Scientists reconstruct millennia-old climate
Dendrochronology (tree-ring dating) offers researchers key information about certain aspects of past ecologies, such as the climate. Taking advantage of the opportunity to evaluate tree-rings, an international team of researchers has reconstructed the climate in northern Europe over the last 2,000 years. The findings, presented in the journal Nature Climate Change, reveal that the long-term trend over this period has been towards climatic cooling.
Led by Johannes Gutenberg University Mainz (JGU), researchers from Finland, Germany, Switzerland and the United Kingdom used tree-ring density measurements from Lapland (Finland)-based sub-fossil pine trees to reconstruct the climate as far back as 138 BC.
'We found that previous estimates of historical temperatures during the Roman era and the Middle Ages were too low,' said lead author, Professor Jan Esper from the Institute of Geography at JGU. 'Such findings are also significant with regard to climate policy, as they will influence the way today's climate changes are seen in context of historical warm periods.'
Is our planet warmer today than what it was during the Roman and Medieval times? Palaeoclimatology gave researchers the chance to find out. In general, analysis of ice cores and ocean sediments help experts draw a picture of the past. But tree-rings reveal key information as well, giving scientists a glimpse into how warm and cool past climate conditions were up to 2,000 years ago.
For the purposes of this study, the researchers used density measurements from sub-fossil pine trees way up in the Nordic region to establish the sequence they needed. According to them, because the density measurements are closely linked with the summer temperatures in this area on the edge of the Nordic taiga (i.e. boreal forest), it was possible for them to create a high-quality temperature reconstruction.
The researchers said they clinched a high-resolution representation of temperature patterns in both the Roman and Medieval warm periods, and they observed that cold phases emerged during the Migration Period and the Little Ice Age. Based on their reconstruction, the team showed the trend involves a cooling of -0.3 degrees Celsius per millennium due to gradual changes to the position of the Sun and an increase in the distance between the Sun and the Earth.
'This figure we calculated may not seem particularly significant,' Professor Esper said. 'However, it is also not negligible when compared to global warming, which up to now has been less than 1 degree Celsius. Our results suggest that the large-scale climate reconstruction shown by the Intergovernmental Panel on Climate Change (IPCC) likely underestimate this long-term cooling trend over the past few millennia.'
Related stories: 34374
Data Source Provider: Nature Climate Change
Document Reference: Esper, J., et al. 'Orbital forcing of tree-ring data', Nature Climate Change, published 8 July 2012. doi:10.1038/nclimate1589
Subject Index: Climate change & Carbon cycle research; Coordination, Cooperation; Environmental Protection; Scientific Research