Tree rings provide clues about climates of the past. As a tree grows, it forms pairs of light and dark rings each year. When growing conditions are good - the right mix of temperature and precipitation - trees grow quickly, forming wider rings. Other years, such as during a drought, trees produce thinner rings. Scientists can "read" tree ring patterns and determine what the climate was like in the past - sometimes as far back as 10,000 years! The study of tree rings is called "dendrochronology".