Hi Ken, I agree, analysis of G1 and G2 would have strengthened the argument somewhat. We did analyse the G3 and G3S HadGEM2-ES simulations and found a statistically significant difference in temperature-independent precipitation changes there. We could have done similar tests for G1 and G2 with more models, of course. We also did some radiative transfer calculations including the effect of the stratospheric aerosols, which gave tropospheric heating changes consistent with the magnitude of precipitation reduction we saw. We didn't explicitly consider pattern changes in temperature, but the agreement between the radiative transfer results and the GCM results suggests it's not a major effect. The agreement isn't perfect, though, and we did rather neglect to discuss the possibility of the pattern of temperature changes being a possible cause of this.
We didn't really touch upon regional changes in precipitation. I find Cao et al's results really useful because of their discussion of the regional changes and the fact the regression approach does quite well in simulating them. Cheers Angus On Sunday, 6 March 2016 17:27:12 UTC, kcaldeira wrote: > > It is not clear to me that Ferraro et al (2016) were able to exclude the > possibility that some of these precipitation changes were associated with > changes in the spatial pattern of near-surface air temperatures, and not > due directly to the presence of aerosols. > > It would seem that to make a more compelling case, they would need to do > the same analysis with G1 and G2, using solar dimming instead of aerosols, > and show that their methodology did not project a similar a reduction in > precipitation as a consequence of the reduction in solar intensity. > > I am not questioning the validity of their hypothesis, but rather the > strength of evidence that they supply in support of their hypothesis. It is > seems to me that their methods could yield similar results even if their > hypothesis turns out to be wrong. (Forgive me if they did the same analysis > using G1 and G2 and I somehow missed it.) > > --- > > Now that I have written this, I realize that Long Cao's paper from late > last year is relevant to Ferraro et al's results. > > We looked at simulations that changes solar intensity and CO2 but did not > use aerosols (see attached). > > http://onlinelibrary.wiley.com/doi/10.1002/2015JD023901/abstract > > Our analysis indicated that in our simulations a reduction in solar > intensity (at constant temperature) would lead to a small statistically > insignificant reduction in global mean precipitation but a statistically > significant reduction in land runoff. > > Best, > Ken > > _______________ > Ken Caldeira > > Carnegie Institution for Science > Dept of Global Ecology > 260 Panama Street, Stanford, CA 94305 USA > +1 650 704 7212 [email protected] <javascript:> > website: http://dge.stanford.edu/labs/caldeiralab/ > blog: http://kencaldeira.org > @KenCaldeira > > My assistant is Dawn Ross <[email protected] <javascript:>>, with > access to incoming emails. > > > > On Sun, Mar 6, 2016 at 4:40 AM, Stephen Salter <[email protected] > <javascript:>> wrote: > >> Hi All >> >> The effects of reduced precipitation from stratospheric aerosols >> identified by Farraro are there with marine cloud brightening but it also >> has others. Making the sea cooler than the land makes stronger monsoons to >> move more water to land. Making stronger winds over the sea makes for more >> breaking waves and so more evaporation. Making smaller drops in clouds >> over the sea reduces rainfall over the sea to leave more to fall further >> inland. >> >> Bala and Caldeira in DOI 10.1007/s00382-010-0868-1 say that the overall >> result of the conflicting effects could lead to increased river run off >> with a contribution from reduced evaporation ashore. >> >> Nobody cares much about precipitation over sea except for people living >> on small islands who might be more worried about rising sea levels. It is >> possible to use the power in sea waves to do carbon-free desalination. >> >> Stephen >> >> >> Emeritus Professor of Engineering Design. School of Engineering, >> University of Edinburgh, Mayfield Road, Edinburgh EH9 3DW, Scotland >> [email protected] <javascript:>, Tel +44 (0)131 650 5704, Cell 07795 203 >> 195, WWW.homepages.ed.ac.uk/shs, YouTube Jamie Taylor Power for Change >> >> On 06/03/2016 12:20, Andrew Lockley wrote: >> >> >> >> https://angusferraro.wordpress.com/2016/03/05/can-stratospheric-aerosols-directly-affect-global-precipitation/ >> >> Can stratospheric aerosols directly affect global precipitation? >> >> What is the effect of stratospheric aerosol geoengineering on global >> precipitation? If we were to inject sulphate aerosol into the stratosphere >> it would reflect some sunlight and cool the Earth, but the atmosphere’s >> CO2 levels would remain high. This is important, because CO2 actually has >> an effect on precipitation even when it doesn’t affect surface temperature. >> In a recent paper with a summer student, I’ve shown the aerosols can >> contribute a similar effect. >> >> Three climate models (CanESM2, HadGEM2-ES, MPI-ESM-LR) did simulations of >> the future with and without geoengineering. The simulations with >> stratospheric aerosols (G3 and G4) show greater temperature-independent >> precipitation reductions than the simulations without them (RCP4.5 and G3S). >> >> Precipitation as energy flow >> >> Precipitation transfers energy from the Earth’s surface to its >> atmosphere. It takes energy to evaporate water from the surface. Just as >> evaporation of sweat from your skin cools you off by taking up heat from >> your skin, evaporation from the Earth’s surface cools it through energy >> transfer. Precipitation occurs when this water condenses out in the >> atmosphere. Condensation releases the heat energy stored when the water >> evaporated, warming the atmosphere. Globally, precipitation transfers >> about 78 Watts per square metreof energy from the surface to the >> atmosphere. Multiplying that by global surface area that’s a total energy >> transfer of about 40 petajoules (that’s 40 with 15 zeros after it) of >> energy every second! To put that in a bit of context, it’s about 40% of the >> amount of energy the Sun transfers to the Earth’s surface. >> >> If precipitation changes, that’s the same as saying the atmospheric >> energy balance changes. If we warm the atmosphere up, it is able to radiate >> more energy (following the Stefan-Boltzmann law). To balance that, more >> energy needs to go into the atmosphere. This happens through precipitation >> changes. >> >> Direct effects of gases on precipitation >> >> Now imagine we change the amount of CO2 in the atmosphere. This decreases >> the amount of energy the atmosphere emits to space, meaning the atmosphere >> has more energy coming in than out. To restore balance the atmospheric >> heating from precipitation goes down. This means that the global >> precipitation response to global warming from increasing CO2 has two >> opposing components: a temperature-independent effect of the CO2, which >> decreases precipitation, and a temperature-dependent effect which arises >> from the warming the CO2 subsequently causes. In the long run the >> temperature-dependent effect is larger. Global warming will increase global >> precipitation – although there could be local increases or decreases. >> >> But what happens if we do geoengineering? Say we get rid of the >> temperature-dependent part using aerosols to reduce incoming solar >> radiation. The temperature-independent effect of CO2 remains and global >> precipitation will go down. >> >> Detecting the effect of stratospheric aerosol >> >> CO2 isn’t the only thing that has a temperature-independent effect. Any >> substance that modifies the energy balance of the atmosphere has one. In >> our newstudy, we ask whether stratospheric sulphate aerosol has a >> detectable effect on global precipitation. Theoretically it makes sense, >> but it is difficult to detect because usually there are >> temperature-dependent effects obscuring it. >> >> We used a common method to remove the temperature-dependent effect. We >> calculated the precipitation change for a given surface temperature change >> from a separate simulation, then used this to remove the >> temperature-dependent effect in climate model simulations of the future. We >> did this for future scenarios with and without geoengineering. >> >> As expected, we found a temperature-independent influence which reduced >> precipitation. Importantly, this effect was bigger when geoengineering >> aerosols were present in the stratosphere. This was detectable in three >> different climate models. The figure above shows this. The >> non-geoengineered ‘RCP4.5’ simulation shows a precipitation decline when >> the temperature effect is removed. This comes mainly from the CO2. The >> ‘G3’ and ‘G4’ geoengineering simulations (blue and green lines) have an >> even greater decline. The aerosol is acting to decrease precipitation >> further. >> >> How does aerosol affect precipitation? >> >> The temperature-independent effect wasn’t present when geoengineering was >> done by ‘dimming the Sun’. The ‘G3S’ simulation (orange lines in the >> figure) does this, and it has a similar precipitation change to RCP4.5. So >> what causes the precipitation reduction when stratospheric aerosols are >> used? We calculated the effect of the aerosol on the energy budget of the >> troposphere (where the precipitation occurs). We separated this in two: the >> aerosol itself, and the stratospheric warming that occurs because of the >> effect of the aerosol on the stratosphere’s energy budget. >> >> Black bars show the temperature-independent precipitation changes >> simulated by the models. Orange bars show our calculation of the effect of >> the stratospheric warming. Green bars show our calculation of effect of the >> aerosol itself. Grey bars show our calculation of the total effect, which >> is very close to the actual simulated result. >> >> We found the main effect was from the aerosol itself. The aerosol’s main >> effect is to reduce incoming solar radiation and cool the surface. But we >> showed it also interferes a little with the radiation escaping to space, >> and this alters the energy balance of the troposphere. The precipitation >> has to respond to these energy balance changes. >> >> This effect is not huge. We had to use many model simulations of the 21st >> Century to detect it above the ‘noise’ of internal variability. In the real >> world we only have one ‘simulation’, so this implies the >> temperature-independent effect of stratospheric aerosol on precipitation >> would not be detectable in real-world moderate geoengineering scenario. >> This also means climate model simulations not including the effects of the >> aerosol could capture much of the effects of geoengineering on the global >> hydrological cycle. >> >> This effect could be more important under certain circumstances. If >> geoengineering was more extreme, with more aerosol injected for longer, >> precipitation would decrease more. But, based on these results, the main >> effect of geoengineering on precipitation is that the temperature-dependent >> changes are minimised. This means the temperature-independent effect of >> increasing CO2 concentrations is unmasked, reducing precipitation. >> >> Take a look at the paper for more details – it’s open access! >> >> Ferraro, A. J., & Griffiths, H. G. (2016). Quantifying the >> temperature-independent effect of stratospheric aerosol geoengineering on >> global-mean precipitation in a multi- model ensemble. Environmental >> Research Letters, 11, 034012. doi:10.1088/1748-9326/11/3/034012. >> -- >> 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 [email protected] <javascript:>. >> To post to this group, send email to [email protected] >> <javascript:>. >> 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 [email protected] <javascript:>. >> To post to this group, send email to [email protected] >> <javascript:>. >> Visit this group at https://groups.google.com/group/geoengineering. >> For more options, visit https://groups.google.com/d/optout. >> >> The University of Edinburgh is a charitable body, registered in >> Scotland, with registration number SC005336. >> >> -- >> 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 [email protected] <javascript:>. >> To post to this group, send email to [email protected] >> <javascript:>. >> 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 [email protected]. To post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/geoengineering. For more options, visit https://groups.google.com/d/optout.
