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The most significant effect is typically to weather. It’s not yet clear whether the eruption of Eyjafjallajökull will affect global weather, but major eruptions in the past certainly have. In 1783, for example, a volcanic fissure in Iceland called “Laki” violently erupted. The sulfur dioxide gases carried with its plume caused increased death rates all over Europe over the next month or two, but that was nothing compared to the meteorological effects. The winter of 1784 was one of the worst on record in both Europe and North America — the Mississippi River even froze at New Orleans! Laki, along with another Icelandic eruption shortly thereafter, had severe effects on crops in France over the next several years, which may have been a contributing factor in the French Revolution of 1789. And if you think that was bad, you should know it wasn’t nearly as significant as the 1815 eruption of Mount Tambora in Indonesia, which caused the Northern Hemisphere to experience “The Year Without a Summer” in 1816. Frost was reported in Connecticut in June, famine was widespread in the U.S. and Europe, and — less importantly, but interestingly — the lousy summer caused Mary Shelley and John Polidori (and their friends) to stay indoors while on vacation, resulting in the novel Frankenstein and the short story The Vampyre.
Other than halting air travel, what other effects can ash clouds have?