The volcanic ash cloud has generated a lot of hot air and serious economic fallout, but few facts of substance. Given what we have heard in the media, I am not convinced that the decision to close airspace completely was based on scientific analysis, but rather a manifestation of "nanny state" and the fear of being sued.
We have been reminded repeatedly of the occasions in the last 30 years where aircraft have been affected by volcanic ash. Very serious incidents they were too - Boeing 747s with all engines shut down or damaged. It is certainly the case that aircraft should avoid significant ash clouds. But is it really necessary to shut all the airspace, at all levels, over whole countries, when any ash in the atmosphere is pretty diffuse? At what level of concentration does the ash represent a threat to aircraft engines? Bear in mind that following Mount St Helens, within two weeks the dust from the explosion had circled the earth. So there were measurable concentrations of ash in the atmosphere then. That didn't shut down the world's air traffic.
There are all sorts of threats and risks to aircraft. Aeroplanes can "fall out of the sky"* as a result of flying in cumulonimbus clouds - here's a reminder of the damage that can be caused by weather which wasn't as well avoided as it could have been. CB clouds show up on weather radar, if it is used properly - and pilots work their way around them. Volcanic ash doesn't show up on radar - but dangerous concentrations can be tracked, and the systems have existed for years whereby airspace can be closed dynamically by air traffic control. Similarly, cosmic background radiation and solar magnetic storms - even the Millenium Bug! - have an impact on aviation, but don't result in the wholesale shutdown of airspace.
I'm not saying the decision to close airspace was wrong. However, we have had little information of substance - safe concentrations of ash, what the concentrations actually are at the moment at different levels, anything more sensible than a hand-to-mouth closure of large swathes of airspace. The volcanic ash is being presented to the world not as something for which a proper risk assessment has been carried out, but as an unknown, out-of-control bogeyman. The media have lapped this up - queues at airports and train stations, disruption, cancelled holidays, phlegmatic travellers all make good airtime. But at some stage, someone is going to have to open the airspace again. And given the way it has been presented up to now, the response of an unnecessarily large group of people will be to say - "Hmm, I don't know. It doesn't look any sunnier now than it did when the airspace was closed. How can they tell it's safe ...?"
*Just to be clear, that is journalistic hyperbole. Aeroplanes fall out of the sky if they blow up or their wings drop off. People don't survive such events to write about them. So if you read about an aeroplane falling out of the sky, you should understand that somebody isn't really describing what happened. In the case of the BA 747 which "fell out of the sky" following its encounter with volcanic ash 28 years ago, it actually descended (albeit in terrifying circumstances, in darkness and turbulence) as it would on a descent to an airport from above 35000 feet to 12000 feet, I believe, before the crew managed to restart one of the engines.