E arth's climate is changing.
Geologists know great ice sheets once covered large portions of the continents.
These glaciers have alternately retreated and advanced as the earth has warmed and cooled, in cycles spanning hundreds, thousands, and millions of years. Historical data from ocean sediments and ice cores indicate warm interglacial periods of 15,000 - 20,000 years separate each major ice age. The Earth was a much colder and drier place then. Deserts were more extensive, summers were short, and winters brutal. At the peak of glaciation, oceans were 300 feet lower than they are today, allowing animals and men to walk from Siberia to Alaska across the Aleutian Land Bridge, causing changes to the ecosystem of North America.