Andrew A few comments in response to this and the subsequent comment by
1. This vector representation is useful way to think about trade-offs when the climate response to CO2 and SRM is reasonably linear. This stuff is published as: Juan Moreno-Cruz, Katharine Ricke and David W. Keith. (2011). A simple model to account for regional inequalities in the effectiveness of solar radiation management. Climatic Change, doi: 10.1007/s10584-011-0103-z. (PDF)<http://www.keith.seas.harvard.edu/papers/131.Moreno-Cruz.Inequality.SRM.e.pdf>. Despite the hype about nonlinearity, models are quite linear in the region of interest, see the appendix to the paper. 2. I was surprised by our results. I expected the trade-offs to be much stronger. Doing this work pushed me to realize that SRM can do a substantially better job of compensating CO2-driven climate change than I had expected. (Of course, it does nothing about the geochemical impacts of CO2 such as ocean acidification.) 3. Yes, you can consider quantities other than temperature and precip; and quantities like soil moisture are certainly important. 4. Stephen Salter imply that these results were somehow particular to stratospheric sulfates, saying: I think that you must be referring to geo-engineering with stratospheric sulphur. With tropospheric salt you can vary precipitation in both directions by choosing the time and place to spray. This analysis is applicable to both. It is certainly true that if sea salt aerosol can be effectively used to alter cloud albedo over large areas--a proposition which is still quite uncertain--then it could be used to reduce (them eliminate) the trade-offs. We looked at exactly this in a more recent paper examining how trade-offs can be reduced if you were able to adjust the intensity of SRM forcing at different locations in seasons: Douglas G. MacMartin, David W. Keith, Ben Kravitz, and Ken Caldeira. (2012). Managing tradeoffs in geoengineering through optimal choice of non-uniform radiative forcing. Nature Climate Change, doi: 10.1038/NCLIMATE1722. (PDF<http://www.keith.seas.harvard.edu/preprints/158.MacMartin.etal.ManagingTradeoffsThroughNonRadForc.p.pdf>). Note that this paper explicitly looks at something people in this blog often ask about which is the ability to tune SRM to focus on restoring Arctic sea ice. 5. Finally, I do not understand your argument about soil moisture. Evaporation always equals precipitation the global mean. All else equal-- and it probably will not be--one expects variability to go down (not up) as you weaken the hydrological cycle. So my back of the envelope physics points the opposite way to yours. We look at this in one of the papers with Kate Ricke and found that at least the one case we looked at variability did go down. Of course, model do not equal reality. Yours, David From: geoengineering@googlegroups.com [mailto:geoengineering@googlegroups.com] On Behalf Of Andrew Lockley Sent: Monday, November 26, 2012 7:08 AM To: David Keith; geoengineering Subject: [geo] Your Vector diagram David I remember the excellent vector diagram lecture you gave at Oxford. In it you represented temperature and precipitation on a vector.diagram and showed that both cannot be simultaneously corrected exactly by geoengineering, but that the mismatch was small. However, that would leave us in a world which was either slightly drier or slightly warmer than in a non-geoeng world - or a combination of both. My concern is that things might be a bit more serious than that. If we consider a warmer world with the same level of precipitation, the surface evaporation world be higher and the relative atmospheric humidity would (I think) be lower. As a result, soil wetness may be very much lower, as evapotranspiration would be higher. If rainfall patterns were perturbed, we might additionally get more variability in both wetness and precipitation. So we could end up in a world with much drier soils, and possibly heavier storms, too. Should your vectors therefore be soil wetness vs temperature, not precipitation vs temp? Making a bad call on this could really hit agricultural outputs. A -- 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<mailto:geoengineering@googlegroups.com>. To unsubscribe from this group, send email to geoengineering+unsubscr...@googlegroups.com<mailto:geoengineering+unsubscr...@googlegroups.com>. 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 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.
