How has the world's climate changed over the years?

Over the past few million years, Earth’s climate has changed many times. In the last million years, there have been four ice ages, or glacials, which occurred about every 100,000 years and were interspersed with shorter, warmer interglacials. During the ice ages there was severe cold and large sections of ice spread across the land. The average temperature of Earth was six to eight degrees Fahrenheit below today’s averages. As the ice moved, it made hollows in the land, pushing soil and rocks ahead of it. The sea level dropped and much of the water froze. After the ice ages, the interglacial period brought warmer weather. The ice melted and the huge hollows filled with water and became lakes. About 7,000 years ago the North American and Scandinavian ice sheets melted, and as sea levels rose the coastlines of the continents slowly took on their present shape. Scientists gather evidence about these past climates by studying sediment samples from the beds of the oceans or ice samples taken from Antarctica.