Dendrochronology
Dating wood and reading past climate from the yearly growth rings of trees, matched across many samples.
In places with distinct seasons a tree adds a new layer of wood each growing season: pale, thin-walled earlywood first, then darker, denser latewood. Cut across the trunk and each year shows as one ring. Dendrochronology is the science of dating with those rings; dendroclimatology reads past climate from them.
Ring width records how good each year was for growth. Wide rings suggest favourable conditions; narrow rings may mean drought, cold, disease or other stress. A run of good and bad years leaves a distinctive pattern of wide and narrow rings, and because trees across a region feel the same weather, the pattern works like a barcode.
The key technique is crossdating: matching ring patterns between many trees, rather than just counting rings in one, which can miss a year or count a false ring. Patterns from living trees are matched with older timbers from buildings, then with older wood still from bogs and archaeological sites, overlapping to build continuous year-by-year records. Thermometer records, by comparison, go back only about 150 years.
Tree rings also calibrate radiocarbon dating. And a sudden spike of radiocarbon in a single ring, left by an extreme solar storm, can pin a piece of wood to an exact calendar year.
See it in action
Coming up in future stories
- Corals Glow With the Floods of Centuries Past 1985 · Great Barrier Reef, Australia
- The Charred Beam That Dated a Thousand Years of America 22 June 1929 · Whipple Ruin, Show Low, Arizona
- The Hockey Stick Graph That Went Before Congress 23 April 1998 → 22 June 2006 · Nature, the IPCC and the US National Research Council
- The Oldest Known Tree Was Cut Down for Science Summer 1964 · Wheeler Peak, Nevada
- The Student Who Cut Down the World's Oldest Tree Summer 1964 · Wheeler Peak, Nevada
- The Trees That Witnessed Angkor's Collapse 1330s–1420s · Angkor, Cambodia (tree rings from Bidoup Nui Ba, Vietnam)