The global climate has warmed by over 0.5 C during the last 125 years. Models of the Earth’s temperature response to increasing levels of atmospheric carbon dioxide and other greenhouse gases estimate that the average global surface temperature will rise about 4 C by the mid twenty-first century. High latitudes will warm more than lower latitudes and winters more than summers. Forests will undergo enormous changes as temperatures increase and precipitation patterns shift. It is doubtful that forest movement will meet the rate of climate change. Many sensitive hardwood tree species, such as paper and yellow birch, sugar maple and black ash, may die. Boreal forests will replace tundra and mixed hardwood forest will replace them. Much of the change expected in the location and composition of forests may be detected by means of remote sensing.