Angus (cc Alan / Ben) Despite several papers addressing the comparability of dimming vs aerosol, there doesn't appear to be a clear consensus on the relevance of dimming as an analogue.
As you point out below, there's also a range of issues with aerosol distribution which have not been resolved. These relate to the prescription in the model, but that itself depends on the injection regime. There's a common presumption that injection into the Brewer Dobson ascension will be effective, but other authors suggest meridional distribution directly is more appropriate (I'll dig out references if needed). Furthermore, open questions remain about expected particle size distribution (which again is highly dependent on the injection regime, particularly temporal distribution). It would be great if there could be a body of work similar to GeoMIP to compare a range of injection scenarios and/or particle distributions across a model ensemble, to get greater clarity as to the most benign regime and the most appropriate modelling shortcuts to use (eg dimming masks) when modelling without aerosol physics. Bearing in mind how disparate the predictions and prescriptions are, it seems worth investing effort to resolve - not least because the determination of injection regime is pretty critical to issues of cost, engineering and permissions. A On 23 Mar 2015 11:11, "Angus Ferraro" <[email protected]> wrote: > Hmm, I thought it had been posted before too. The sensitivity of the > tropical circulation (and hence hydrological cycle) to the effects of the > aerosols appears to be rather sensitive to how the aerosol is set up in the > model. Kalidindi et al (2014) > <http://link.springer.com/article/10.1007/s00382-014-2240-3> didn't see > the big effect on tropical precipitation we saw in the ERL paper. I > interpret this as meaning it's pretty important to think about how a > geoengineering aerosol layer might look in the real world. On the one hand, > some (but not all) models predict a relatively uniform AOD with latitude, > whereas we used a distribution with a peak in the Tropics. We also put the > aerosol layer very low (it pretty much sticks to just above the > tropopause). This would mean the effect on tropical circulation might be > overstated in the ERL paper (though the basic physics remains the same). On > the other hand, we assumed 0.1 micron sulphate particles, which was very > likely too small. Larger sulphate particles would be more emissive in the > longwave part of the spectrum, increasing the downwelling longwave > radiative flux from the aerosol layer. > > As long as there are plausible ways to achieve aerosol distributions that > do not have the effect shown in the ERL paper, I don't feel like it's a > 'show-stopper' for solar dimming vs aerosols differences. > > Cheers > > Angus > > On Sunday, 22 March 2015 11:15:46 UTC, andrewjlockley wrote: >> >> Poster's note : another paper showing solar dimming vs aerosols >> differences. I thought this had been discussed on the list previously, but >> I can't find it in the archives. >> >> http://iopscience.iop.org/1748-9326/9/1/014001/article >> >> Weakened tropical circulation and reduced precipitation in response to >> geoengineering >> >> OPEN ACCESS >> Angus J Ferraro et al 2014 Environ. Res. Lett. 9 014001 >> doi:10.1088/1748-9326/9/1/014001 >> Published 8 January 2014 >> >> Abstract >> Geoengineering by injection of reflective aerosols into the stratosphere >> has been proposed as a way to counteract the warming effect of greenhouse >> gases by reducing the intensity of solar radiation reaching the surface. >> Here, climate model simulations are used to examine the effect of >> geoengineering on the tropical overturning circulation. The strength of the >> circulation is related to the atmospheric static stability and has >> implications for tropical rainfall. The tropical circulation is projected >> to weaken under anthropogenic global warming. Geoengineering with >> stratospheric sulfate aerosol does not mitigate this weakening of the >> circulation. This response is due to a fast adjustment of the troposphere >> to radiative heating from the aerosol layer. This effect is not captured >> when geoengineering is modelled as a reduction in total solar irradiance, >> suggesting caution is required when interpreting model results from solar >> dimming experiments as analogues for stratospheric aerosol geoengineering. >> > -- > 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 http://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. 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