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.
