Warm summers were experienced during Roman times, up to the 3rd century, followed by generally cooler conditions from the 4th to the 7th centuries. A generally warm medieval period was followed by a mostly cold Little Ice Age from the 14th to the 19th centuries. The pronounced warming early in the 20th century and in recent decades is well captured by the tree-ring data and historical evidence on which the new reconstruction is based.

The evidence suggests that past natural changes in summer temperature are larger than previously thought, suggesting that climate models may underestimate the full range of future extreme events, including heat waves. This past variability has been associated with large volcanic eruptions and changes in the amount of energy received from the sun. The new research finding that temperatures over the past 30 years lie outside the range of these natural variations supports the conclusions reached by the Intergovernmental Panel on Climate Change (IPCC) that recent warming is mainly caused by anthropogenic activity.

Fredrik Charpentier Ljungqvist. Foto: Malin Gard.
Fredrik Charpentier Ljungqvist. Foto: Malin Gard.

"Summer temperatures in Central Europe and the Mediterranean has been exceptionally high in the last 20 years. In contrast, temperatures were as high in Scandinavia during Roman times and the times of the vikings. It is not surprising that the consequences of the greenhouse effect will first be seen in southern Europe because the natural variability of the climate is much less there than here in Scandinavia", says Fredrik Charpentier Ljungqvist, Department of History and Bolin Centre for Climate Research, Stockholm University, one of the researchers behind the study.

“We now have a detailed picture of how summer temperatures have changed over Europe for more than two thousand years and we can use that to test the climate models that are used to predict the impacts of future global warming,” says the coordinator of the study, Professor Jürg Luterbacher from the University of Giessen in Germany.

The interdisciplinary study involved the collaboration of researchers from Past Global Changes’ (PAGES) original European 2k Network working group, Euro-Med2k. PAGES, a core project of the International Geosphere-Biosphere Programme (until 2015) and Future Earth (2015-), is funded by the U.S. and Swiss National Science Foundations and the U.S. National Oceanic and Atmospheric Administration.