A couple of points: 1. On the angle issue, this is of course taken into consideration in calculating how much solar radiation reaches the Arctic at any given time‹and rays just passing tangentially through will not count much at all. That the actual incident light in high latitudes in summer, integrated over the day, is roughly as much as at the equator (William Sellers book of roughly 4 decades ago on climate has a nice diagram) means that there is plenty of time when aerosols could backscatter solar radiation to space. 2. On the backscattering from reflected light, roughly speaking sulfate aerosols forward scatter about ten times as much as they back scatter. That would likely still be the case for the upward scattered radiation. The cancellation would only then occur if the surface albedo were about 1, and if it is this high, then we should not be using aerosols anyway as the natural system is doing just fine on its own. The time to use aerosols is when the surface-troposphere albedo is dropping lower, so the cancelling effect would be smaller. And I don¹t think one would have to do the mentioned iteration, again, unless the surface-troposphere albedo is high. I guess what would be really nice to have is a particle that has a corner reflector on top (so perfect reflection with no scattering) and is absorbing below, so the energy is taken up high in the atmosphere where most is likely to be radiated to space by the layer¹s CO2 (some does go down, but one is above most of the water vapor, so its GH effect is very small).
Mike On 5/12/09 9:53 AM, "Andrew Lockley" <andrew.lock...@gmail.com> wrote: > You'd have to calculate this across the whole globe, surely? If the whole > atmos was affected, then this would mean the Earth turned from being a sharp > round disc to a bigger, hazy one? But, the evidence from Pinatubo surely > demonstrates that this doesn't cause a problem, it still cools down. > > However, can I ask if the backscattering from reflected light has been > considered? Over the tropics, where it's not snowy, this is not very > important, but over the ice, where about 90pc of the light comes back, then > it's massively important and (seems to) cancel out 90% of the aerosol's > effects (you'd have to iterate that a few times, of course). That tangental > ray effect could then end up being very significant, and if it's more than 10% > of the net effect then aersols will heat, not cool the arctic. > > Or perhaps I'm just being thick. > > A > > 2009/5/12 Bonnelle Denis <dbonne...@ra.ccomptes.fr> >> I agree that my point wasn't considering seasonal changes in the earth's >> orientation relatively to the sun rays (I was in fact dealing with equinox >> times), and that mid-summer conditions are much more favorable for the most >> polar locations. >> >> However, at each time of the summer, there exist locations where the lowest >> point of the sun's daily trajectory is very low above the horizon, and in >> such locations the effect of aerosol creation would be a notable increase in >> the received luminous power during several hours around midnight. It is far >> from sure that this would be offset by the reduction in the received heat >> around midday (remember my point that a tangential ray would propagate >> through many hundreds km of the stratosphere, when an oblique one would only >> get through some tens km or air). >> >> At mid summer (and during at least several weeks before and after the 21st of >> June), these "dangerous" locations are the ones just north of the arctic >> polar circle (a central slice of Greenland, and lands near the Northern >> coasts of Canada and Siberia - mind the permafrost). >> >> If these regions are to be avoided, would it be possible to control very >> precisely the location (are there significant shifts of air masses from one >> latitude of the stratosphere to another?) and the time (the particle size >> control issue) of the aerosols to be created? >> >> Denis Bonnelle. >> >> -----Message d'origine----- >> De : John Gorman [mailto:gorm...@waitrose.com] >> Envoyé : mardi 12 mai 2009 11:25 >> À : xbenf...@aol.com; Bonnelle Denis; geoengineering@googlegroups.com >> Objet : Re: [geo] Re: Balancing the pros and cons of geoengineering >> >> Although I was initially worried by Denis's point that arctic aerosols will >> capture some rays that would otherwise just pass tangentially through the >> stratosphere, I have now done some geometry and believe that this will only >> apply to about 0.2% of the incident sunlight on the Arctic at midsummer. >> >> This is because the atmosphere is thin in comparison with the radius of the >> earth. >> >> This applies of course to all aerosols SO2 or SiO2. My main argument for >> suggesting silica (Greg's diatoms) is that we might be able to control >> particle size much more exactly. >> >> John Gorman >> >> >> ----- Original Message ----- >> From: <xbenf...@aol.com> >> To: <gorm...@waitrose.com>; <dbonne...@ra.ccomptes.fr>; >> <geoengineering@googlegroups.com> >> Sent: Monday, May 11, 2009 4:33 PM >> Subject: [geo] Re: Balancing the pros and cons of geoengineering >> >> >> >> All: >> >> Bonnelle Denis is right that a detailed study of aerosol reflections >> needs doing. Someone may wish to use research time on it, but without >> any funding it's difficult to mount a determined attack on the many >> parameters that need varying. >> >> The issue of particle size demands some actual experiments, to see what >> happens to candidate aerosols at the actual altitudes considered. How >> much particle growth occurs, under what conditions of humidity, >> pressure, etc? What's the true fallout time vs altitude and particle >> size? There's a whole agenda here. >> >> I do wonder how much Lowell Wood and collaborators are doing on this, >> but Lowell is mum. >> >> Gregory Benford >> >> -----Original Message----- >> From: John Gorman <gorm...@waitrose.com> >> To: Bonnelle Denis <dbonne...@ra.ccomptes.fr>; >> geoengineering@googlegroups.com >> Sent: Mon, 11 May 2009 1:59 am >> Subject: [geo] Re: Balancing the pros and cons of geoengineering >> >> I have to admit I hadnt thought of that aspect of >> aerosols in the arctic. >> >> To Gregory Benfold -What do you think >> ? >> >> John Gorman >> >> ----- Original Message ----- >> From: >> Bonnelle Denis >> To: gorm...@waitrose.com ; andrew.lock...@gmail.com ; John Nissen ; >> geoengineer...@googlegroups.com >> >> Sent: Monday, May 11, 2009 9:42 AM >> Subject: [geo] Re: Balancing the pros and >> cons of geoengineering >> >> >> >> Dear >> all, >> >> (please >> forgive me if the following >> geometrical arguments have already been >> discussed). >> >> The >> positive feedback (albedo, methane, etc.) rationale for focusing >> about the >> Arctic is doubtlessly great. But the geometry is not very favorable, >> especially if very tangential sun rays are concerned, which is more >> often the >> case near the poles than near the equator. >> >> The >> most dramatic case is the one of the most tangential rays which: 1 - >> without >> geoengineering - would have traveled horizontally through the >> stratosphere, >> unharmed, and which: 2 - would be diffracted by the silica, half >> upwards but >> also half downwards, giving their heat to the earth. Seen from the >> sun, the >> relevant cross-section is around 10 or 20 km (the considered >> stratospheric >> layer's thickness) >> multiplied by 2000 or 3000 km (the considered bow length). Such a >> result >> (several 10,000 km²) is not negligible when compared to the whole >> target >> cross-section (the same 2000 or 3000 km, multiplied by 300 or 400 km >> which is >> the width, seen from the sun, of the true useful target region). In >> addition, >> the effect in our x0,000 km² region will be more intense, as the rays >> which >> travel quite horizontally through the stratosphere will meet much >> more silica >> than those which make a larger angle with the >> horizontal. >> >> And >> even >> in the latter case (i.e., in all the target region, but mainly >> for sun >> rays which will reach the atmosphere with a quite small angle with >> the >> horizontal), an effect of the silica will be to increase the >> proportion of >> such rays which will be redirected towards the ground in a rather >> vertical >> direction, instead of coming quite tangentially (the blue sky will be >> brighter). Thus, various effects will have to be considered: lesser >> absorption >> in various layers of the atmosphere, lesser reflexion on the ocean >> surface, >> deeper penetration into the ocean, etc. It doesn't seem clear to me, >> whether >> such undesired effects will be lower than the desired fact that half >> of such >> diffracted rays will be redirected upwards, i.e. outwards of the >> earth >> climatic machine. >> >> Best >> regards, >> >> Denis >> Bonnelle. >> denis.bonne...@normalesup.org >> >> >> >> >> De : >> geoengineering@googlegroups.com >> [mailto:geoengineer...@googlegroups.com] De >> la part de John Gorman >> Envoyé : lundi 11 mai 2009 >> 09:45 >> À : andrew.lock...@gmail.com; John Nissen; >> geoengineer...@googlegroups.com >> Objet : [geo] Re: Balancing the >> pros and cons of geoengineering >> >> >> I am thinking of >> how to get funding for in-lab Evaluation of Tetra >> Ethyl Silicate Dissolved in Aviation Kerosene As a Means of 0D >> Distributing >> Stratospheric Aerosols for Geoenginering. >> The two >> points below are relevant to this discussion but a bit muddled as >> this is a >> rehash of my submission to the Royal Society >> 1)Possible >> Advantages of Silica. >> Particle >> size. At these submicron sizes it is the size of the particle which >> defines the wavelength of light which is reflected/diffracted. There >> have been several papers, which have pointed out the difficulty of >> controlling >> sulphuric acid droplet size and the problem of agglomeration of the >> droplets. (Papers include that by Tilmes/Robock in the Royal >> Society's >> Philosophical Transactions) >> It seems >> logical that the concentration of Tetra ethyl silicate in aviation >> fuel would >> define the size of silica particles produced on burning. If so, the >> particle size could be selected for maximum reduction in net >> radiation. >> There would then be less material and fewer particles/droplets for >> the same >> level of global cooling. >> >> >> 2)The most likely >> first application of a stratospheric aerosol sunscreen is that >> proposed by >> Gregory Benfold "Saving >> the Arctic". >> Combined with the >> aircraft distribution system, the proposal would be to spread the >> aerosol by >> aircraft flying between 40 and 60,000 ft. from the time of first >> Arctic >> daylight (April approximately) >> until late July >> approximately. >> >> >> Ideally for very >> long stratospheric life, aerosols need to be injected at about 80,000 >> ft. If >> they are only injected at 50,000 ft. they will fall out of the >> atmosphere in >> about three months. (Ken Caldera's lecture available on U tube). >> In this case that is exactly what we want so that they would fall out >> by the >> end of the Arctic summer and would not be present during the winter >> --. >> >> >> Most of the arguments that aerosols will damage the ozone layer >> assume that >> the aerosols are injected high in the stratosphere for long life. In >> this case most of the injection would not reach the ozone layer. In >> addition the aerosols would no longer be present in winter when the >> effect is >> greatest. >> >> It seems very likely that implementation of this type would succeed >> in "saving >> the Arctic". In particular the target would be to eliminate >> significant >> melting of the Greenland ice sheet or sudden loss of parts of it. The >> same >> principle could then be applied to Antarctica. >> >> >> The target should be zero sea level rise. If this could be achieved >> the >> saving in=2 >> 0costs of construction, relocating populations and lives >> lost in >> flood disasters would be absolutely enormous. >> john >> Gorman >> >> ps >> this is a really good discussion -by everyone. >> >> >> >> >> >> > > > > > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "geoengineering" group. To post to this group, send email to geoengineering@googlegroups.com To unsubscribe from this group, send email to geoengineering+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/geoengineering?hl=en -~----------~----~----~----~------~----~------~--~---
