Scientists forecast decades of ash clouds - Times Online

...They have reconstructed a timeline of 205 eruptions in Iceland, spanning the past 1,100 years, and found that they occur in regular cycles — with the relatively quiet phase that dominated the past five decades now coming to an end.
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Grimsvotn, another highly active volcano, lies under the huge Vatnajokull glacier in Iceland’s southeast. An eruption in 1996 saw much of this glacial ice melt, causing a flood that washed away the country’s main ring road.

It is linked to the massive Laki fissure volcano whose 1783 eruption ejected so much ash into the atmosphere that it cooled the entire northern hemisphere for nearly three years. The resulting low temperatures caused crop failures and famines that killed 2m people and helped trigger the French Revolution.