During the past billion years, the Earth’s climate has changed from warm, ice-free periods to cold periods where the Earth was covered with many huge glaciers, and back to warm again. We are currently near the end a warm period in between ice ages. It is believed that there were many ice ages, 36, 15, and 3 million years ago. The last ice age is estimated to have been about 20,000 years ago. Approximately 55 million years ago the whole earth was warm. The difference in temperature of the equator and the arctic was much less than it is now. Tropical conditions existed into the Central United States. It was so warm that trees grew at both poles, and alligators lived in Ellesmere Island, 78 degrees North. This warm period was called the Eocene. The Eocene was followed by cooling.Between 52 and 36 million years ago icecaps began to develop in Antarctica and the temperature around the ice caps began to drop. About 28 million years ago sheets of ice began to emerge at the poles, cooling the surrounding ocean even more. Eighteen million years ago it became somewhat warmer, but it did not last long. After the brief warming, the temperatures fell again, so that by 7 million years ago Greenland was completely covered with glaciers. The glaciers started to creep down into Scandinavia and the North Pacific. Once again it became warmer.Between 5 and 3 million years ago the sea around North America and the Arctic was much warmer. Plants grew in Europe that can now only survive in the tropics. We are now experiencing another cooling period that started about 3 and a half million years ago....
