If the volcano in Iceland continues to blow ash into the stratosphere for a significant amount of time the natural thing to happen is a cooling of the climate. It's happened several times in history. Most notably, the year without a summer in 1816 was caused by a volcano eruption in Indonesia. Hundreds froze to death in Europe and crops failed all over the world.
Another example is Krakatoa. The explosion of Krakatoa in 1883 dropped global temps 1.2C in the year following the explosion
This is likely not a repeat of 1815 or 1883, but the effects of a volcano explosion has happened at least once that I remember and was affected by. I believe it was in 1983 that a volcano erupted in Mexico. We had volcanic ash on our cars that year. I remember hearing not to wipe it off with your hands as it would scar the paint on your car. The winter of 83-84 was brutal. It got down to near zero in December of 83. The winter of 84-85 was just as bad. It was an El Nino year and the San Antonio area had 13" of snow in January 85.
So, if the Iceland volcano produces a mini-ice age as the Mexico volcano did, or worse as in 1816 or 1883, the global warming deniers will capitalize on it to the fullest extent possible. If it gets cold quickly this fall, it may even be an issue in November because of Cap and Trade votes.
The Iceland eruption looks fairly powerful. We'll see what happens.
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