Several observational studies (see Chapter 5) support the existence of the
first aerosol indirect effect on low - level clouds and a negative sign for the associated radiative forcing, but these studies do not give indications on what a (negative) upper bound of the forcing would be.
Additional runs for 2000 with 1850 climate and for 2030 and 2100 (RCP 8.5) with 2000 emissions are designed to separate the effects of climate change on constituents and for
isolating aerosol indirect effects more cleanly using the clear - sky / all - sky flux diagnostics.
6.15.1 Past to Present 6.15.2 SRES Scenarios 6.15.2.1 Well - mixed greenhouse gases 6.15.2.2 Tropospheric ozone 6.15.2.3 Aerosol direct effect 6.15.2.4 Aerosol indirect effect
Quantitative estimates of
sulphate aerosol indirect effects on cloud properties and hence RF were sufficiently well established to be included in assessments, and carbonaceous aerosols from biomass burning were recognised as being comparable in importance to sulphate (Penner et al., 1992).
Additionally, changes in anthropogenic sulfate aerosol forcing have been proposed as the dominant cause of the AMV and the historical multidecadal variations in Atlantic tropical storm frequency, based on some model simulations
including aerosol indirect effects.
adding two variables that were requested in the ACCMIP Word document but not explained in the spreadsheet: the longwave and the shortwave cloud radiative forcing with reference (fixed) composition, for diagnosis
of aerosol indirect effect.
«We found that
aerosol indirect effect on deep convective cloud systems could lead to enhanced regional convergence and a strong top - of - atmosphere warming.»
These results confirm the need to use global high resolution models to study
the aerosol indirect effects.
However, I am not optimistic — the specifics of the small scale physics (
aerosol indirect effects on clouds, sea ice formation, soil hydrology etc.) are so heterogeneous that I don't see how you can do without calculating the details.
If this argument is sound, it seems to suggest that
aerosol indirect effects may actually be stronger than first thought?
Yet if there is less water vapor available due to the combined effect of «global dimming» and global warming, then it would seem to me that the more vapor - starved clouds that result would have to rely entirely on
the aerosol indirect effect to overcome the diminished vapor availability.
Still there remains a question in my mind about the relative importance of the GHG warming and
the aerosol indirect effects on «global dimming».
So the burden would seem to be * completely * on
the aerosol indirect effects, since they must first counteract the moisture starvation / thinner cloud effect, then go on to actually reduce the total observed solar radiation.
While this does not invalidate
the aerosol indirect effect at all, it underlines the limitations in using satellite observed changes in droplet size to compute the aerosol indirect forcing.
Assuming a Northern Hemisphere to Southern Hemisphere ratio of 2:1 for
the aerosol indirect effect, this would imply a globally - averaged forcing of -1.5 Wm - 2.
A few days ago a paper (Sato et al) dealing with some aspects of the «Aerosol Cloud Interactions», (ACI, also called «
aerosol indirect effects») was released.
Aerosol indirect effect, while it affects TOA energy balance, is technically a feedback rather than a forcing from the perspective of a GCM.
Menon, S., A.D. Del Genio, D. Koch, and G. Tselioudis, 2002: GCM simulations of
the aerosol indirect effect: Sensitivity to cloud parameterization and aerosol burden.