Ancient wood uncovers a lost era of rains that fueled early China

Two thousand years ago, a pine coffin lowered into a Han dynasty tomb carried more than the remains of the dead. Its wooden planks quietly recorded the climate its tree once lived through – year by year, ring by ring.

A new analysis of those rings, led by Chun Qin of the Chinese Academy of Sciences Tree Ring Laboratory (CAS), reveals that northern China was roughly one-fifth wetter around 2,200 years ago. That was wet enough to support expanding farms and a growing empire.

Ancient coffins often contain the only surviving old-growth timber in regions where forests have vanished.

The study shows that these planks can serve as high-precision climate archives that trace monsoon shifts, agricultural booms, and even the rise of dynasties.

Coffin wood with a climate record

Dendrochronology – the science of dating events using tree rings – treats each narrow ring of wood as a tiny yearly report.

Ring width depends on temperature and moisture. Small chemical changes in the wood document how much water and sunlight the tree received.

Across much of China, old forests have vanished, so coffin planks sometimes hold the only detailed traces of paleoclimate – climate before modern instruments.

“Existing coffins often provide the only high-resolution evidence of regional environmental change,” said Qin.

“Coffins, a ready source of ancient wood, provide an immense amount of data,” said Pearce Paul Creasman, an Egyptologist and dendrochronologist.

Creasman used cedar coffins and a funeral boat from Egypt to refine dates for rulers who lived 4,000 years ago.

A wetter north and a growing empire

Qin and his team read rings from Han dynasty coffin planks and nearby living pines in north-central China. Together those trees form a continuous record of rainfall from 270 BCE to 77 BCE.

The team’s reconstruction shows that the Asian summer monsoon, a system of summer winds and rains, delivered much more precipitation than today.

On average, the region was between 18 and 34 percent wetter. That was enough to push rain-fed fields many miles farther into the dry northwest.

Modeling suggests that this wetter hydroclimate, the mix of rainfall and soil moisture, let farmers work land about 40 to 60 miles farther northwest.

Those new fields helped feed more people and support frontier towns and forts across areas that today stand on the edge of the desert.

Coffin wood captures medieval shifts

The same approach is reaching into the Middle Ages, as researchers compile coffin planks to build a two- thousand-year record for eastern China.

One focus is the Medieval Climate Anomaly (MCA), a warm period from about 950 to 1250 CE, when climate patterns changed in many regions. In one set of tombs, Chinese fir planks have been dated between 941 and 1388 CE using a method called wiggle matching.

That technique lines up changes in radiocarbon, a radioactive form of carbon in tree rings, with a standard atmospheric record built from dated samples.

“This study provides a critical chronological framework for the development of our long-term tree-ring chronology,” said Yesi Zhao, a paleodendrochronologist at Jiangsu Second Normal University.

A sharp spike in cosmic rays, high-energy particles from space, that hit Earth in 993 CE provides a distinctive marker in these rings. The same event appears in tree rings worldwide. It helped date Viking wood at L’Anse aux Meadows in Newfoundland to 1021 CE.

New clues from old wood

Coffin-based tree ring records are now being combined with other proxies, indirect climate clues such as lake mud and cave minerals.

By comparing them, researchers can see which patterns appear across different archives and which might reflect only local quirks.

Long coffin sequences from China and Egypt could anchor histories of drought, flood, and harvest success to the calendar years used by historians.

That precision helps researchers see how climate shocks or calm periods aligned with major turning points in past societies.

The challenge of coffin wood

Rich families sometimes imported lumber from distant forests, so coffin planks may not record the weather at their burial sites.

Untangling that provenance – the original source of the wood – is now a key step in any serious climate study that uses coffins.

In southeastern China, researchers used coffin timber to build a ring chronology, a dated sequence of tree growth, that tracks droughts and wet spells.

The results show that coffin wood can fill major gaps in climate records for lowland regions where few old trees survive. Humid years during the Qin and Western Han dynasties offer an analogue – a past comparison – for the wetter trend in parts of northwestern China.

The study is published in the journal PNAS.

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