How Do Preserved Tree Rings Indicate Changes in Climate?

Ring Configuration

• Each tree ring has two bands, one light, one dark, The lighter colored area is produced when the tree grows rapidly at the start of the growing season. The darker wood is produced later in the growing season, when growth has slowed down. These two areas together represent one year of tree growth. The ring on the outside of the tree, closest to the bark, is the youngest ring. If the tree was just cut down, that ring represents the growth during the most recent growing season. From there, you can work backward and match each ring to its year of growth. Knowing the species of tree and the location where it was growing will help in interpreting the data. Cross-dating known trees with preserved trees by comparing ring width patterns will provide accurate dates for trees that have been dead for many years.

Narrow Rings

• Narrower rings indicate a bad growing year. In the trees first few years, narrow rings can simply be a consequence of the tree being shaded by larger trees and not receiving adequate sunlight for more vigorous growth. As the tree ages, narrow rings indicate a bad growing season. In northern areas with a short growing season, narrow rings could indicate lower temperatures and a shortened growth period. In other areas, narrower rings could indicate a lack of moisture.

Wide Rings

• Thicker growth rings are indicative of good growing seasons. In northern areas, it likely means higher temperatures. For other areas, it could mean adequate moisture.

Changes in Climate

• By looking at ring patterns, it is possible to deduce changes in climate going back beyond the point of instrumental records. The larger the number of samples, the more accurate the results will be. Comparing average ring widths for each year will reveal a pattern of changes in the climate. Where rings are approximately the same width for several years, the climate was roughly the same. Where the rings become narrower or wider, a change in the climate is likely. From this, researchers can deduce when temperatures or precipitation changed and the degree of change. Researchers at the University of Hawaii, for example, used tree ring data to study el Nino events going back 1,100 years, and discovered changes in the intensity of the climatic event.