DOUGLASS , ANDREW ELLICOTT

Douglass came from a family of academics in Windsor, Vermont, with both his father and grandfather being college presidents. He graduated from Trinity College, Hartford, Connecticut in 1889 and in the same year was appointed to an assistantship at Harvard College Observatory. In 1894 he went with Percival Lowell to the new Lowell Observatory in Flagstaff, Arizona, moving to the University of Arizona in 1906 as professor of astronomy and physics.

Douglass's first interest was the 11-year sunspot cycle. In trying to trace its history he was led to the examination of tree rings in the hope that he would find some identifiable correlation of sunspot activity with terrestrial climate and vegetation.Soon the tree rings became the center of his studies.

The only previously established method of dating the past, except by inscriptions, was the geological varve-counting technique, which was developed from 1878. But this was of no use if there were no varves (thin seasonally deposited layers of sediment in glacial lakes) to be found. Douglass soon found that he could identify local tree rings with confidence and use them in dating past climatic trends. He thus founded the field of dendrochronology. By the late 1920s he had a sequence of over a thousand tree rings with six thin rings, presumably records of a severe drought, correlated with the end of the 13th century. In 1929 he found some timber that contained the six thin rings and a further 500 in addition. This took him to the eighth century and over the years he managed to get as far as the first century. This was extended still further and by careful analysis scholars have now established a sequence going back almost to 5000 BC.

The dated rings of Arizona and New Mexico were found however not to correlate with sequences from other parts of the world: the tree-ring clock was a purely local one. The search for a more universal clock continued, and the method of radiocarbon dating was developed by Willard Libby in 1949.