Pithwell / 1816
The year without a summer, in wood
A volcano in Indonesia cooled the whole northern summer of 1816. Trees kept the record, unevenly, and the unevenness is part of what they tell us.
What did the 1816 summer do to trees?
After Tambora erupted on 10 April 1815, dust and ash spread round the world and summers stayed cold for several years. In many places trees answered with narrow rings. The record is uneven: some trees barely reacted, and some narrow rings in later years have other causes, such as insect outbreaks.
Mount Tambora erupted on 10 April 1815. Roughly 60 million tonnes of sulfur reached the stratosphere, making it the third-largest volcanic event of the last 1,500 years by atmospheric loading. Snow fell in Quebec, Maine and New York in June 1816, frosts reached as far south as New Jersey, and crops failed across North America and parts of Europe.
Instrument records are thin, which is why trees matter. The few thermometers then running suggest Northern Hemisphere temperatures dipped by 0.7 to 0.8 °C in 1816. In the Alps, summers from 1813 to 1816 were exceptionally cold, and growth was held back for several years.
Did 1816 leave a narrow ring in every tree?
No, and that is why dendrochronologists read many trees, not one. A 700-year-old larch in the Alps, shown by the Swiss research institute WSL, reduced its growth only partly and after a delay. Its narrow ring in 1821 may come from a larch budmoth outbreak that year instead of from the cold.
Look at one tree and the story is ambiguous. Look at hundreds and it is not. That is the job of a chronology: it averages away the larch budmoth, the shaded decade and the lightning scar, and leaves the signal that trees in one region share.
Is the ring for 1816 missing in some trees?
No. In 2012 Mann and colleagues argued that trees at their cold limit might have skipped 1816. Tests on European and American records rejected it, and a radiocarbon spike dated to AD 774–775 turns up in tree rings from several continents at the same year. The Year Without a Summer was not a year without a ring.
Mann and colleagues (2012) argued that trees near their cold limit may not have formed a ring in 1816, which would put some older tree-ring dates a year out. Esper and colleagues (2013) tested this by deleting 1816 from several density records, re-dating every earlier ring by one year, and checking against thermometer records and a five-century documentary reconstruction. The altered series stopped matching either.
St. George and colleagues (2013) then surveyed 2,359 ring-width records across the Northern Hemisphere. Locally absent rings were frequent in the American Southwest during severe droughts and extremely rare at high-latitude and high-elevation sites, exactly where the “missing 1816” idea needed them to be common.
None of this makes dating infallible. It is a reason to cross-check every sample against several others and to say so when it cannot be done.
Where the facts come from
- WSL Dendro Expo: 1816, the year without a summer
- St. George and Anchukaitis, “On the AD 1815 Tambora eruption and the matter of misplaced tree rings”, PAGES Magazine 23(2), December 2015
- Esper, Büntgen, Luterbacher and Krusic, “Testing the hypothesis of post-volcanic missing rings in temperature sensitive dendrochronological data”, Dendrochronologia 31 (2013)
- Laboratory of Tree-Ring Research, University of Arizona: What is dendrochronology?