Katla (myjarfaraskoll(sp?), can be tracked on Iceland site, en.vedur, google seismic activity Iceland volcanos. In past 2000 years, Katla has erupted 18-30 months after little sister, each and every time, plus a few times without the warning. Has averaged 20x larger than the 2010 12day eruption.
1918 the last eruption: Here in Maine, menfolk went out to cut wood for winter on 6" of ice on the lakes the day after Thanksgiving (my family recalls)
Ice that year in the bays froze clear out to the islands (sea ice).
Figure 1-2 climate zones colder for 1-2 seasons, cutting out significant % of N. hemispher northern most grain belt, depending on the time and extent of the eruption.
Grain and meat prices will be VERY adversely affected. Seed corn will last 10 years well stored, much longer long term stored. Roy's Calais flint was the only corn that survived in New England 1816, when a major volcano blew (Java?) Other good corns include OP painted Mountain, and similar. I have kucyk, but seed supply is out for this year.