The transcript of the BBC programme on Global Dimming starts like this:
NARRATOR: September 12th 2001, the aftermath of tragedy. While America mourned, the weather all over the country was unusually fine. Eight hundred miles west of New York, in Madison, Wisconsin a climate scientist called David Travis was on his way to work.
DR DAVID TRAVIS (University of Wisconsin, Whitewater): Around the twelfth, later on in the day, when I was driving to work, and I noticed how bright blue and clear the sky was. And at first I didn't think about it, then I realised the sky was unusually clear.
NARRATOR: For 15 years Travis had been researching an apparently obscure topic, whether the vapour trails left by aircraft were having a significant effect on the climate. In the aftermath of 9/11 the entire US fleet was grounded, and Travis finally had a chance to find out.
DR DAVID TRAVIS: It was certainly, you know, one of the tiny positives that may have come out of this, an opportunity to do research that hopefully will never happen again.
NARRATOR: Travis suspected the grounding might make a small but detectable change to the climate. But what he observed was both immediate and dramatic.
DR DAVID TRAVIS: We found that the change in temperature range during those three days was just over one degrees C. And you have to realise that from a layman's perspective that doesn't sound like much, but from a climate perspective that is huge.
http://www.bbc.co.uk/sn/tvradio/programmes/horizon...
The site realclimate.org ran two articles on the show, the first one contains:
Though there are serious issues with the quality of some of the data (birds drinking out of uncovered evaporation pans, drift and inhomogeneities in the solar radiation measuring instruments), in the most global assessment, Beate Liepert http://www.ldeo.columbia.edu/~liepert/papers/liepe... estimated that there was globally a reduction of about 4% in solar radiation reaching the ground between 1961 and 1990.
A change of that magnitude in the incoming solar radiation itself is not possible since satellite observations would have seen it. Thus, it must be something that is happening in the atmosphere to intercept solar radiation. There are only a few possibilities: clouds, water vapour or aerosols http://www.realclimate.org/index.php?p=49 .
First of all it is important to note that even pure greenhouse gas forcing will lead to a slight decrease in surface solar radiation (due to the concurrent increased humidity) and potential cloud feedbacks. Cloud cover and thickness are both like to vary as a function of climate change.
Contrails (those wispy trails left behind high flying jets) have increased over the period and may be important. But estimates of their global effect, even making very generous assumptions about their spread are small (Minnis et al, 2004 http://www-pm.larc.nasa.gov/sass/pub/journals/Minn... ). Aerosols are also known to have increased over this time, and so they are a natural candidate. However, simulations using the relatively straightforward 'direct effect' of aerosols (the increase in albedo of the planet due to the particle brightness) do not match the inferred changes. The final candidates are numerous interactions of aerosols with clouds, the so-called 'indirect effects'.
There are an ever increasing number of these 'indirect effects', but the two most discussed are the aerosol/cloud opacity interaction (more aerosols provide more sites for water to condense in clouds, thus cloud droplets are smaller and clouds become more opaque), and the cloud lifetime effect (smaller droplets make it more difficult to make drops big enough to rain, and so clouds live longer). Estimates of the importance of such effects vary widely, and while they are thought to be significant, the uncertainty associated with them is very large. These effects are nevertheless a necessary part of the suite of human-related forcings that are being assessed in order to understand the climate of the 20th Century.
It should however be stressed that there are as yet no completely convincing explanations that quantitatively match the (admittedly uncertain) observations of this phenomena (Liepert and Lohmann, 2004 http://www.ldeo.columbia.edu/%7Eliepert/papers/200... ).
http://www.realclimate.org/index.php?p=105
And the second article contained:
Why is solar radiation changing? From observations we can separate cloud-free skies and cloudy conditions. We can hence infer clouds or atmospheric transparency as possible causes for the dimming. In my study of the US data I identified clouds as the main reason for the dimming of sunlight. Only about a fifth of the dimming could be observed during cloud-free conditions.
Why should the atmospheric transparency change at all in cloud-free conditions? V. Ramanathan explained it in the BBC documentary. Sunlight is reflected by air pollution or absorbed in the atmosphere before it reaches the ground. Field campaigns like INDOEX http://www-indoex.ucsd.edu/ show this clearly (well, not "clearly" in the literal sense!). Advanced climate models include this "direct" aerosol effect and base their inputs on experiments like INDOEX.
Why should clouds change? Global warming for example. Surprised? Most climate simulations predict some "global dimming" due to the water vapor and cloud feedback of greenhouse gas forced global warming. Global warming, however, affects the entire atmosphere whereas global dimming is only a surface and near-surface phenomena. Hence global warming and global dimming are not exclusive or contradictory. (Incidentally, the decline of solar energy at the surface inferred in my study is about 60% of the increasing longwave radiation in a typical global warming climate simulation (Feichter et al. 2004 http://www.ldeo.columbia.edu/~liepert/biblio.html )). With global warming, atmospheric moisture increases and this makes the atmosphere slightly less transparent to sunlight. Furthermore once clouds are formed, they tend to hold more water and therefore look a little darker.
http://www.realclimate.org/index.php?p=110