I would like to share my thoughts (and clarify them in the process !) on the issue of the apparent paradox of global warming and dimming, raised by Gerald Stanhill in EOS (v. 88, no. 5, 30 January 2007).
Global warming refers to the observed increase in average air and ocean temperatures since the pre-industrial era.
Global dimming refers to the observed decrease in solar radiation received at the surface from the late 1950s to the late 1980s, after what it started to increase, refered to as global brightening.
In a provocative article, Stanhill says that there is a paradox between observed reductions of incoming solar radiation at the Earth's surface of 20 W/m^2 (W=watts) over the 1958-1992 period, and the increase in longwave radiative forcing of 2.4 W/m^2 since the industrial era due to fossil and biofuel combustion (green-house effect). The former should have cooled the surface of the Earth more than the latter could have warmed it, yet observations show an unequivocal increase in surface temperatures. Stanhill argues that this puts into question the validity of the explanation of global warming as caused by green-house gases, and criticizes the Intergovernmental Panel on Climate Change (IPCC) for failing to mention global dimming in their first 3 assessment reports.
I have not read the first 3 reports, so I cannot confirm that this topic was omitted, but I started to read the 4th assessment report (released this year), and there is a whole section on global dimming, without any mention of a paradox of any kind. Why is that so ?
Liepert et al., and Schmidt et al., recently responded to Stanhill's article in EOS (v. 88, no. 45, 6 November 2007). They pointed to the fact that Stanhill made 2 errors: first he used a rather limited selection of 30 sites to infer his "global" values of dimming, that were dominated by large urban areas, where the observed dimming is strongest, while it is weaker at rural sites or even missing altogether over remote areas (IPCC 4th report, section 18.104.22.168). Therefore more realistic global values are much weaker than Stanhill's, around 7 W/m^2.
But this is still stronger than the effect of green-house gases.
This is where the second error comes into play: the observed dimming values are at the surface of the Earth, while the longwave radiative forcing is computed at the tropopause (between 10 and 15 km altitude). A lot of things can happen between the tropopause and the surface of the Earth, so this is like comparing apples and oranges !
The causes of global dimming are not well understood, but possible mechanisms include increases of clouds coverage (although different observations and analyses give different trends with sometimes opposite signs), and of aerosols due to human activity. The latter directly scatter more light in every direction, reducing the amount reaching the surface, and indirectly affect clouds structures (reducing the droplets sizes and increasing their number, hence making the clouds brighter, i.e. more reflective of incoming solar radiation, and increasing their lifetime by reducing precipitation likelihood). An increase in clouds coverage also increases the green-house effect (water vapor is one of the most efficient green-house gases !), so it would not be a paradox to have a decrease in incoming solar radiation at the surface at the same time as an increase in surface temperatures. Another explanation could be that the energy balance at the surface is not local but involves the 3-D circulation of the atmosphere.
We see that if we correct for the 2 errors made by Stanhill, and take into account the various feedback mechanisms available within the climate system, there is no paradox between global warming and global dimming, and if anything, global dimming should have masked somewhat the effects of greenhouse warming up to the late 1980s, when dimming gradually transformed into brightening and temperature rise rate increased, revealing the full dimension of the greenhouse effect (Wild et al., Geophysical Research Letters, 2007).
As Schmidt et al. put it in their reply to Stanhill:
"Understanding anthropogenic climate change depends on multiple lines of evidence and a variety of observations. «Global dimming» is an integral part of that evidence, not a contradiction to it."