Tree-rings indicate climate was WARMER in Roman and Medieval times than it is now - and world has been cooling for 2,000 years
Tree rings are dubious thermometers but the "Hockeystick" is based on them so this looks like a case of the Warmists being hoist with their own petard
And the desperation in the response from Michael "Hockeystick" Mann is amusing. "Mann argues that Esper's tree-ring measurements come from high latitudes and reflect only summer temperatures".
So guess where most of Mann's tree ring data came from? Northern Russia! A high latitude!
How did the Romans grow grapes in northern England? Perhaps because it was warmer than we thought. A study suggests the Britain of 2,000 years ago experienced a lengthy period of hotter summers than today.
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 the very warm period during the years 21 to 50AD has been underestimated by climate scientists.
Lead author Professor Dr Jan Esper of Johannes Gutenberg University in Mainz said: ‘We found that previous estimates of historical temperatures during the Roman era and the Middle Ages were too low.
‘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.’
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 which took it further away from the Sun.
The study is published in Nature Climate Change. It is based on measurements stretching back to 138BC.
The finding may force scientists to rethink current theories of the impact of global warming
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.
In so doing, the 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.
Professor Esper said: '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 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.