Each year, as a tree grows, it adds a light ring of new growth to its trunk during the spring and early summer, when growing quickly, and a thinner, dark ring in the fall, when growth is slower. Because climate affects the amount a tree grows and the thickness of the rings, scientists use tree rings to learn about past climate.

During years when the temperature and rainfall are ideal for a species of tree, the trees grow faster and their rings are wider. Other years, when it is too hot or cold or too wet or dry, the trees grow more slowly and rings are much thinner.

The science of tree rings is called dendrochronology. The study of climates of the past is known as paleoclimatology. Climate scientists use clues from ice cores, layered sediment deposits in lakes and seas, the structure of coral reefs, as well as tree ring sequences to learn about paleoclimates. The use of tree ring records to decode Earth's climate history is called dendroclimatology.

Climate scientists collect data from many trees to get a clear picture of past climate because the growth of any single tree can be influenced by its specific location such as being in the shade or near a stream. Scientists collect and average data from many trees to reduce the influence of these sorts of variations. Likewise, different species of trees grow at different rates depending on the temperature, precipitation, and other factors. Scientists must therefore collect tree ring data from many different types of trees to formulate the most accurate record of climates of the past. The trees don’t need to be cut down to see their rings. Instead, a special tool is used to bore out a thin, pencil-sized core from a living tree. Removing such a core does little or no damage to the tree.

Old trees tell us about conditions on Earth long before people started measuring and recording the weather. Climate scientists can extend these dendroclimatology records back more than 10,000 years by comparing ring patterns of living trees with the rings in dead but not-yet-decayed trees that have fallen. Scientists match patterns from the early stages of a living tree's rings with the sequence formed in the latter parts of the lives of older, dead trees to assemble an unbroken paleoclimate record extending back thousands of years.

As a tree grows, it forms new wood near the outer part of its trunk, just beneath the bark. In a cross-section of a tree, the oldest rings are smaller and near the center, while the youngest rings are larger and close to the outer edge. Trees that grow in mid- to high-latitude regions form rings that are easy to spot because they have a distinct growing season when a light colored ring forms. As growth slows in the late summer or fall, wood forms more slowly and is darker in color. This is not the case in the tropics where trees grow year-round adding wood to their trunks at an almost constant rate. Trees from the tropics generally show little or no alternating dark and light growth patterns and so are not very useful for dendrochronology studies.