Even before the early ancestors of modern humans began walking on 2 legs, climate change was a constant and ongoing process. During an earlier warm era, trees and plants now common in areas of the United States were found near the north pole.1 Only 18,000 years ago most of the North American Continent was covered in ice.2 From historical records a recent cooling period known as the little ice age occurred from the 15th to the 19th century. This cooling caused a decline in human existence because of massive crop failures and hypothermia-related disease.3 The subsequent warming trend that began in the 19th century has been good for humans; however, for the first time in history our activities may be affecting the environment in ways that are contributing to global climate change.