A Weblog monitoring coverage of environmental issues and science in the UK media. By Professor Emeritus Philip Stott. The aim is to assess whether a subject is being fairly covered by press, radio, and television. Above all, the Weblog will focus on science, but not just on poor science. It will also bring to public notice good science that is being ignored because it may be politically inconvenient.

Thursday, January 15, 2004

Aerosols cool the climate debate.....

I'm sure that everyone remembers the Intergovernmental Panel on Climate Change (IPCC) diagram (IPCC 2001, Figure 6.6) which examines our degree of knowledge, and uncertainties, about different agents possibly involved in global mean radiative forcing (if you can't find the original, the much-abused Dr. Lomborg most helpfully reproduces the diagram as Figure 139 in The Skeptical Environmentalist).

On the cooling side of the diagram, there is something called the 'aerosol indirect effect', our understanding of which is classed as VL, i.e. 'Very Low'. Intriguingly, it appears that this might possibly more than balance out, on the warming side, carbon dioxide, about which our understanding is classed as (how surprisingly!) H, i.e. 'High'.

Now read this, just published in Nature - 'Pollutants that cool' ['Observational evidence of a change in radiative forcing due to the indirect aerosol effect.' Joyce E. Penner, Xiquan Dong & Yang Chen, Nature 427, 231–234 (2004)]:

"Anthropogenic aerosols (tiny particles that contribute to smog and haze) can enhance cloud reflectivity by increasing the number concentration of cloud droplets. Many small droplets scatter more sunlight back into space than fewer large ones, so in theory an accumulation of aerosol particles can produce cooling, known as the indirect aerosol effect. The Intergovernmental Panel on Climate Change has concluded that the indirect effect of aerosols might be negligible, in part because of a lack of direct evidence that this mechanism affects the balance between incoming solar radiation and outgoing infrared. New data from sites of polluted (Oklahoma) and clean (Alaska) air show that aerosol pollutants affect cloud optical depth significantly, at a magnitude expected from the indirect aerosol effect, and hence that this effect is an important factor in climate."

Well! Well! Something I have been saying for yonks. It is also worth recalling that no less that 66% of the factors/agents in the famous Figure are, like the 'indirect aerosol effect', classed as VL, i.e. about which we have only a very low level of understanding, while for three others we possess only L, low, or M, medium levels of understanding. One agent alone is classed as H, for which our understanding is thought to be high - now I wonder what that little package might constitute? Remember, as well, that water vapour and clouds remain the big modelling problems!

So, with the famous recent work on temperature, cloud cover and the cosmic ray flux (CRF), followed by this on aerosols, things are looking distinctly.....


Philip, off for a late cold snack for lunch.

[New counter, June 19, 2006, with loss of some data]

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