Over thousands and millions of years, the Earth’s climate has changed many times. About 20,000 years ago, much of the United States and Canada was covered in glaciers. Today, the United States has a warmer climate and few glaciers.
During ice ages, the water level in the oceans dropped because so much water was frozen on land as ice and snow. The water level in the oceans became higher during warm periods because much of the ice and snow on the land melted.
Some content courtesy of NASA Climate Kids.