Tree rings unveil temperatures of the last millenium

High on mountains across Alaska, Canada, Europe and Russia near the upper or high latitude tree line, even the hardiest of conifers are at the edge of survival. Their annual rings tell a story of extreme cold that limit their growth and of warmer years that allow them to flourish. By analysing samples from these living trees and fallen timbers, researchers are able to reconstruct past temperature change.

A new international consortium of scientists, led by Dr Rob Wilson of the Department of Earth & Environmental Sciences, has collated a network of such tree-ring archives to derive a history of temperature fluctuations across the entire Northern Hemisphere. The N-TREND consortium (N-TREND stands for Northern Tree-Ring Network Development) was created to develop a global database of tree-ring research that improves on previous efforts for developing large-scale temperature reconstructions across the hemisphere. The consortium was devised to provide a collective platform where participants are all on the same page with respect to identifying not only gaps in the hemispheric network, but also facilitating the communication and training of new methodological approaches that can further improve tree-ring based temperature reconstructions.

The consortium’s first research paper, appearing in the current issue of Quaternary Science Reviews, provides a view of past Northern Hemisphere temperature changes over more than 1,000 years. It reveals a longer and warmer Medieval period than previous temperature reconstructions suggested, from around 850 to the end of the 11th century, with a peak in the 1160s. It also shows how the two coldest decades—1812-1821 and 1832-1841, both during a period known as the Little Ice Age—are followed by near continuous warming until present. The new paper takes a close look at some of the challenges of previous historic temperature reconstructions, explaining why reconstructions using multiple proxy archive sources—such as tree rings, ice cores, lake sediments etc, or mixing seasons expressed by different archives — can end up with ambiguous results when trying to understand past climatic variability and forcing.
A second paper will soon be submitted that will look at spatial temperature variations across the Northern Hemisphere for the last millennium. Such spatial analyses will help answer questions about the spatial extent of Medieval Warm Period, for example, and through comparative analyses with global climate models could help attribute the forces behind such past temperature ch
ange.

Columbia University links: Lamont-Doherty Earth Observatory, Blog
"Last millennium northern hemisphere summer temperatures from tree rings: Part I: The long term context", Quaternary Science Reviews Volume 134, 15 February 2016, Pages 1–18, doi:10.1016/j.quascirev.2015.12.005