Since the turn of the century, most of Europe has experienced record-hot summer months that at times were deadly.
Just last year – now known as the world’s hottest year on record — France had to take measures to prevent the catastrophe it experienced during the summer of 2003, when a heat wave killed between 15,000 and 19,000 people. Madrid and many German cities also set their all-time heat records. Even parts of the Arctic state of Sweden saw temperatures reach almost 90 degrees Fahrenheit last July.
These summer heat waves were called historic more than once. Yet following a new study of ancient climate published Thursday in Environmental Research Letters, scientists say we may have underestimated just how hot today’s European summers are, compared to the region’s history.
“The past 30 summers — 1986-2015 — were warmer than any other 30-year period since at least 138 BC,” said Nick McKay, a researcher from the School of Earth Sciences and Environmental Sustainability at Northern Arizona University, in an email to ThinkProgress. McKay, who was not part of the study, added the study is “the most state-of-the-art” attempt to reconstruct European summer temperatures over the past 2,000 years.
“Reconstructing temperature variability of the past two millennia is incredibly important for understanding the changes we’re witnessing now,” he wrote.
The study, completed by the international Euro-Med2k Working Group, is based on tree-ring data and other records from ten European locations. With funding from the U.S. and Swiss National Science Foundations, and the National Oceanic and Atmospheric Administration, more than 40 researchers combined their expertise to generate climate models and reconstruct, using tree density and width, how climate has changed since Roman times.
Researchers documented weather patterns going from unusually cold to unusually warm over the centuries. The mid-13th century was characterized by cooling in northeastern Europe, according to the study, but warming in southwestern regions. Then an exceptionally cold period occurred in the late 16th century and early 17th century, with negative temperature anomalies over nearly half of Europe.
“When you look at, say, the 20th century,” said Jason Smerdon, one of the authors and an associate research professor at Columbia University’s Lamont-Doherty Earth Observator, “there were [past] periods that were at least comparable in warmth to the 20th century. But even with that sort of enhanced variability of the last 2,000 [years], these last years really stand out still as a period of warmth that is unmatched.”
Recent unusual warmth is particularly clear in southern Europe, according to the report, where the signals of man-made climate change are expected earlier. Indeed, researchers not associated with the study reached by ThinkProgress said the climate system described in the report shows a combination of human-caused climate change, as well as decades of natural climate oscillations.
“European summer temperatures over the past 2,000 years reflect a combination of variability related to changes in solar and volcanic activity, recent increases in greenhouse gases, and internal natural climate system variability,” said Kevin Anchukaitis, a paleoclimatologist at the University of Arizona, who was not involved in the study.
But while the study adds to years of research showing the planet is warming following man-made greenhouse gas pollution, Smerdon said the climate story the group created has many unanswered questions. Climate models and weather reconstructions didn’t fully explain the change that happened during the Medieval Age and the so-called Little Ice Age that ended in the 1700s, he said.
These last years really stand out still as a period of warmth that is unmatched
“So the question is: why aren’t the models reproducing the magnitude of temperature difference between the (Medieval Climate Anomaly) and the Little Ice Age?” Said Smerdon. One possibility is that the weather reconstructions accounting for volcanic activity and other factors affecting climate might need adjustments, and then there is how to deal with what Smerdon called the “natural randomness of the climate.”
Some level of uncertainty when recreating paleoclimates is, however, unavoidable. That’s because most paleoclimate reconstructions, and especially those based on tree-ring records, have less and less data to work with as researchers reach further back in time. But researchers – even those not associated with the study – said there is much value in Thursday’s study, and in paleoclimate in general.
Paleoclimate studies “let us begin to answer questions like: Is recent warming similar to other warming that’s occurred in the recent past? Can recent warming be explained by natural causes?” said McKay. But natural causes for recent warming in Europe and elsewhere is for most out of the question. In fact, most in the scientific community say that although warming has occurred in the past, recent accelerated warming can only be explained by human involvement in planetary processes. Just this Monday, a study published in Scientific Reports found that recent record temperatures are roughly 600 to 130,000 times more likely to have occurred under conditions of anthropogenic causes, than in their absence.
Still, for some scientific uncertainty in some areas causes skepticism or flat out denial of the role humans play in climate change. Smerdon is cognizant of that, yet said those uncertainties can cut both ways. “It might make a person skeptical and say we can’t model this very effectively … so we shouldn’t worry about it. But one of the things these uncertainties do speak to is that the models may not be fully characterizing the risks that we face, so it cuts both ways.”