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A study of tree ring data by a group of scientists at the Institute of Geography Johannes Gutenberg University Mainz has found that the world has actually cooled for the last 2000 years and is not in a period of warming as many would have us believe.
Analysis of tree ring density from sub-fossil pine trees shows that during Roman and medieval times the world was warmer than it is today.
Using a method known as Orbital forcing of tree-ring data researchers have been able for the first time to precisely demonstrate that the long-term trend over the past two millennia has been towards climatic cooling.
The tree ring data has allowed the international group, led by professor Dr. Jan Esper, to construct a temperature data model going right back to 138BC.
"We found that previous estimates of historical temperatures during the Roman era and the Middle Ages were too low," says Esper. "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."
The annual growth rings in trees are the most important witnesses over the past 1,000 to 2,000 years as they indicate how warm and cool past climate conditions were.
Researchers from Germany, Finland, Scotland, and Switzerland examined tree-ring density profiles in trees from Finnish Lapland. In this cold environment, trees often collapse into one of the numerous lakes, where they remain well preserved for thousands of years.
The international research team used these density measurements from sub-fossil pine trees in northern Scandinavia to create a sequence reaching back to 138 BC. The density measurements correlate closely with the summer temperatures in this area on the edge of the Nordic taiga.
The researchers were thus able to create a temperature reconstruction of unprecedented quality. The reconstruction provides a high-resolution representation of temperature patterns in the Roman and medieval warm periods, but also shows the cold phases that occurred during the migration period and the later, so called, little ice age.
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 tree-rings to precisely calculate 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 to the position of the sun and an increase in the distance between the earth and the sun.
"This figure we calculated may not seem particularly significant," says Esper. "However, it is also not negligible when compared to global warming, which up to now has been less than 1°C. 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."
The study has been published on the Nature Climate Change website: Orbital forcing of tree-ring data
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