sulfate aerosol geoengineering modelled by solar dimming

[Tamas Bodai]

Dear All,

I would like to ask for some useful references about sulfate aerosol geoengineering. Assuming some uniform aerosol coverage around the globe, at some height, with a certain vertical layer thickness, i would imagine that at higher latitudes the radiative forcing exerted by the aerosols is larger due to the longer distance of travel of sun rays through the aerosol "cloud". As a consequence, the latitude-dependence of the downward-directed radiative forcing should have an even larger gradient than solar irradiance. Therefore, I’m wondering how big mistake it is to model such a geoengineering scenario by dimming the sun.

Any feedback or reference would be much appreciated.

Thank you,

Tamas

[Olivier Boucher]

Dear Tamas,

there are typically 3 effects that govern RF by stratospheric aerosols as a function of latitude for a given aerosol burden. Let's think in terms of solar zenith angle (one has then to integrate over SZA which is a function of latitude and season)

1/ insolation decreases with SZA as cos(theta) where theta is the SZA

2/ air mass increases with  SZA as 1/cos(theta), of course the effect this has breaks down at some point because of multiple scattering

3/ upscattering function also increases with SZA (because more forward scattering contributes to upscattering).

You could assume 1/ and 2/ cancel each other at first approximation, so because of 3/ there is indeed more RF at larger SZA. In fact there is an optimum around SZA=60° but that depends on the AOD and how much multiple scattering there is.

Now life is a bit more complicated, as transport and aerosol size varies also.

In any case, the climate response is not a copy-paste of the spatial distribution of the RF. It matters but not too much. And it matters more for rapid adjustments than for feedbacks. See eg https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2013JD021110

Regards,

Olivier

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[Stephen Salter]

Hi All

But you also have to consider outgoing long wave radiation especially in winter.

Stephen

Emeritus Professor of Engineering Design. School of Engineering, University of Edinburgh, Mayfield Road, Edinburgh EH9 3DW, Scotland S.Sa...@ed.ac.uk, Tel +44 (0)131 662 1180 WWW.homepages.ed.ac.uk/shs, YouTube Jamie Taylor Power for Change

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[Olivier Boucher]

Hi Stephen,

you're correct and I'd think the negative SW RF is more offset by the positive LW RF in the tropics than in the high latitudes (alike the pattern of RF by WMGHG). But again, the pattern of a not-too-inhomogeneous forcing is only moderately important.

Regards

Olivier

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[Douglas MacMartin]

Agreed that the pattern of response isn’t as inhomogeneous as the forcing is, though it is still true that a uniform aerosol layer will overcool the tropics and undercool the poles, and that choosing your injection locations so that the aerosol layer is not perfectly uniform does actually maintain temperature gradients better.   (But the undercooling of the poles of course is still small compared to the warming that would be there without geoengineering.)

 

doug

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[Tamas Bodai]

Dear Olivier, Stephen and Douglas,

I highly appreciate your responses. Olivier’s point 1/, that the sulfate aerosol geoengineering forcing depends on the preexisting radiation, made me realise that the there should be a latitude-dependence of the green house forcing too. I believe Stephen’s comment relates to this. However, I do better appreciate now, as Douglas emphasized, that rather different forcing distributions can result in similar response in terms of e.g. the temperature field. This is quite a surprising fact. 

Best wishes,

Tamas

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