Where is the European Warm Period in the Bristlecone Record?

Eli has always wondered how Malcolm Hughes avoided the storm associated with the Mann, Bradley and Hughes papers. Hughes, of course, was the dendrologist of the group, and head of the Arizona Laboratory of Tree-Ring Research.

A recent paper by Mathew Salzer and Malcolm Hughes in Quaternary Research 67 (2007) 57–68 provides additional information. The paper has modest, but interesting goals

Many years of low growth identified in a western USA regional chronology of upper forest border bristlecone pine (Pinus longaeva and Pinus aristata) over the last 5000 yr coincide with known large explosive volcanic eruptions and/or ice core signals of past eruptions. Over the last millennium the agreement between the tree-ring data and volcano/ice-core data is high: years of ring-width minima can be matched with known volcanic eruptions or ice-core volcanic signals in 86% of cases. In previous millennia, while there is substantial concurrence, the agreement decreases with increasing antiquity. Many of the bristlecone pine ring-width minima occurred at the same time as ring-width minima in high latitude trees from northwestern Siberia and/or northern Finland over the past 4000–5000 yr, suggesting climatically effective events of at least hemispheric scale. In contrast with the ice-core records, the agreement between widely separated tree-ring records does not decrease with increasing antiquity.

Tree ring widths are affected by temperature, precipitation and length of growing season (which may be correlated with the other two). High elevation trees are affected by large volcanic eruptions for several years. This allows Salzer and Hughes to differentiate between drought and the cooling brought about by eruptions. Looking at the tree ring index one can clearly see many large eruptions, the little ice age, but no European Warm Period, often called medieval.