Europe has almost certainly experienced warmer summers in the last three decades than at any other time since the Roman empire, according to a study published on Friday in the Environmental Research Letters journal.
Since 1986, mean summer temperatures have been about 1.3C hotter than they were two millennia ago, while heatwaves have been longer, more frequent and more persistent, the study says.
The paper was compiled by 40 prominent academics, using tree-ring analysis, climate modelling and historical documentary evidence from the notes of doctors, priests and monks.
“This degree of warming is unprecedented in the last two thousand years,” Professor Jürg Luterbacher, the report’s coordinator, told the Guardian. “It is exceptionally high and cannot be explained by natural variability, tropical volcanoes or solar changes. It is because of anthropogenic [manmade] climate change.”
While the paper offers new insights into the effects that volcanic eruptions, solar variability and land use change can have on climate, its range is limited to periods between June and August.
Much of the data for the period before 755 comes from analysis of tree rings and density information from three pine tree species in Finland, Austria and Sweden. These trees grow in warm weather but are dormant in the cold, meaning that their rings, density – and the outside temperatures – can only be measured in summer.
After 755, more pine trees in countries such as Switzerland, France and eventually Spain become accessible to the scientists, allowing spatial variations – or regional differences – to be more accurately recorded, and comparisons to be drawn.
“The anomalous recent warmth is particularly clear in southern Europe, where variability is generally smaller, and where the signal of anthropogenic climate change is expected to emerge earlier,” the report says.
The report finds evidence that past swings in summer temperatures were larger than previously thought. It says that summers were warmer between Roman times and the third century, before cooling until the 7th century.
A warmer medieval interlude was then punctured by a ‘Little Ice Age’ that lasted from the 14th to the 19th centuries. As the 20th century dawned, the effects of climate change became progressively more pronounced.
“Summer temperatures during the last 30 years have been anomalously high and we find no evidence of any period in the last 2,000 years that has been so warm,” the paper says.