This book describes thoroughly the North American Climate of the past 65 million years, with special emphasis on the last 21,000 years, as revealed by paleoclimatic observations and climate models. It analyzes weather observations over the past century and satellite measurements of the last few decades to develop a picture of more recent climatic trends. It explains how global climate models are used to simulate and project climate, and presents the application of these models to reproduce recent climate variations and predict future North American climate. It answers the critical question of whether observed climate change is due to natural variations or human activity.