What will the volcano in Iceland mean for us?
Back in the early 90s, Mount Pinatubo in the Philippines erupted, ejecting 10 billion metric tons of magma and 20 million tons of sulfuric acid into the atmosphere. Not only was the system devastating to Luzon, the island where the eruption occurred, but it affected the entire planet for about a 12 month period. The hemispheric average temperature was down an entire degree Fahrenheit after the eruption, thanks to the concentration of ash in the atmosphere, reflecting the suns radiation. This is a dramatic drop in temperature. Consider, that's the same temperature change that is being discussed in regards to global warming. Even then, the degree change is somewhat misleading, as the lowest temperature change would be over the Pacific, where there are fewer recording stations, and the change in temperature would not have a dramatic impact.
Now, in 2010, we are poised for a similar event. The Eyjafjallajokull (and that's the last time I'm typing that) volcano in Iceland hasn't put out as much ash, or sent it as high in the atmosphere as Pinatubo did, but the Icelandic volcano has a history of longer eruptions. That said, the global temperature drop will likely be imperceptible when taken on a large scale. That said, this stands to have a greater impact on more people than Pinatubo did. Remember, that Pinatubo's most dense ash was over the Pacific. Iceland's ash will end up over Europe, as it already is. Even as the ash clears up at flight levels and air travel picks up again, temperatures for the summer and perhaps even into the window are going to be down in Europe. Here in the United States, it is extremely unlikely we see any temperature changes, unless the volcano somehow becomes more explosive. Even then, we won't see a substantial drop in temperature stateside as we did with Pinatubo, as the ash would have to wrap its way all the way around the globe.