THE QUATERNARY PERIOD, the most recent geologic interval, represents the last 1.8 million years of time. Its most striking feature is that the Earth had cold polar regions, which led to periodic development of continental glaciers. The prolonged Ice Ages, comprising the main part of the Quaternary interval, ended 10,000 years ago, when the continental glaciers had melted. Compared to the many cold periods (glacial intervals) of the Quaternary, the last 10,000 years of the present interglacial has been comparatively warm.

The changes in climate over the past 150 years have shown a varied history. The world endured a historic cool period during the late 1800s (the Little Ice Age), followed by a warm period of the 1930s (the Dust Bowl years), and since about 2000, ...