Hello Stephen,
Thanks for responding. I wrote rather carelessly I realise. My aim is to explore what we are up against, in trying to save the Arctic sea ice. Firstly I was thinking of stratospheric aerosols, rather than marine cloud brightening. I am glad that you have allowed something for higher cloud than your marine cloud obscuring the effectiveness of your brightening, increased albedo. But for stratospheric aerosols there will be plenty of cloud cover underneath, say x% - Jeff assumes 50%, although I have heard it mentioned that summer cloud cover might increase - do you know about that? This cover will obscure sunlight by x%, reducing the albedo change warming effect, but will also reduce the cooling effect of stratospheric aerosols by x%, thus cancelling out in the calculation of how much aerosol is needed. Secondly, my aim is to save the Arctic sea ice, rather than counter global warming - although Jeff included that in his calculation. Thus I am interested in the regional warming from the sea ice albedo effect. I am assuming that the aerosols are applied regionally. Of course they could be applied for the whole area above say 70 degrees latitude, whereas the sea ice is only a fraction of that area. So there is a bit of leverage. For the marine cloud brightening, you can only do it at sea, however you can most importantly cool the Gulf Stream flowing into the Arctic ocean. So may have a larger leverage, in a very benign way as it would effect little human habitation, except Iceland. Nevertheless, the albedo effect, when all the sea ice is gone, is huge. And I think it would be suicidal to delay the application of geoengineering techniques while the albedo effect is building up so rapidly towards this huge amount. Thirdly, Jeff takes an amazingly low figure of 0.46 for the albedo of ice, before dividing by 2 for cloud effect (regardless of cloud height); however he ignores the small albedo for water (slightly higher than usual because of low average sun angle). I assume the calculation is looking at pre-industrial pristine ice/snow albedo, at 80% to 90% albedo, rather than thin sea ice which could have a much lower albedo - although I find Jeff's 0.46 surprisingly low - I don't know where he got that from - nor his 0.23 "generally agreed by a number of authors". (I've not researched that.) Then we contrast the pre-industrial sea ice cover with open sea throughout summer - with albedo 10% or less. Hence I reckon 70% (=80%-10%) for the albedo change is reasonably conservative - almost exactly 3x Jeff's figure of 0.23. This gives 90 W/m-2 rather than Jeff's 30 W/m-2. Do we have an albedo expert in the house? There is a question, that I'd like answered, about the cloud cover blanket effect, keeping temperatures up, so noticeable at night. Does marine cloud brightening increase the blanket effect at night? Would aerosols increase the blanket effect at night? And lastly, there is precipitation - which could have an overall cooling effect if snow landing on sea ice through thickening, but on the other hand would leave clear skies for greater insolation in summer/daytime - less blanket in winter/nighttime. What effect would marine cloud brightening and stratospheric aerosols have on precipitation? BTW, these calculations blow away the idea that Arctic sea ice could conceivably be saved by reducing CO2 emissions. And they suggest that much of the "polar amplification of temperature" (polar warming at least twice as fast as global warming) could be due to the albedo effect. Cheers, John ----- Original Message ----- From: "Stephen Salter" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Wednesday, December 10, 2008 5:37 PM Subject: Re: [geo] Geoengineering - cloud effects > John > > I agree that high clouds will reduce the effectiveness of albedo control > of low clouds. We use a figure from Charlson Lovelock et al. that 0.18 of > the oceans have low but not high cloud and our predictions are based on > that. > > I do not understand your 70%. If the problem of double CO2 is 3.7 > watts/m2 and there is a 24-hour world of average of 340 watts/m2 we have > to increase reflection by 3.7 parts in 340 which is 1.088 %. If we have > the right clouds in only one fifth of the surface we have to increase > their reflectivity by 18.5 parts in 340. > > Stephen > > John Nissen wrote: >> Jeff Ridley calculated albedo effect, as follows: >> The effect on global temperatures can be calculated as follows >> [quote] >> >> 1. The albedo of summer Arctic sea ice is 0.46 >> 2. Clouds are the same albedo as ice and partially obscure the ice >> reducing to an effective albedo of 0.23 (a figure reached by a >> number of authors). >> 3. The summer solar input at the Arctic is about 400 W/m2, thus if all >> the ice were removed then the balance at the surface would be >> 0.23 x 400 = 92 W/m2. This is a large local effect - roughly >> equivalent tot the change in heat from a cloudy day to a clear day. >> 4. Because of the annual cycle and no sunlight in winter we take a third >> of this 30 W/m2 on the annual average >> 5. Summer sea ice covers only 1.6% of the world's surface so the global >> effect is 30 x 0.016 = 0.48 W/m2 This is not insignificant! >> 6. A rule of thumb is that 1 W/m2 forcing is equivalent to 1 Kelvin >> temperature rise. >> >> Thus the direct effect of removal of all Arctic sea ice would raise >> global temperatures by about 0.5 C. The removal of say 20% extra ice in >> 2007 would thus be expected to have a 0.1 C impact on global >> temperatures. >> [end quote] >> I would use 70% as the albedo flip, between ice/snow (~80%) and water >> (<10%). But Jeff reduces the albedo change to about a third, at 0.23%, >> taking into account cloud cover. >> When we apply aerosols in the stratosphere, they will become less >> effective due to clouds in the troposphere. Thus to exactly counter the >> albedo flip where it happens, aerosols would have to reduce insolation by >> 70%, I think. >> Any comments? >> John >> >> >> > > > -- > Emeritus Professor of Engineering Design > School of Engineering and Electronics > University of Edinburgh > Mayfield Road > Edinburgh EH9 3JL > Scotland > tel +44 131 650 5704 > fax +44 131 650 5702 > Mobile 07795 203 195 > [EMAIL PROTECTED] > http://www.see.ed.ac.uk/~shs > > The University of Edinburgh is a charitable body, registered in > Scotland, with registration number SC005336. > > --~--~---------~--~----~------------~-------~--~----~ 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] For more options, visit this group at http://groups.google.com/group/geoengineering?hl=en -~----------~----~----~----~------~----~------~--~---
