New kiwi blog Sir Humphreys opens with an attack on "global dimming" - and in doing so displays a shameful level of scientific illiteracy. The idea that aerosols both increase albedo and promote cloud formation (further reducing the amount of sunlight reaching the earth's surface) is uncontentious. What is contentious is how great the effect is, and how it will feed into other climate effects, such as that of increased CO2 emissions.
Wikipedia has some interesting facts here. The size of the effect has been estimated at:
- 5.3% (9 W/m2) over 1958-85 (Stanhill and Moreshet, 1992)
- 2%/decade over 1964-93 (Gilgen et al, 1998)
- 2.7%/decade (total 20 W/m²) up to 2000 (Stanhill and Cohen, 2001)
- 4% over 1961-1990 (Liepert 2002)
with the largest reductions being found in the northern hemisphere. As for Sir Humphrey's "explanation" - "sunspot activity" -
the value of the solar radiation at the top of the atmosphere has not changed by more than a fraction of this amount
The change is clearly due to the atmosphere, not the sun.
As for consequences, this may help counteract some of the expected global warming (due to increased albedo) - or amplify it due to trapping heat (something clouds are very good at). But I'd like to see some modelling done before dismissing it out of hand.