An international team including scientists from Johannes Gutenberg University in Mainz, Germany, analyzed annual growth rings in trees as important witnesses during the past 1,000 to 2,000 years to how warm and cool past climate conditions were.
They looked at tree-ring density measurements from fossil pine trees from Finnish Lapland to produce a reconstruction reaching back to 138 B.C., a university release reported Monday.
The reconstruction provides a high-resolution representation of temperature patterns in the Roman and Medieval Warm periods, researchers said, but also showed the cold phases that occurred during the Migration Period and the later Little Ice Age.
In addition to the alternating cold and warm phases, the new climate curve shows a much longer-term cooling trend that has been playing out for the past 2,000 years, they said.
This trend involves a cooling of -0.3 degree Celsius per millennium.
"This figure we calculated may not seem particularly significant," Gutenberg researcher Jan Esper said, "however, it is also not negligible when compared to global warming, which up to now has been less than 1 degree Celsius.
"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," he said.