If we wanted to understand the deep history of earth’s climate, we could examine an oft-overlooked place: the inside of a tree. Tree rings—particularly from trees that have been around for over thousands of years—can be particularly telling.  Traumatic events affecting growth, such as severe storms or fires, can leave their mark on the annual ring of the year in question, as well as on the structure of the tree rings in the next generation.

Tree rings are formed by changes in a tree’s growth rate over the course of a year. When trees grow, the thin layer of living tissue beneath their bark lays down new cells on top of the older ones. As the bark expands, the oldest cells die off, leaving their hardened skeletons behind—this is what we count as a single ring when examining the cross-section of a tree. Tree rings grow rapidly in warmer, moister climates, and subsequent rings grow tighter together in drier, more stressful periods.

In 1901, American astronomer A.E. Douglass had the idea that changes in climate affected the size of a tree’s rings. Writing in The Scientific Monthly, he wrote:

Nature is a book of many pages and each page tells a fascinating story to him who learns her language…. The forest is one of the smaller pages in nature’s book, and to him who reads it too tells a long and vivid story…. The trees composing the forest rejoice and lament with its successes and failures and carry year by year something of its story in their annual rings. The study of their manner of telling the story takes us deeply into questions of the species and the individual.

In 1920, after studying approximately 85,000 rings from 275 trees, Douglass published a study where he looked at how tree growth correlated with annual ring fall. Douglass, however, “greatly regretted” at the time that no data was available from the Giant Sequoia trees, which would give us “the longest uninterrupted series of annual climatic effects.”

Recent studies of tree rings have provided some illuminating insights on climate. For instance, in 2009, researchers constructed a 3,000 year chronology of fire events in the Giant Seqouia trees. They found that the years between 800 and 1300, otherwise known as the “Medieval Warm Period,” had the most frequent fires on record. It was important to study how Giant Sequoia trees responded to that 500-year warm spell, because global warming and current patterns in climate could once again subject the trees to similar conditions. Last year, scientists found that the rings of California’s blue oak trees corresponded to low snowpack in the Sierra Nevada—the state’s primary water supply—which confirmed that California was, indeed, experiencing the worst drought in 500 years.

These studies remind us that a technique, nearly a century old, can provide some of the best historical data about climate patterns on planet earth. They also show that no matter how the weather behaves, the climate’s legacy will remain permanently etched in one of nature’s most bountiful resources.