from The Great Debate UK:
Within the space of just over a year, aircraft have now been grounded in Europe twice by ash blowing in from Iceland. This has caused many millions of pounds of disruption.
A key question uppermost in many minds is whether the frequency of eruptions in Iceland is increasing. The short answer here is ‘yes, probably’. But, it is not just the frequency of eruptions that matters. To impact the airspace of the United Kingdom and continental Europe, the ash has to be ejected high enough, and be fine enough that it can remain airborne for days. Then the winds have to conspire to push it towards Europe, and the winds blow over Iceland from the north only a small fraction of the time.
Studies by researchers at the University of Iceland have noted that the frequency of eruptions from volcanoes beneath the Vatnajökull ice cap (including Grímsvötn, which erupted in May this year) seems to wax and wane with a cycle of about 140 years.
Prior to the 1980s, there were no such eruptions from these volcanoes for over 40 years and there have now been four within 15 years. The reason for the apparent cyclicity is not established but, in any case, we appear very likely to have entered a phase of more eruptions.