Authored by Alex Kirby, re-posted from Climate News Network
The unusually hot summers in Europe over the last three decades are further evidence that human activities are largely responsible for recent global warming, according to new research.
The scientists say they have found no 30-year periods in the last 2,000 years that have exceeded the mean average European summer temperature of the years from 1986 to 2015.
This new data adds to the fears expressed by scientists only a week ago that parts of the Mediterranean and Arctic regions will heat up by 3.4˚C and 6˚C respectively above pre-industrial levels.
Despite this, they report in Environmental Research Letters journal that the 1st century AD, and possibly also the 10th century, may have been slightly warmer than the 20th century, although the difference is not statistically significant.
The 45 scientists, from 13 countries, say their research now puts the current warmth in the context of the last 2,100 years, using tree-ring information and historical documentary evidence. Their interdisciplinary study involved the collaboration of researchers from Past Global Changes (PAGES), a core project of the global sustainability science programme, Future Earth.
During Roman times, up until the 3rd century, there were warm summers, 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 scientists say the pronounced warming early in the 20th century and in recent decades is well represented by the tree-ring data and historical evidence on which their reconstruction is based.
“We now have a detailed picture of how summer temperatures have changed . . . and can use that to test the climate models used to predict the impacts of future global warming”
They also say the evidence suggests that past natural changes in summer temperature are greater than previously thought, suggesting that climate models may underestimate the full range of future extreme events, including heatwaves.
This past variability has been associated with large volcanic eruptions and changes in the amount of energy received from the sun.
The scientists say their finding that temperatures over the last 30 years lie outside the range of these natural variations supports the conclusion reached by the Intergovernmental Panel on Climate Change that recent warming is mainly caused by human activity.
“We now have a detailed picture of how summer temperatures have changed over Europe for more than 2,000 years and we can use that to test the climate models that are used to predict the impacts of future global warming,” says the co-ordinator of the study, Professor Jürg Luterbacher, director of the department of geography at the Justus Liebig University of Giessen, Germany.
Professor Luterbacher co-authored a 2014 report titled “The year-long unprecedented European heat and drought of 1540 – a worst case”, published in Climate Change journal. The report drew on more than 300 first-hand documentary weather report sources.
He and his colleagues wrote then that Europe was affected in 1540 by “an unprecedented 11-month-long megadrought . . . We found that an event of this severity cannot be simulated by state-of-the-art climate models.”
They concluded: “Given the large spatial extent, the long duration and the intensity of the 1540 heat and drought, the return of such an event in the course of intensified global warming involves staggering losses.”