Thursday, 26 July 2012

Tree ring data enhances understanding of millennial scale climate change

A team of international scientists, including Dr Rob Wilson of the Department of Earth and Environmental Sciences, has published a reconstruction of past summer temperatures for northern Europe reaching back to 138 BC based on the information provided by tree-rings. Researchers from Germany, Finland, Scotland, and Switzerland examined tree-ring density profiles in trees from Finnish Lapland. In this cold environment, trees, when they die, often fall into one of the numerous lakes, where they remain well preserved for thousands of years. The researchers were thus able to create a temperature reconstruction of unprecedented quality using many hundreds of samples. The reconstruction provides a high-resolution representation of summer temperature patterns in the Roman and Medieval Warm periods, but also shows the cold phases that occurred during the Migration Period and the later Little Ice Age.

Dr Wilson sampling a tree in Finland. 
In addition to the cold and warm phases, the new climate curve also exhibits a phenomenon that was not expected in this form. For the first time, researchers have now been able to use the data derived from the density data to precisely capture a much longer-term cooling trend that has been playing out over the past 2,000 years. Their findings demonstrate that this trend involves a cooling of -0.3°C per millennium due to gradual changes in summer insolation related to the position of the sun and an increase in the distance between the Earth and the sun. This long-term trend, however, is not seen in the more traditionally used ring-width parameter - measured from the same samples.

These findings suggest that large-scale near-surface air-temperature reconstructions, relying predominantly on ring-width data, may underestimate pre-instrumental temperatures including warmth during Medieval and Roman times. [Nature article]. Dr Wilson is leading a similar project based in the Scottish Highlands.