<div> Western North America is a unique area to use as a laboratory for climate change. Aridity makes it highly sensitive to changes in precipitation and the majority of its water resources come from mountain streams fed by snowfall. Water availability in the West is dependent on both precipitation and evapotranspiration loss. Any change to water availability has a direct affect on agriculture and livestock ranching throughout the region.<BR> Climate change is a subject of extreme breadth and complexity. This book focuses largely on climate change during the twentieth century and the associated environmental effects in western North America. It also uses models based on twentieth-century changes to project twenty-first-century changes. The scientific evidence presented here does not engage in policy advocacy, but provides policy makers useful information in planning for the future. </div>