During the 2.4 million-year lifetime of the last ice epoch, about two dozen ice ages occurred. That means that the earth's average annual temperature fluctuated upwards and downwards to a very significant extent about two dozen times during the 2.4 million-year period. In each case, a period of significant cooling was followed by a period of significant warming—an interglacial period after which cooling once more took place.
Scientists know a great deal about the cycle of cooling and warming that has taken place on the earth over the last 125,000 years, the period of the last ice age cycle. They have been able to specify with some degree of precision the centuries and decades during which ice sheets began to expand and diminish. For example, the most severe temperatures during the last ice age were recorded about 50,000 years ago. Temperatures then warmed before plunging again about 18,000 years ago.
Clear historical records are available for one of the most severe recent cooling periods, a period now known as the Little Ice Age. This period ran from about the fifteenth to the nineteenth century and caused widespread crop failure and loss of human life throughout Europe.
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