http://www.bbc.co.uk/sn/tvradio/programmes/horizon/dimming_qa.shtml

Horizon, BBC, January 2005 - GLOBAL DIMMING

Questions and Answers

Why would particles in the atmosphere cause global dimming?
It's very simple - they reflect sunlight back into space. This fact has been well-known for many decades (it is why volcanoes, which can throw vast numbers of sulphate particles into the upper atmosphere, can have a strong temporary cooling effect on the Earth).

What came as a surprise was the so-called indirect effect, whereby particles in the atmosphere change the optical properties of clouds. This happens because the presence of man-made particles in the atmosphere increases the number of sites where water droplets can form. The effect is that up to six times as many water droplets form in a polluted air mass as would naturally, but since the amount of water vapour in the atmosphere does not change, the droplets have to be smaller than they would naturally be.

Since the reflectivity of the clouds depends on the surface area of the droplets, these polluted clouds are more reflective than unpolluted ones (since many small droplets have a bigger surface area than fewer big ones). By reflecting more sunlight back into space these more reflective clouds cut down the sunlight reaching the surface - hence contributing to global dimming.

Is global dimming a daytime/daylight phenomenon or a round-the-clock greenhouse/infra-red phenomenon?
Dimming is a daytime effect. At night the sun's radiation is obviously completely blocked by the Earth!

How does anthropogenic particle emission compare with natural sources, for example volcanoes, deserts, sea salt etc?
That is very difficult to say. Emissions from volcanoes can have a big global effect on the climate, but it is transitory - the Earth may go many decades between major eruptions. Sea salt and natural sulphur compounds emitted by plankton are also very important for cloud formation. But the results from the Indian Ocean Experiment suggest that the effects of anthropogenic particles in the atmosphere dominate natural effects, in the Indian Ocean at least.

At the moment the scientific consensus is that global dimming is a man-made phenomenon. Another important aspect of man-made particle pollution is that the evidence is growing that its direct impact on human health is very serious - causing literally hundreds of thousands if not millions of deaths annually. Sea salt doesn't give you cancer!

If the 9/11 contrail evidence suggests warmer nights due to the air travel, isn't that global warming rather than dimming?
The 9/11 study showed that removing contrails resulted in a large increase in the daily temperature range - in other words warmer days and cooler nights. The study does not really provide a clear-cut answer to the question of whether the overall effect of the contrails is a net warming or a net cooling averaged over the whole 24 hours. This question is controversial. But what seems clear is that contrails contribute to a reduction in the amount of daytime solar radiation reaching the surface, and that this has significant effects on temperature.

Given the effect of the air traffic, is there any way to reduce the pollution they cause? For example, changing altitude, fuels and engines or economic measures?
Contrails form at altitude, so in theory flying lower would reduce them - but at the cost of burning even more aviation fuel and therefore making a still greater contribution to global warming.

Why has progress been made on particulate emissions more easily than gaseous emissions?
There are several reasons. Firstly, people can see, smell and taste particle emissions and they have very obvious damaging effects on human health. Also, sulphate emissions cause acid rain, which again has very obvious and immediate effects on vegetation and fish. So the political pressure to reduce particle emissions is strong. Then, the cure for particle emissions is technologically fairly straightforward - it is a question of adding equipment to existing plant rather than replacing plant.

This is a big contrast with what is required to tackle greenhouse gas emissions. Firstly, carbon dioxide is invisible and tasteless, and its catastrophic long-term effects on the environment are not immediately apparent. Secondly, tackling the problem means changing some of our behaviours and radically re-tooling our energy generation systems. These issues have yet to be properly confronted. But the good news is that we already have the technologies needed to solve the problem of greenhouse gases (principally energy conservation measures and nuclear power); what we so far lack is the political will to use them.

Are scientists often sceptical of findings that go against the current orthodoxy?
Yes, for good reason - usually the orthodoxy is correct. A famous physicist once told me that if you doubt every new idea in science you will be right 90% of the time, but you will be wrong the only time it matters.

Is there a possibility that the pan-evaporation method is too crude to be reliable?
Rather the reverse. It is the very simplicity of the pan-evaporation experiments that makes them reliable and comparable between different locales and over long periods of time. That is why many climate scientists regard the pan-evaporation data as the most convincing evidence of solar dimming.

What's the range in the figures on global dimming? Is there a consensus or are they as open to interpretation as global warming figures?
I don't think the figures on global warming are open to interpretation. It is an established fact that global temperatures have risen by 0.6°C over the past century. It is also an established fact that carbon dioxide levels have risen by about 100 parts per million over the same period due to human activity. It is a matter of the basic laws of physics that an increase in carbon dioxide will trap more heat in the Earth's atmosphere, which is why almost no respectable and independent scientist doubts the causal link between these two established facts.

The only surprise is that the warming has not been greater - which is where global dimming comes in. Unfortunately, there is still a very large range in the estimates of the cooling effect of dimming - by up to a factor of four. What seems to have been established already, however, is that the cooling effect of dimming is far larger than previously thought. This may explain why the world has not already warmed more strongly - the cooling effect of particle pollution has been offsetting the warming from carbon dioxide. If so, then we are in for far faster warming in the future as particle emissions are brought under control while greenhouse gas emissions continue to rise.