The acid deposition problem may well carry less weigh than the contribution to trop chemistry and air pollution.
D From: [email protected] [mailto:[email protected]] On Behalf Of Mike MacCracken Sent: Monday, March 19, 2012 10:43 AM To: Alan Robock Cc: Stephen Salter; Ken Caldeira; Andrew Lockley; Geoengineering; [email protected] Subject: Re: [geo] Source on SRM causing warming Hi Alan-Well, I got the 2 months number from your paper-and used that. Interesting that a more detailed evaluation indicates that the lifetime in summer is longer. I think longer times than a week might well be possible in the troposphere by choosing injection times and meteorological conditions, so I'll correct to ratio of 10 to 20 to 1 for stratosphere, but noting that there might not be a need for the aerosols to be there for 4 months, so the longer stratospheric time might be real, but not necessarily relevant. On the issue of the amount of pollution, a couple of comments. Aside from arguments over whether it is the sulfate or things with the sulfate causing the health effects that have been associated with sulfate from coal-fired power plants (for any sulfate injection it would be pure SO2 or whatever without all the other combustion products-or perhaps one might use sea salt or something else), due to past coal use in Europe and Soviet Union, we have a reasonable sense of what the impacts from sulfate might be. With summer only injections, one would avoid much of the acid deposition problem (shorter season, and not accumulating on snow and running off all at once). One would also be choosing emissions times to have air flows that carry the SO2/sulfate over the Arctic and not over the land. So, yes, will be some impacts, but can possibly be moderated to be less than, as your study suggested, the unintended side effects of stratospheric SO2. I am all for considering and comparing the full range of possible approaches (stratospheric, tropospheric, surface, etc.--separately and/or in combination). With some sense of what might be able to be done and the potential impacts, the next step is a comparative risk evaluation, as for all climate engineering. Without doing something, it is hard to see how the Arctic can be kept from very extensive thawing and loss of the climate that we have. With it, yes, some different types of impacts due to the engineering effort, but, assuming it works, a good deal less, or slowed climate impact on the Arctic, and if loss of glacier/ice sheet mass can be slowed (or reversed-as Caldeira-Wood study suggested), then a benefit to the global community. With some sense of relative risks of various choices, it becomes a political decision, with its many considerations. I happen to think that, if any climate engineering is to be considered, having a focused goal such as limiting polar warming and associated impacts would be more likely to be considered as a first step than jumping straight to a global counter-balancing approach, but that is just my opinion. In any case, rather than saying what is or is not acceptable, it seems to me our responsibility is to explore and evaluate options and then it is the governance system that decides about the tradeoffs of pollution versus un- (or under-) moderated Arctic change (and everything else). Mike On 3/19/12 12:03 PM, "Alan Robock" <[email protected]> wrote: Dear Mike, I don't know how you do this 6 to 1 calculation. We found that the e-folding time for stratospheric aerosols in the Arctic s 2-4 months, with 4 months in the summer, the relevant time. (see http://climate.envsci.rutgers.edu/pdf/2008JD010050small.pdf ) If we compare this to the lifetime of tropospheric aerosols, on week, and add a week to the 4 months for their tropospheric time, the ratio is 130 days to 7 days, which is 19 to 1, not 6 to 1. Furthermore, the health effects of additional tropospheric pollution are not acceptable, in my opinion. Alan [On sabbatical for current academic year. The best way to contact me is by email, [email protected], or at 732-881-1610 (cell).] Alan Robock, Professor II (Distinguished Professor) Editor, Reviews of Geophysics Director, Meteorology Undergraduate Program Associate Director, Center for Environmental Prediction Department of Environmental Sciences Phone: +1-732-932-9800 x6222 Rutgers University Fax: +1-732-932-8644 14 College Farm Road E-mail: [email protected] New Brunswick, NJ 08901-8551 USA http://envsci.rutgers.edu/~robock On 3/18/2012 5:49 PM, Mike MacCracken wrote: 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]> <mailto:[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]> <mailto:[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]> <mailto:[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]<mailto:[email protected]>. To unsubscribe from this group, send email to [email protected]<mailto:[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]. For more options, visit this group at http://groups.google.com/group/geoengineering?hl=en.
