Aerosols again

Having just said that not much is happening in the climate world, there was an interesting paper in Nature just before Christmas in Nature: Bellouin et al., about possible underestimates of the aerosol effect. Oddly enough I haven't heard much about this... maybe its because it was just before Crimbo (someone has just posted it to sci.env though). Or maybe its because no-one wants to talk about it: from the Grauniad: "We found that aerosols actually have twice the cooling effect we thought," said Nicolas Bellouin, a climate modeller at the Met Office. The consequence is that as air quality improves and aerosol levels drop, future warming may be greater than we currently think.". Thats not really anything the skeptics are going to want to tell you. And if you're a... opposite-of-skeptic I suppose, then the idea that the GCMs have got the aerosol forcing wrong is nothing to trumpet either.

I did read the Nature article and I remember thinking it seemed Fair Enough (though I can't re-read it because the evil information-hoarding folk at Nature won't share it... I can't even find a free abstract); you can read the UKMO press release. There was some kind of let-out clause (that maybe, to compensate, the *indirect* effect could be weaker).

Someone over at wiki got round to asking someone at NCAR, who said:

The exact magnitude of various forcings is uncertain. The new estimates you refer to for aerosols are larger than what some models use (the magnitude of what models compute, for example, for sulfate aerosols varies depending on the nature of their sulfur cycle models or types of sulfate aerosol concentrations they use) but not out of the range of uncertainty for aerosol forcing used across all of the more than 20 models currently being assessed in the IPCC AR4. This accounts for some of the range of model responses to the simulation of 20th century climate. Even with this uncertainty in aerosols, the GHGs are still the largest forcing by far, and are the big driver for late 20th century warming and estimates of 21st century warming. The latest simulations will be assessed in the IPCC AR4, but many modeling groups are publishing their latest findings in the peer reviewed literature now (for example, from our group see: Meehl et al., 2005: How much more warming and sea level rise? Science, 307, 1769—1772). [1]

so its possible that another answer is as she says: the new numbers may be different to previous UKMO ones but are within the uncertainty range.

[Update: the pdf fairy has been to visit, so I've read it again (or rather, for the first time: I now realise I only read the commentary; reading the paper I don't see the bit about indirect effect, so maybe that was invented by the commentary...). They estimate a (clear sky) RF of -1.9 W/m2 whereas the TAR (graph estimates about -0.4, with a range up to nearly -1.0. Though the TAR value isn't clear-sky... but B et al. say the model value is -0.5 - -0.9, which is similar.

Anyway, here is the abstract:

Atmospheric aerosols cause scattering and absorption of incom­
ing solar radiation. Additional anthropogenic aerosols released
into the atmosphere thus exert a direct radiative forcing on the
climate system 1 . The degree of present­day aerosol forcing is
estimated from global models that incorporate a representation
of the aerosol cycles 1--3 . Although the models are compared and
validated against observations, these estimates remain uncertain.
Previous satellite measurements of the direct effect of aerosols
contained limited information about aerosol type, and were
confined to oceans only 4,5 . Here we use state­of­the­art satellite­
based measurements of aerosols 6--8 and surface wind speed 9 to
estimate the clear­sky direct radiative forcing for 2002, incorpo­
rating measurements over land and ocean. We use a Monte Carlo
approach to account for uncertainties in aerosol measurements
and in the algorithm used. Probability density functions obtained
for the direct radiative forcing at the top of the atmosphere give a
clear­sky, global, annual average of 21.9 Wm 22 with standard
deviation, 60.3 Wm 22 . These results suggest that present­day
direct radiative forcing is stronger than present model estimates,
implying future atmospheric warming greater than is presently
predicted, as aerosol emissions continue to decline

I'm sure Nature won't mind that.]