Tim De Chant in ars technica:
Ask any second grader what you can do with the rings on a tree, and they'll respond, “Learn the age of the tree!” They're not wrong, but dendrochronology—the dating of trees based on patterns in their rings—is more than just counting rings. The hundred year-old discipline has given scientists access to extraordinarily detailed records of climate and environmental conditions hundreds, even thousands of years ago.
The ancient Greeks were the first people known to realize the link between a tree's rings and its age but, for most of history, that was the limit of our knowledge. It wasn’t until 1901 that an astronomer at Arizona's Lowell Observatory was hit with a very terrestrial idea—that climatic variations affected the size of a tree's rings. The idea would change the way scientists study the climate, providing them with over 10,000 years of continuous data that is an important part of modern climate models.
A. E. Douglass, the astronomer in question, is revered as the father of dendrochronology even though one of the field's basic concepts—crossdating, or the matching of ring patterns between trees—was independently discovered on four earlier occasions. (Pioneering computer scientist Charles Babbage was among that group.) Douglass was the first to apply truly scientific rigor to the study of tree rings, using a quantitative approach to tie variations in ring width to available climate records.
For the next dozen years, Douglass scoured Arizona for Ponderosa pine—dead or alive—to construct his first chronology. Completed in 1914, Douglass's chronology stretched back nearly 500 years, a feat accomplished by crossdating. Months later, Douglass teamed with an anthropologist to date timbers in pueblos in the American Southwest. For the rest of his life, Douglass continued to develop the science of dendrochronology. Though he was never able to tie sunspot activity to ring patterns—his original inspiration—his new field found favor with climatologists.