The movement of the tectonic plates and subsequent tsunami off the coast of Japan reminds us that we live on a thin crust subject to the whims of nature. In 2004, a similar event in the Indian Ocean killed 230,000 people in dozens of countries. Devastating as these tsunamis were, they had little effect on the atmosphere, unlike that of volcanoes. I am reminded of Krakatoa between Sumatra and Java, Indonesia in 1883, which killed 36,000 people. In the same area, Tambora, in 1816 produced an ash cloud that resulted in a year without a summer, and stunted harvests and created hunger in Europe and Russia.
David Keys in his book, Catastrophe, presents evidence of a much larger eruption in 835 that blew away the island upon which it sat. He associates the major climatic change to follow (floods and drought) with the disintegration of the Roman Empire, waves of bubonic plague, westward movement of the Avars out of Mongolia, and the rapid rise of Islam. When food sources change dramatically, political and social changes follow.