March 26, 2002 Tree Rings Show a Period of Widespread Warming in Medieval Age By KENNETH CHANG A new study of old tree rings shows that 1,000 years ago, long before power plants and sport utility vehicles, temperatures across North America, Europe and Asia rose in a period of unusual warmth. In warm weather, trees thrive and grow a thick ring of wood in their trunks for that year. In cold years, growth slows and the tree ring is thin. Temperatures were known to be warm in Europe between 900 and 1100, what is known as the Medieval Warm Period....