Ice Ages

Ice ages
Throughout Earth's history, there have been times when conditions have been alternately warmer and colder than they are today. There have also been several periods, called ice ages, during which large portions of the planet's surface have been covered with ice. Ice ages, or glacial periods, have occurred at irregular intervals for more than 2.3 billion years.
The Precambrian period (from 4.6 billion years ago to 570 million years ago) had four ice ages. The first occurred somewhere between 2.7 billion and 2.3 billion years ago. Then the Earth warmed up and was free of ice for almost a billion years. The second ice age occurred between 950 and 890 million years ago; the third between 820 and 730 million years ago; and the fourth between 640 and 580 million years ago. In each case, some area of the Earth was iced over for about 100 million years.
The Mesozoic Era (225 to 65 million years ago) experienced a number of temperature swings, ending in an ice age. This resulted in the extinction of about 70 percent of the living species on Earth, including the dinosaurs. That ice age may have been a result of a collision between an asteroid with the Earth, which created a dust cloud that blocked out the sun.
From 2.4 million years ago to 11,000 years ago, the period known as the "Great Ice Age" (which most people think of as the ice age), there were two dozen times when the global temperature has plummeted. And for seven different intervals over the last 1.6 million years, up to 32 percent of the Earth's surface has been covered with ice. Scientists estimate that throughout this time period, new ice ages have started about every 100,000 years and have been separated by warmer, interglacial periods, each lasting at least 10,000 years.
The most recent glacial period peaked between 22,000 and 18,000 years ago. At its height, about 27 percent of the world's present land area was covered by ice (compared with 10.4 percent today). There were glaciers up to 10,000...