Hi Stephen--My wording must have been confusing. For stratospheric injections at low latitudes, the lifetime is 1-2 years. The aerosols do move poleward and are carried into the troposphere in mid and high latitudes. This is one approach to trying to limit global climate change, and, as David Keith says, studies indicate that these cool the polar regions, though perhaps not in the stratosphere.
Your cloud brightening approach is also to limit global warming. I'd also suggest that we could offset some of the global warming by sulfate aerosols out over vast ocean areas instead of sulfate's present dominance over, now, southeastern Asia, China, etc.--so keeping or modestly enhancing the present cooling offset. [And reducing cirrus may also be a viable approach.] A third approach is to cool the poles (and this might be good for regional purposes alone), but cooling also pulls heat out of lower latitudes and helps to cool them somewhat. The Caldeira-Wood shows it works conceptually (they reduced solar constant) and Robock et al. injected SO2 into stratosphere to do (but the full year injection of SO2/SO4 likely spread some to lower latitudes and the monsoons were affected). One thing Robock et al. found was that the lifetime of sulfate in the polar stratosphere is about two months, and so that means that the potential 100 to 1 advantage of stratospheric sulfate is not valid, and we're down to 6 to 1 compared to surface-based approaches such as CCN or microbubbles to cool incoming waters, sulfate or something similar over Arctic area, surface brightening by microbubbles, etc.--noting that such approaches are only needed (and effective) for the few months per year when the Sun is well up in the sky. As David Keith also says, there is a lot of research to be done to determine which approaches or alone or in different variants might work, or be effective or ineffective and have unintended consequences, much less how such an approach or set of approaches might be integrated with mitigation, adaptation, suffering, etc. Best, Mike MacCracken On 3/18/12 12:52 PM, "Stephen Salter" <[email protected]> wrote: > Mike > > I had thought that the plan was stratospheric aerosol to be released at > low latitudes and would slowly migrate to the poles where is would > gracefully descend. If you can be sure that it will all have gone in 10 > days then my concerns vanish. But if the air cannot get through the > water surface how can the aerosol it carries get there? It will form a > blanket even if it is a very low one. > > A short life would mean that we do not have to worry about methane > release. But can we do enough to cool the rest of the planet? Perhaps > Jon Egil can tell us about blanket lifetime. > > Stephen > > Mike MacCracken wrote: >> The Robock et al simulations of an Arctic injection found that the lifetime >> of particles in the lower Arctic stratosphere was only two months. In that >> one would only need particles up during the sunlit season (say three months, >> for only really helps after the sea ice surface has melted and the sun is >> high in the sky). During the relatively calm weather of Arctic summer, the >> lifetime of tropospheric sulfate, for example‹and quite possibly sea salt >> CCN--emitted above the inversion is likely 10 days or so. It is not at all >> clear to me that the 6 to 1 or so lifetime advantage of the lower >> stratosphere is really worth the effort to loft the aerosols. >> >> And on the temperature rise in the polar stratosphere, I would hope any >> calculation of the effects of the sulfate/dust injection only put it in >> during the sunlit season‹obviously, there would be no effect on solar >> radiation during the polar night, so, with a two month lifetime of aerosols >> there, it makes absolutely no sense to be lofting anything for about two >> thirds of the year. And so likely no effect on winter temperatures (although >> warming the coldest part of the polar winter stratosphere might well help to >> prevent an ozone hole from forming). >> >> So, I think a tropospheric brightening approach is likely the better option. >> Whether it can be done with just CCN or might also need sulfate seems to me >> worth investigating (what one needs may well be not just cloud brightening, >> but also clear sky aerosol loading). >> >> Best, Mike >> >> ***** >> >> On 3/17/12 8:41 PM, "Ken Caldeira" <[email protected]> wrote: >> >> >>> That is just misleading. The third attachment is a top-of-atmosphere >>> radiation balance on the email I am responding to shows shortwave radiation. >>> >>> The attached figure shows the corresponding temperature field from the same >>> simulation for the same time period. Note Arctic cooling. >>> >>> Also, we should not focus on individual regional blobs of color in an >>> average >>> of a single decade from a single simulation. >>> >>> The paper these figures came from is here: >>> http://www.atmos-chem-phys.net/10/5999/2010/acp-10-5999-2010.pdf >>> >>> _______________ >>> Ken Caldeira >>> >>> Carnegie Institution Dept of Global Ecology >>> 260 Panama Street, Stanford, CA 94305 USA >>> +1 650 704 7212 [email protected] >>> http://dge.stanford.edu/labs/caldeiralab @kencaldeira >>> >>> YouTube: >>> <http://www.youtube.com/watch?v=a9LaYCbYCxo> Climate change and the >>> transition from coal to low-carbon electricity >>> <http://www.youtube.com/watch?v=a9LaYCbYCxo> >>> Crop yields in a geoengineered climate >>> <http://www.youtube.com/watch?v=-0LCXNoIu-c> >>> >>> >>> >>> >>> On Sat, Mar 17, 2012 at 1:31 PM, Andrew Lockley <[email protected]> >>> wrote: >>> >>>> Hi >>>> >>>> Here are some model outputs which Stephen sent me. These appear to show >>>> localized arctic warming in geoengineering simulations. This could be due >>>> to >>>> winter effects. >>>> >>>> I assume this is the source for the controversial figure in the BBC quote >>>> >>>> A >>>> >> >> -- 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.
