" or other sunlight scattering materials. The altitude at issue here has never been clearly stated, but I assume it is to be around 40,000-45,000 ft, close to the magic 53,000, but not above it and high enough to guarantee aerosol lifetimes of several months. If you want to use airplanes like the 747, the KC-135, the "extender" or even the B-52, that's about as high as you are going to be able to go. Even though various people over the last several years including myself have talked about 40,000-50,000 ft as an altitude at which aerosol lifetimes are in the months and not years, I'm not aware of any studies that confirm this. Add to this the fact that in the Arctic, the stratospheric air is descending and not rising, so even at above 53,000 ft, this fact, coupled with removal of aerosol via tropopause folding makes the lifetimes uncertain. If, for example, a true "stratospheric" aerosol program is carried out above the Arctic circle, the litetimes of these aerosols will likely be less than those produced in the tropics, perhaps only 6 months instead of 12 or 18. The recent volcanic eruptions in Alaska (Redoubt and Kasatochi) are good examples of what may actually happen with tropospheric areosols. http://www.avo.alaska.edu/volcanoes/volcact.php?volcname=Kasatochi&eruptionid=605 "The eruption was characterized by three distinct explosions that were detected by the seismic network on Great Sitkin Volcano, at approximately 2:01 PM, 5:50 PM, and 8:35 PM AKDT. The first two events produced relatively ash-poor, but gas-charged, eruption clouds that reached 45,000 - 50,000 feet above sea level and apparently no or very little local ash fall. The third event generated an ash- and gas-rich plume that also rose to 45,000 - 50,000 feet and produced several inches of ash and lapilli fall over the ocean and on islands southwest of Kasatochi, including minor amounts on Adak Island, the closest island with a year-round population, about 50 miles from the volcano. Boats in the vicinity of the volcano reported 4-5 inches of coarse grained ash fall, darkening skies, and lightning, likely caused by static electricity in the ash plume. The third event was followed by about 17 hours of continuous ash emission as determined from satellite data. The cumulative volcanic cloud from Kasatochi (Fig. 1<http://www.avo.alaska.edu/image.php?id=15049>) contained a large amount of sulfur dioxide gas that was detected by the Ozone Monitoring Instrument on NASA's EOS-Aura satellite for more than a week after the eruption as the cloud circled the globe. The ash and gas cloud drifted east and interfered with air travel between Alaska and the conterminous US causing at least 40 flight cancellations and stranding many thousands of travelers. The cloud was visible for thousands of miles downwind and apparently was the cause of some brilliant sunsets over the Midwestern US. http://www.avo.alaska.edu/image.php?id=15049 This shows the spread of the SO2 from Kasatochi last August. Approx. 1.2 Mt of S were injected into the altitude range from 35,000-45,000 ft. Below 53,000 ft, the winds tend to blow from west to east and the reverse is true above 53,000 ft. Note the irregular pattern of dispersal, the southern reach into Missouri and Iowa and how, coincidentally perhaps, none of it made it to Greenland or the Arctic sea. Can we expect any better spread with a manmade release over the Arctic? Granted, Kasatochi is at 52N, so a true Arctic program would likely be carried out farther north. The excerpt from the website notes that the satellite only detected SO2 for about a week after the eruption, but since it only measures SO2 and not H2SO4, the fate of the SO2 is unknown. I think most of it descended as aerosol within a few days over the Atlantic and had little or no effect on solar forcing as the spread pattern looks suspiciously like that of typical west-to-east weather fronts that begin to show up in August. Alan stated that this eruption showed that there is no clear threshold for a climate impact from aerosols as 1.2Mt of S from this one had none. As I previously noted and as the reporting on this eruption show, this eruption is irrelevant to making such predictions for a stratospheric aerosol program. Pinatubo's 6Mt of stratospheric S caused a global temperature decrease of about 1 degree F, so as a crude approximation, 600Kt of S similarly dispersed should decrease temperature by 0.1 degree F if the relationship is linear. Temperature variations smaller than this are measureable, but due to the lack of volcanic eruptions reaching the stratosphere in recent years, there hasn't been enough data to confirm the actual temperature/S burden relationship. Ditto for other effects on climate such as precipitation and stream flow. Mt. Laki did produce both tropospheric and stratospheric aerosols and was more like what a human program would resemble with incremental injections, but the total mass of S was much greater than what we could do and I would have to review the data for this eruption more closely to see if much of the stratospheric was also the result of tropopause folding (the air goes in both directions). --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "geoengineering" group. 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