Hi Stephen, The first number I found when I re-googled this was 13%, Table 1 of https://journals.ametsoc.org/doi/full/10.1175/2011JCLI3972.1. But regardless, since the statistics are not uniform across the ocean, the patchiness doesn’t average out, and I think it is fair to say that the radiative forcing from MCB will be more spatially heterogeneous than that from SAI, and hence there is more potential for the response to also be. If the goal is to keep the climate as close as possible to the climate one would have had with the same global mean temperature but lower CO2 (certainly a plausible goal, not the only plausible goal), then not obvious without actually conducting research which method leads to better compensation; presumably a combination of MCB and SAI would do better than either alone (obviously). I think we would both agree that it would be nice to have research so that we can start answering questions like this instead of just guessing.
Agree completely that there are many more advantages and disadvantages than could possibly be listed in a table (we were considerably over-length as it was, so I wound up having to condense the table, only listing one key advantage or disadvantage relative to stratospheric sulfate). We were trying to keep this high-level and not get in to a lengthy discussion of pros and cons that is, unfortunately, almost entirely hypothetical at this point. Two major advantages of MCB that you didn’t list are: 1) More likely to be societally acceptable (this is related to your first point, but different in that the perceived risk might be much more important than any actual risk), and 2) More testable; as an engineer I would view that as a pretty major plus. (That is, for MCB you can do a full-scale radiative forcing over a small area, whereas for SAI the time constants involved mean that any test that is “full-scale” in radiative forcing is also global, and hence not acceptable pre-deployment; this has implications for research strategy (paper currently being drafted so don’t expect to see it soon).) Insofar as both techniques would lead to less of a temperature drop in the event of a volcanic eruption than would have happened absent any deployment, I don’t really think that’s a big advantage compared to the open question about how well either technique can compensate for climate change. I also doubt that the fossil fuel usage to bring material to the stratosphere is a significant factor in choosing between them (assuming we ever are in a position to choose between them). doug From: geoengineering@googlegroups.com [mailto:geoengineering@googlegroups.com] On Behalf Of Stephen Salter Sent: Tuesday, April 03, 2018 9:46 AM To: geoengineering@googlegroups.com Subject: Re: [geo] Solar geoengineering as part of an overall strategy for meeting the 1.5°C Paris target Hi All In the recent Phil. Trans. Roy. Soc. special issue on Solar Geoengineering to meet Paris the Paris target at <http://rsta.royalsocietypublishing.org/content/376/2119/20160454> http://rsta.royalsocietypublishing.org/content/376/2119/20160454 table 1 lists, as a disadvantage relative to stratospheric sulphur, that marine stratospheric clouds cover 10% of the Earth which means that marine cloud brightening is ‘patchy’. No reference to this number is given and it contradicts the 18% ‘low not high clouds’ mentioned by Charlson and Lovelock in their 1987 paper about the CLAW hypothesis and the effects of dimethyl-sulphide from phytoplankton. There is also the Jones Hayward Boucher paper of 2009 which concluded that marine cloud brightening over the best 3% of the oceans would offset about half the thermal damage since preindustrial times. However as clouds move the patchiness is smoothed out. Furthermore the life of condensation nuclei will be approximately half the mean time between rain showers so clear skies mean longer nuclei lifetimes. The Twomey effect is logarithmic so it is better to have half the dose over double the area and spraying under clear skies gives nuclei a chance to spread. I argue that some short-term patchiness is much less serious if the patches are our patches. Finally the recent paper by Ahlm et al. at doi:10.5194/aco-2107-484 suggests that marine cloud brightening works much better than I would have expected with no clouds. The Royal Society table did not have any space for the disadvantages of stratospheric sulphur relative to marine cloud brightening and I am reluctant to knock any technology about which I am not an expert. However if I was forced to suggest entries for the contents of a disadvantage table they would be as follows: Nasty acid everywhere compared with medicinally beneficial salt mainly over sea. Very little control over the areas affected. Very long shut-down times in the event of a volcanic eruption leading to over-cooling. Reflection of outgoing infra-red during polar winters leading to warming. Use of fossil fuel from aircraft release rather than the use of energy from the wind. Even though recent climate modelling has not yet used monodisperse spray with sizes of both liquid drops and dry salt residues both in the Greenfield gap and has not varied spray patterns with seasons or surface temperature anomalies, the majority of ensemble results show that as well as cooling there is a trend for dry places to become wetter and wet ones to become drier. We can hope that we can learn how to improve on this trend. I suggest that the choice of time and place to do marine cloud brightening will confer advantages similar to the use of an accelerator, steering and brakes on road vehicles. On 02-Apr-18 9:16 PM, Andrew Lockley wrote: https://keith.seas.harvard.edu/publications/solar-geoengineering-part-overall-strategy-meeting-15%C2%B0c-paris-target Solar geoengineering as part of an overall strategy for meeting the 1.5°C Paris target Citation: Douglas G. MacMartin, Katharine L. Ricke, and David W. Keith. 4/2/2018. “ <https://keith.seas.harvard.edu/publications/solar-geoengineering-part-overall-strategy-meeting-15%C2%B0c-paris-target> Solar geoengineering as part of an overall strategy for meeting the 1.5°C Paris target.” Philosophical Transactions of the Royal Society, 376, 2119. <https://keith.seas.harvard.edu/publications/solar-geoengineering-part-overall-strategy-meeting-15%C2%B0c-paris-target> Download Citation Download <https://keith.seas.harvard.edu/files/tkg/files/macmartin_ricke_keith_ptrs.pdf> macmartin_ricke_keith_ptrs.pdf 1.03 MB Abstract: Solar geoengineering refers to deliberately reducing net radiative forcing by reflecting some sunlight back to space, in order to reduce anthropogenic climate changes; a possible such approach would be adding aerosols to the stratosphere. If future mitigation proves insufficient to limit the rise in global mean temperature to less than 1.5°C above preindustrial, it is plausible that some additional and limited deployment of solar geoengineering could reduce climate damages. That is, these approaches could eventually be considered as part of an overall strategy to manage the risks of climate change, combining emissions reduction, net-negative emissions technologies and solar geoengineering to meet climate goals. We first provide a physical science review of current research, research trends and some of the key gaps in knowledge that would need to be addressed to support informed decisions. Next, since few climate model simulations have considered these limited-deployment scenarios, we synthesize prior results to assess the projected response if solar geoengineering were used to limit global mean temperature to 1.5°C above preindustrial in an overshoot scenario that would otherwise peak near 3°C. While there are some important differences, the resulting climate is closer in many respects to a climate where the 1.5°C target is achieved through mitigation alone than either is to the 3◦C climate with no geoengineering. This holds for both regional temperature and precipitation changes; indeed, there are no regions where a majority of models project that this moderate level of geoengineering would produce a statistically significant shift in precipitation further away from preindustrial levels. This article is part of the theme issue ‘The Paris Agreement: understanding the physical and social challenges for a warming world of 1.5°C above pre-industrial levels’. See also: <https://keith.seas.harvard.edu/researchareas/solar-geoengineering> Solar Geoengineering, <https://keith.seas.harvard.edu/people/david-keith-0> David Keith, <https://keith.seas.harvard.edu/publications-type/academic> Academic Publication -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To unsubscribe from this group and stop receiving emails from it, send an email to geoengineering+unsubscr...@googlegroups.com <mailto:geoengineering+unsubscr...@googlegroups.com> . To post to this group, send email to geoengineering@googlegroups.com <mailto:geoengineering@googlegroups.com> . Visit this group at https://groups.google.com/group/geoengineering. For more options, visit https://groups.google.com/d/optout. -- Emeritus Professor of Engineering Design School of Engineering Mayfield Road University of Edinburgh EH9 3DW Scotland -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To unsubscribe from this group and stop receiving emails from it, send an email to geoengineering+unsubscr...@googlegroups.com. To post to this group, send email to geoengineering@googlegroups.com <mailto:geoengineering@googlegroups.com> . Visit this group at https://groups.google.com/group/geoengineering. For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To unsubscribe from this group and stop receiving emails from it, send an email to geoengineering+unsubscr...@googlegroups.com. To post to this group, send email to geoengineering@googlegroups.com. Visit this group at https://groups.google.com/group/geoengineering. For more options, visit https://groups.google.com/d/optout.
