The rings inside trees are beginning to tell a different story

In a basement in Tucson, Ariz., slices of tree trunks stand on shelves. They are ragged with age, scarred from fire, with bark like the crust of a lava flow. But on the face of each enormous slab is a pattern of concentric rings like a vast ripple. Run a hand over the surface. Those are years under your fingers.

Every spring in much of the world, the cells under a tree’s bark wake up and start to divide. This forms a visible ring marking each year of the organism’s life. In a tree sensitive to moisture, the rainier the year, the more cells the tree makes and the thicker the ring. So if you know the year a tree was cut down, you can count backward to figure out exactly what year a catastrophic drought began. And by matching up the rings in multiple long-dead trees, you can understand the rhythms of climate much farther back than any human records go. This is because when trees grow under certain constraints, each ring faithfully reports its local conditions in a way that’s relatively easy for scientists to interpret.

Or at least, that used to be true.

Climate change is altering the story that trees are telling. Scientists are having to adjust, coming up with new approaches to interpret the changing signal. And yet, as the scientists are shifting their techniques, it may be possible for them to design methods that get even more precise about the past — and in turn will help us better adapt to the future.