When considering what will be the trigger for the next BIG unscheduled air space closure? Consider:
BBC News - UK 'can cope with solar superstorm'
But the experts stress that it is the sum of a number of issues all happening at once rather than one or two big calamities that will test society's ability to cope.
"It will be perhaps comparable to the Icelandic volcano eruption [in 2010], or something similar, where there will be severe disruption to our way of life for a while, but it will be something we believe we can deal with," Prof Paul Cannon, the report chairman, told BBC News.
My emphasis.