The last 15 years have not only seen an explosion in cheap airline travel - spawning new tourist industries in once-forgotten European cities - but there has been an increase in the use of air transport for goods, mail, soldiers and much else besides. What would happen if this is ground to a halt in Europe not for a weekend or weeks but months?
First, there would be a serious impact on the affected industries - airlines, tax-free outlets, hotels and ultimately the cities that have sprung to life after the arrival of no-frills travel - like Poznan or Faro.
Second, there would be a real cost in terms of jobs, goods and services as companies who rely on transport by air go under. It is not just that fresh fruit would take a lot longer to reach Britain but that entire supply-chain systems would have to rethought. As people use cars to jet across Europe to make their appointments it is not unlikely that the volcano will extract a hidden toll in increased traffic-related deaths.
Then there is the conceptual shift which would occur. If you cannot easily travel to somewhere in Europe (besides Paris and Brussels) how will this impact neighbourly relations on the continent.
European diplomacy would certainly have to change. Forget the once-a-month European summits - if EU leaders have to travel to Brussels by train they are unlikely to meet more than twice a year. Even the transatlantic link may be altered if the continent's leaders can take a train to Moscow but only sail to the US.
There would also be an adverse impact on defence. The expeditionary model depends on strategic lift, most of which is by air. If you can only ferry troops to the front by ship, train or trucks warfare may become more akin to that of the First World War than the Post-Cold War i.e. determined by train-tables.
On the other hand, if airplanes stop flying Europe's train system will probably be improved very quickly and environmental groups will be happy that the pollution caused by airplanes would disappear.
Right now everyone assumes that the volcanic ash will not remain a problem for long. But if it does, the world will end up looking very different than it does today.