Scientists gather climate change evidence from centuries-old sources, as well as modern ones. Tall, wide trees can show temperature and precipitation levels over hundreds of years, thanks to information stored in their wooden centers. Studying old trees is a growing field, with labs around the world learning more about historical patterns of seasonal weather, climate variations and the effects on humans. The field "has exploded," says Edward Cook, director of a Columbia University tree ring lab in New York. Data from living trees' core samples and the concentric rings of trees that fall naturally is far more extensive than satellite images of forests, carbon dioxide measurements, jet stream records and computer models based on about 30 years of data.

Trees grow outward from the center, creating a distinct circle of dead wood annually around the trunk. Those rings have information about water, temperature, fires and other natural factors each year. For example, tree rings usually grow wider in warm, wet years. They're thinner in years when it is cold and dry. If the tree has experienced stressful conditions, such as a drought, the tree might hardly grow at all in those years. "This data helps the modeling of climate change become more reliable," says Valerie Trouet, an assistant professor at the University of Arizona’s Institute of the Environment. Weather instrument readings date back only to the 1890s or so, while some big trees have been around much longer. Trees are "giant organic recording devices," The New York Times put it last week.

About a dozen large labs around the world analyze data from 4,000 sites on all continents except Antarctica. The information is stored in the International Tree Ring Data Bank, an online library for all researchers. It's based in Tucson at the University of Arizona, which has the largest collection of tree ring samples. They show huge environmental shifts. Climate change in the past six or seven decades has few, if any, comparisons in the distant past, researchers say.

Researcher says: "We can learn a lot from the memories of trees." – Neil Pederson, Harvard University senior ecologist.

Earth is hotter and drier: "We keep breaking records year after year. It's a little worrisome to see the most extreme years right near the present." – Professor David Meko, University of Arizona Laboratory of Tree-Ring Research

Magazine headline: “Tree rings contain secrets from the forest” – Popular Science, March 2019