Large-scale tree planting is advocated to provide additional atmospheric cooling and further reduce global warming. This raises a question about the present time: do trees cool or warm the atmosphere? This question does not have a simple yes or no answer. Examination of the greenhouse effect, global warming and the carbon cycle, and how trees and forests function provides the basis for understanding how forests might cool or warm the atmosphere. Results from research and models indicate that cooling or warming depends on where forests are located and the type and color of trees. Cooling generally prevails over warming, but this may change. This book will appeal to anyone interested in climate change, ecology and conservation.