Andrew - I agree that if one were to only pick two variables, then temperature and soil moisture (or P-E as a reasonable proxy for it) might even be better ones to pick than temperature and precipitation. However, given that geoengineering will change both precip and evaporation, it isn't obvious to me whether the degree of compensation on P-E or soil moisture will be obviously better or worse than the degree of compensation on precipitation alone; it will certainly be different. This is worth looking at.
More generally, one could include more regionally specific concerns (e.g. whether the local crops are more prone to heat stress or to water stress, whether this is more true in summer or winter, etc). Though that is a big and challenging subject that would be hard to get agreement on. At any rate, ideas like the vector diagram are still applicable regardless of what one takes for metric, and regardless of whether one uses stratospheric aerosols or cloud brightening or whatever. d From: geoengineering@googlegroups.com [mailto:geoengineering@googlegroups.com] On Behalf Of Andrew Lockley Sent: Friday, November 30, 2012 5:04 PM To: david_ke...@harvard.edu Cc: geoengineering Subject: RE: [geo] Your Vector diagram David Thanks for your response. My comments regarding soil moisture related to precipitation and evaporation over land specifically. Land isn't a closed system, as river flows provide a balancing term. Let us consider geoengineering to get us back to a ore industrial temperature or precipitation level. With less rain than baseline and a temperature which is the same as baseline, soils will tend to be drier. My point is simply that the change in soil moisture may be dramatically more than the change in precipitation. Obviously, the global fluxes have to balance - but equilibrium may be reached with much drier soils than in the baseline case, and with a proportional change much greater than that of precipitation. In the alternative case, where temperature is higher than baseline and precipitation (over land) constant as compared to baseline, relative humidity is lower, evaporation is stronger and soils are still drier than in the baseline case. I'm pretty confident of the sign - the precipitation reduction is widely acknowledged. My point is simply that the soil moisture effect might be a lot worse than than the precipitation effect - to the extent where ecosystem and agricultural effects are severe. Am I missing something? Surely precipitation doesn't actually matter so much - it's sustained soil moisture which plants actually use. Runoff only really affects rivers and floodplains. Thanks A On Nov 30, 2012 10:26 PM, "David Keith" <david_ke...@harvard.edu> wrote: 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.p df> . 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.ManagingTrad eoffsThroughNonRadForc.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. 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