Saturday, February 11, 2006

1940 - 1980: Why No Warming?

Wolfgang asks a question which really deserves an answer from James, William, or some other smart guy:

The CO2 concentration in the atmosphere has steadily increased during the last century.
But the average surface temperature did not and shows three distinct phases.
An (almost) linear increase in temperature from 1910 to 1940, a flat to slightly falling temperature from 1940 to 1980 and then again an (almost) linear increase until now.
I found this page which argues:
An argument against the idea that global warming is due to mankind's emissions of carbon dioxide goes as follows: the warming this century occurred mostly between 1910 and 1940, when the carbon dioxide concentration grew slowly from 293 to 300 ppm. On the other hand, the temperature remained steady between 1940-1980, while the carbon dioxide concentration increased from 300 to 335 ppm. The most likely answer to this inconsistency is atmospheric aerosol. Aerosols are emitted by industrial processes, transport, etc, and their increased concentration offset simultaneous warming due to increasing greenhouse gases. However the warming overtook the cooling by mid 1970's (8).

Tropospheric aerosols reduced solar radiation to the ground by about 0.5 W/m2, as a global average, between 1940-1992 (5). Unlike greenhouse gases, which are generally long-lived, aerosols fall out of the atmosphere fairly rapidly, either dry ('sedimentation') or within rain (as condensation nuclei). Therefore aerosol concentrations are not uniformly mixed across the globe. They have been so high in some regions that the cooling effect may have exceeded the warming due to greenhouse gases. In fact the lack of warming between 1940-1980 is only found in the northern hemisphere, where most manmade aerosols are emitted.
The Northern Hemisphere point looks good, but I would like the opinion of some guys with climate smarts.