Image copyright AFP Image caption Millions of air passengers were stranded by the Eyjafjallajökull eruption in 2010

Potentially disruptive volcanic ash clouds across Northern Europe occur more frequently than previously thought, according to new research.

Scientists investigated known and newly identified records of ash fall deposits over the past few thousand years and concluded the average return rate to be about 44 years.

Previous research had put the recurrence at roughly 56 years.

The source of the ash is almost always from Iceland.

In 2010, the island’s Eyjafjallajökull volcano erupted, throwing some 250 million tonnes of fine particles into the atmosphere that grounded planes across Europe.

The eruption of Grímsvötn the following year also disrupted air traffic - albeit on a much smaller scale.

But despite these two recent, closely spaced events, the team behind the latest research says the general frequency of volcanic ash clouds over Northern Europe is still generally quite low.

"The more data you have the more confidence you can have in your estimates, and that’s what we set out to do in this study," Dr Ivan Savov from Leeds University told BBC News.

And lead author Dr Liz Watson added: "Reliable estimates of the frequency of volcanic ash events could help airlines, insurance companies and the travelling public mitigate the economic losses and disruption caused by ash clouds in the future."

Image copyright NASA Image caption Only the big, explosive eruptions that put ash high into the atmosphere will send a cloud across Northern Europe. This fine-grained material will eventually fall to the ground where it may be preserved in lake and peatland sediments

With written records of ash fall across Europe extending back over only a few hundred years, scientists must look to the environment if they want to understand the real, long-term return rate of such events.

There is a reasonable archive of European samples showing where and and when ash has fallen in the past, but Dr Watson and colleagues wanted to check whether there were any gaps in the databases.

The team therefore set about examining peatlands and lake beds in England, Wales, Sweden and Poland, drilling sediment cores to try to find traces of the tiny glassy shards produced by volcanoes - so-called tephra.

Everywhere the scientists looked, they found new ash layers, indicating that gaps in the archive were more reflective of past research effort than of the actual incidence of volcanic ash dispersal.

For many of the layers, the group could match the tephra to historical records or to the existing geological archives that catalogued specific eruptions.

Looking back over 7,000 years, the study found evidence for 84 ash clouds spreading over Northern Europe. Almost exclusively these were Icelandic in origin, although Alaskan and Russian traces were evident also.

Image copyright Leeds University Image caption The scientists searched for the tiny fragments of volcanic glass

Looking at the better preserved record of the past 1,000 years, the team estimated an average recurrence of 44 years, give or take seven years. Put another way, the group says there is about a one-in-five chance of a disruptive cloud occurring in any one decade.

"To do the statistical modelling, you need a lot of eruptions," explained Dr Savov.

"The older the horizons we discovered, the more eroded or dissolved or more uncertain they became. But we have a pretty large database, and a large degree of certainty when it comes to the last 1,000 years, and so our model is based on this period."

The new study is published in the journal Earth and Planetary Science Letters. The team included co-workers from the universities of St Andrews and South Florida.

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos