Tree rings from ancient coffins offer clues to Earth’s past

About 2200 years ago, a wealthy Han soldier was entombed in a hillside grave on the frontier of the expanding Han Dynasty, in what is now western China. His tomb was filled with gold coins and emblazoned with ornate calligraphy. But what most interested Bao Yang at the Chinese Academy of Sciences (CAS) and head of its Tree-Ring Laboratory was the wood of his coffin.

For dendrochronologists like Yang, coffin wood can be a critical source of tree rings, which can help scientists date sites—sometimes to precise calendar years—and offer details about the region’s environment and climate during the tree’s lifetime. The thickness of rings from the Han soldier’s pine coffin and hundreds of others like it, for example, revealed that from 270 B.C.E. to 77 C.E. average humidity levels were 18% to 34% higher than today’s, which may have allowed the western Han to expand westward into what before—and is again today—a barren desert.

That insight into China’s climate more than 2000 years ago, reported in December 2024 in the Proceedings of the National Academy of Sciences, is one of many from the burgeoning study of wood in ancient coffins. Dendrochronologists typically piece together their records from long-lived trees and logs either naturally buried or preserved in ancient structures. But wooden coffins can provide data in places where other records are missing. “Due to historical deforestation and a lack of long-lived timber, existing coffins often provide the only high-resolution evidence of regional environmental change,” says Chun Qin of CAS, a member of the Tree-Ring Laboratory and lead author of the Han Dynasty paper.

As trees grow, their annual growth rings vary in width depending on temperature and moisture. They also capture the fluctuating levels of radioactive carbon-14 in the atmosphere and can record stresses such as earthquakes. “When solid timbers are preserved, we need to measure every tree ring on a cross section of the timber,” says Kristof Haneca, a dendrochronologist who specializes in coffin woods at the Flanders Heritage Agency. By matching ring widths and other properties, researchers can piece together records from wood of varying ages into highly detailed chronologies that can extend thousands of years into the past.

Coffins, a ready source of ancient wood, “provide an immense amount of data,” says Pearce Paul Creasman, an Egyptologist and dendrochronologist at the American Center of Research and strong advocate for the use of coffin woods to clarify ancient Egypt’s past. In 2014, Creasman and his colleagues used 4000-year-old cedar planks from the Ipi-ha-ishutef coffin in Egypt and tree rings from a similarly aged funeral boat to sharpen our understanding of the pharaohs and when they ruled.

Other researchers are mining coffin wood for insights into ancient climate. Yesi Zhao, a paleodendrochronologist at Jiangsu Second Normal University and member of the Tree-Ring Laboratory, is compiling coffin woods to construct a 2000-year-long tree ring chronology of eastern China—a project that has been underway for 5 years. One recent focus of this work is to determine whether coffin tree rings from eastern China can capture the so-called Medieval Climatic Anomaly, an aberrant warming period from approximately 950 to 1250 C.E. In Western Europe, the warming may have aided the Viking colonization of Greenland and Newfoundland in 985 and 1021 C.E., respectively. Zhao wants to learn whether eastern China experienced a similar warming, and how it affected society.

He and his colleagues assessed coffin wood planks of Chinese fir from three tombs across central and eastern China. By radiocarbon dating cellulose samples cut from multiple tree rings within each plank and comparing them with a known record of atmospheric radiocarbon stretching back tens of thousands of years—a technique called “wiggle matching”—they were able to confirm the coffin tree rings formed between 941 and 1388 C.E. They validated the calendar dates for the rings with the help of a well-known surge in radiocarbon that occurred in 993 C.E., the result of an unusual barrage of cosmic rays. It shows up as a prominent radiocarbon spike in rings of that age. “This study provides a critical chronological framework for the development of our long-term tree-ring chronology,” says Zhao, who plans to submit the findings for publication next month.

A preliminary analysis of the tree rings indicates that sites across southern China experienced wild swings in atmospheric moisture over that time period, suggesting extreme weather events such as droughts and floods that presently plague the region likely also did so in medieval times. That’s “a useful reference for assessing the respective contributions of natural versus human-induced warming,” Zhao says.

This research will help “sharpen the focus on how long-term climatic regimes influenced social prosperity across China’s historical periods,” Yang adds.

Ensuring that the rings capture conditions where the coffins were found can be tricky, because coffin wood was often imported from other areas. Ancient Egyptian coffins were often made from Lebanese cedar. Medieval coffins in Belgium were often imported from Germany. Still, Zhao expects to learn many more tales—not from the dead, but from the boxes that held them. “Archaeological excavations in eastern China are progressing rapidly,” he says, and “a large number of coffin woods have been unearthed.”

Cover photo:  Scientists can analyze tree rings from coffins—like this one belonging to an ancient Egyptian army commander named Ipi-ha-ishutef—for clues to past climates