Tree rings speak to ancient climate change

As every school child knows, counting the growth rings in a tree tells you how old the tree is. But some samples of wood can tell you even more than age. That’s because some trees live in difficult environments. They grow best only when there is a good year in terms of precipitation, temperature, and the like, so they have growth rings that are quite uneven. Some are thick, representing good years for growth, while others are quite thin from when times were tough.

In the southwest U.S., a lot of work has been done with tree rings. Indeed, the whole science of what’s called dendrochronology was...