Cooling Trend the Last 2,000 Years, Says Research Team

The researchers’ “high-precision density data” was derived from the rings of living and subfossil northern Scandinavian pines (P. sylvestris) in Finnish Lapland.

A team of mostly northern European researchers led by Professor Dr. Jan Esper, department of geography, Johannes Gutenberg University, Mainz, Germany, have used tree-ring density measurements from living and subfossil northern Scandinavian pines in Finnish Lapland to produce a climate reconstruction reaching back to 138 B.C.

The research findings, published in the July 8 Nature Climate Change journal, assert the tree ring data allowed the researchers “for the first time to precisely demonstrate that the long-term trend over the past two millennia has been towards climatic cooling.”

The researchers believe their findings prove the world was much warmer than previously thought, and that the earth has been slowly cooling for 2,000 years at a rate of 0.3°C (3.74°F) per millennium.

The annual growth of tree rings, said the researchers, is the most important witnesses over the past 1,000 to 2,000 years to how warm and cool past climate conditions were.

Esper says this cooling rate may not seem that significant, “…however, it is also not negligible when compared to global warming, which up to now has been less than 1°C.”

He adds: “We found that previous estimates of historical temperatures during the Roman era and the Middle Ages were too low. 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 2000 years of cooling, according to the researchers, is the result of “long-term oscillations of orbital configurations,” i.e., gradual changes to the position of the sun and an increase in the distance between the Earth and the sun.

“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,” says Esper.

The researchers came from Johannes Gutenberg University (Germany), Swiss Federal Research Institute, Oeschger Centre for Climate Change Research, University of Bern, Finnish Forest Research Institute, Institute for Coastal Research (Germany), School of Geography and Geosciences, University of St Andrews, Scotland, Department of Geography, Climatology, Climate Dynamics and Climate Change, Justus-Liebig University (Germany) and the Max Planck Institute for Meteorology in Hamburg.

Ref: Esper J, Frank D, Timonen M, et al. Orbital forcing of tree-ring data. Nature Climate Change, 08 July 2012.

Click here to read the study.