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.

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