evidence: Earth has also experienced several major ice ages—at least four in the past 500,000 years. During these periods, Earth’s temperature decreased, causing an expansion of ice sheets and glaciers. The most recent Ice Age began about two million years ago and peaked about 20,000 years ago. The ice caps began retreating 18,000 years ago. They have not disappeared completely, however. Their presence in Antarctica and Greenland suggests Earth is still in a sort of ice age. Many scientists believe we are in an interglacial period, when warmer temperatures have caused the ice caps to recede. Many centuries from now, the glaciers may advance again. [...] Climate changes occur over shorter periods, as well. For example, the so-called Little Ice Age lasted only a few hundred years, peaking during the 16th and 17th centuries. During this time, average global temperatures were 1 to 1.5 degrees Celsius (2 to 3 degrees Fahrenheit) cooler than they are today. [...] One way scientists have learned about the Little Ice Age is by studying the rings of trees that are more than 300 years old. The thickness of tree rings is related to the amount of the trees’ annual growth. This in turn is related to climate changes. During times of drought or cold, trees could not grow as much. The rings would be closer together.