IPCC is there to assess the manmade influence in climate (change)

Why would you like to know the effect of natural aerosol?

N.B. an important influence is cloud formation: without natural aerosol there are no low clouds?!

"Based on a combination of global aerosol models and observation-based methods, the best radiative effect estimate of the aerosol–radiation interaction in Assessment Report 5 (AR5) is –0.35 (–0.85 to +0.15) W.m–2 (global annual mean between 1750 and 2011 http://www.climatechange2013.org/images/report/WG1AR5_Chapter08_FINAL.pdf page 24). As far as I understand, this is an estimate for all 7 aerosols conisdered in AR5 without distinction between natural and anthropogenic.

For the major natural global aerosol, Mineral Dust, the global mean radiative forcing in AR5 is -0.10 (-0.30 +0.10) W.m-2. The RE of other natural aerosols such as sea-salt was not provided as such in AR5 as far as I know.

Victor

The values are for the ANTHROPOGENIC part of the aerosol types

Beware of the differences between "radiative forcing" and "radiative effect"; the first is per definition anthropogenic and represents the difference between the total radiative effects and the natural radiative effects. J. Quaas (and possibly others) pointed that out in one of his recent papers. Hence the forcing of sea salt aerosols is 0, but the radiative effect is not. 

I have never come across estimates of radiative effects of natural aerosols, but I guess it would be interesting to know their impact, if only to put the anthropogenic forcing into perspective. And local effects may be substantial, particularly for desert dust.

And let me ask you this: how natural are dust aerosols if they are caused by increased erosion due to land use changes?

@Marloes Penning de Vries

I am not sure the radiative forcing per say imbibes anthropogenic forcing. In fact one of the figure in IPCC WG1 2007 reports categorically mentions natural forcing by solar irradiance. However, I agree that most people confuse radiative effects with radiative forcing.

http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-1.html

thanks Jonathan, this is from 2007, though. But since I have not found anything on 2013, I will use that one.

Marloes

There are estimates of the reflection of solar energy by seasalt. And also of a forcing effect by an increase in wind velocity and production of seaspray/seasalt. This wind velocity was one of the model outcomes in future climate chaage. The draft 2004 IPCC gave this as the most important forcing factor?!

does anyone know if they considered effect of expected changes in R.H. on scattering? 

Thanks for pointing that out, Harry! However, in the current IPCC report, I don't find anything on sea salt forcing  --- but then I must admit I'm only looking at the technical summary. Am I missing or misunderstanding something?

Marloes

It is in Ar4 but I do not see an entry in AR5

In ch 8 of AR5 the following:

A robust trend is an increase in sea salt emission in the high latitudes because of a decrease in sea ice cover with global warming, which can results in atmospheric cooling through scattering sunlight and increasing cloud albedo with acting as cloud condensation nuclei