Iceland could really mess things up...

Because of Bárðarbunga.  Daily Kos's Rei tells the story:

The winter of 1783-1784. The Revolutionary War had just ended, and Benjamin Franklin was puzzling over the nation's bizarre weather. Congress had been delayed getting to Annapolis to vote for the Treaty of Paris because the Chesapeake Bay just wouldn't melt. The Mississippi River froze down to New Orleans, and ice was reported floating in the Gulf of Mexico. Reports from Europe were of a bizarrely hot summer with thick fog that was choking people to death in Scotland, massive hailstones, lightning, and crop failures. The sun was blood-red at noon. Mass starvation that would ultimately kill 1/6ths of Egypt's population took hold due to a historic drought of the Nile. As many as six million people would die from the bizarre weather.

Franklin was one of the few scientists of the era to (almost) correctly speculate as to its cause:

"The cause of this universal fog is not yet ascertained [...] or whether it was the vast quantity of smoke, long continuing, to issue during the summer from Hekla in Iceland, and that other volcano which arose out of the sea near that island, which smoke might be spread by various winds, over the northern part of the world, is yet uncertain."

He, however, had mixed up his Icelandic volcanoes, for it was not Hekla that erupted that year, causing the planet-altering weather, but Laki (Eldgjá). A rift 23 kilometers long opened up in places up to 100 meters wide with lava fountains at times reaching over a kilometers into the air - and it continued erupting for 8 months.

The total quantity of lava erupted - 14 cubic kilometers - was not that much more than Mount Pinatubo (largest eruption of the 20th century)'s 10 cubic kilometers. But the eruption kicked out a staggering 120 million tons of sulfur dioxide, compared to Pinatubo's 17 million - nearly supervolcano levels. Also unusually, Laki emitted 8 million tons of hydrogen fluoride - normally a trace volcanic gas. These gasses created the "Laki Haze" across Europe. In Iceland, the consequences were most severe - a quarter of the population starved or died of fluoride poisoning, and most of the livestock died. Denmark considered evacuating the entire island.

Is Laki threatening to go off? No. Then why do I mention him?

Because his big sister IS threatening to go off.

Bárðarbunga (BOWR-thar-Boon-kah, "Bárður's Bulge") is part of the same volcanic system, but is much larger than Laki. Barðárbunga stretches out over 200 kilometers long. It has a large eruption every 250-600 years. One of its eruptions before settlers arrived was 21-30 cubic kilometers of lava. Like her little brother Laki, she's associated with massive amounts of toxic gas release.

Update:
Rei's update.