Chronicles of the Rings: What Trees Tell Us

Studying the historical data stored in centuries-old trees is a burgeoning field, with labs around the world learning more about historical patterns of weather and climate and the effects on humans.

Wood samples for research at the Laboratory of Tree Ring Research at the University of Arizona. Adriana Zehbrauskas for The New York Times

TUCSON — From the early 1700s until the 1960s, the fast moving river of wind known as the North Atlantic Jet Stream, which drives weather extremes over Europe, was pretty steady on its course.

Then it became less predictable. But instrument data alone can’t tell the jet stream’s movements for comparison over the centuries, given that scientists began keeping records of weather events via instruments only in the late 19th century.

The rings of trees, however, offer a far more complete historical picture of climate variations. As they age, trees grow outward from the center, and each year a new, distinct circle of dead wood is created around the trunk of most trees. In that ring, one can find information about precipitation, temperature and other data about that year.

A team led by Valerie Trouet, a dendrochronologist, sampled 400 trees from the Balkans and 200 in Scotland — including what might be the oldest known tree in Europe, a Bosnian pine in Greece named Adonis, which is 1,075 years old. The jet stream flows between these two regions, and trees revealed the range of temperatures in their rings and the frequency of fires over time, an expansive chronicling of jet stream behavior.

“More extreme positions create more extreme climate events, especially heat waves and storms,” in Europe, Dr. Trouet said. And the tree rings show “big fires happen in the Balkans when the jet is in its southerly position.”…