A study of semi-fossilised trees has proven that world climate was warmer in Roman and Medieval times than it is in the modern industrial age.
German researchers used data from tree rings – a key indicator of past climate – to claim the world has been on a ‘long-term cooling trend’ for two millennia until the global warming of the twentieth century.
This cooling was punctuated by a couple of warm spells.
These are the Medieval Warm Period, which is well known, but also a period during the toga-wearing Roman times when temperatures were apparently 1 deg C warmer than now.
They say climate scientists have underestimated the very warm period during the years 21 to 50AD.
“We found that previous estimates of historical temperatures during the Roman era and the Middle Ages were too low,” the Daily Mail quoted lead author Professor Dr Jan Esper of Johannes Gutenberg University in Mainz as saying.
“This figure we calculated may not seem particularly significant, however it is not negligible when compared to global warming, which up to now has been less than 1 deg C,” Dr Esper noted.
In general the scientists found a slow cooling of 0.6C over 2,000 years, which they attributed to changes in the Earth’s orbit that took it further away from the Sun.
The finding, published in Nature Climate Change, is based on measurements stretching back to 138BC.
Professor Esper”s group at the Institute of Geography at JGU used tree-ring density measurements from sub-fossil pine trees originating from Finnish Lapland to produce a reconstruction reaching back to 138 BC.
The study has for the first time precisely demonstrated that the long-term trend over the past two millennia has been towards climatic cooling.
“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,” said Professor Esper.
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 the cold environment of Finnish Lapland, trees often collapse into one of the numerous lakes, where they remain well preserved for thousands of years.
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 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.