Dendroclimatology

Source: Wikipedia

Trees lay down new tissue in circular patterns called “rings”. Scientist can look at these rings to see what was happening with the climate while the tree was growing. Scientists examine a grove of trees, so there are a wide variety of samples. To collect samples from living trees, scientists use a tree corer, similar to an apple corer, known as an increment borer. The samples are sanded down and examined underneath a microscope.