Historical data on global warming subject to misrepresentation

Every time there is a heat wave in France, the same copy-and-paste text about precedents since 1152 is circulated online. Although extreme heat events did indeed take place, they were much less intense and frequent than those in the 21st century. Romain Geoffroy discusses in an article on Le Monde website.

Global warming: Beware of online misrepresentations of historical data

“In 1152, the heat was so intense that you could cook eggs in the sand. (…) For periods in 1303 and 1304, it was possible to cross the Seine, the Loire, the Rhine and the Danube on foot (…) In 1748, 1754, 1760, 1767, 1778 and 1788, the summer heat was excessive.” With each heat wave in 2019, 2020 and now in 2022, the same text has been circulated on blogs or Facebook to try to demonstrate that the current heat waves are not at all exceptional.

What the publication says

The often copied list is said to have appeared in “an English newspaper, the Hampshire Advertiser of Southampton, on July 17, 1852.” In fact, the British weekly had reprinted an article from Paris-based English-language daily newspaper, Galignani’s Messenger, dated July 12, 1852, where it listed, rather inaccurately in some instances, heat waves that had occurred in the past.

“This is really a newspaper excerpt from 1852 and not from some conspiracy site,” the Facebook post insists. According to these self-described “climate realists,” all these examples of “extreme heat waves” and “catastrophic droughts,” “at a time when there was not yet the slightest hint of the beginning of the industrial revolution” show that, “obviously, there is nothing new under the sun.”

This viral text argues that historical examples prove that man-made global warming is not the cause of the current heat waves and that there is nothing extraordinary about them. In their argument, these climate skeptics cite an interview given by renowned climate history specialist Emmanuel Le Roy Ladurie to the Libération newspaper during the 2003 heat wave. He put the “the current hot summer” into context, explaining that “this kind of great drought has not been lacking in French history”, and cited “series of consecutive hot summers, climatic microeras: 1331-1334, four summers in a row, 1383-1385, three summers.” He also recalled the “spectacular” death rate of pre-industrial heat waves, citing the figure of “700,000 deaths” in 1718-1719.

WHY IT’S MISLEADING

While there has been no shortage of heat waves in history, they need to be put into context. “In all eras, people have experienced periods of extreme heat, but they are always relative to the average climate of the time considered, and these average temperatures have largely increased today,” explained Françoise Vimeux, climatologist at the French Research Institute for Sustainable Development (IRD).

The scientist cites the heat waves that hit Paris in the early 17th century. We have as reference points a series of monthly temperatures in the Paris region since 1676, developed by meteorologist Daniel Rousseau, as well as information recorded by scientists or newspapers at the time, which can be cross-checked against dates of grape harvest. In 1705, the average temperature from June to August was 18.9°C. In 1706 and 1707, it was 19.7°C, which means both years were considered to have abnormally hot summers for the time.

In comparison, the summer of 2021, which did not experience any severe heat waves, exceeded the levels of two centuries ago with an average of 20.2 °C in Paris between June and August. “These heat waves were not as frequent, not as long and not as intense,” said Ms. Vimeux. “They were not as out of line with the average temperatures of the time.”

‘The current warming extends much further’

In the same interview in Libération cited by the viral post, Mr. Le Roy Ladurie, far from denying current climate change, stated on the contrary that “the [current] worsening of the greenhouse effect phenomenon is a break in the history of the climate that could open a new climatic era with a difference of one or two degrees. This would have incalculable consequences.”

In another 2009 interview in the Regards croisés sur l’économi magazine, the historian also dismissed the theory of a return to the Medieval Climate Optimum (MCO) – also called “medieval climatic anomaly” – during which Western Europe experienced slightly milder temperatures, between the year 900 and the year 1250. “The current warming extends much further,” he stressed, arguing:

“At the height of the MCO, average temperatures were, in Scandinavia at least (…), 0.7°C above their minimum level during the Little Ice Age. In 2001-2007, we had an increase of 1.6°C in France compared to the beginning of the 20th century… Of course, we don’t know everything. In particular, it is absolutely not proven that the first warming (1911-1950) was linked to CO2. For the second (post-1986), there is no longer any doubt.”

As for the impressive figure of “700,000 deaths” in the summers of 1718-1719 mentioned by Mr. Le Roy Ladurie in 2003, it does not prove that heat waves in the past were more serious because health conditions have evolved in two centuries. “At the time, mortality was mainly due to the spread of epidemics, dysentery or typhoid fever, due to the consumption of unsanitary water,” explained climatologist Ms. Vimeux. In the 18th century, we did not have the infrastructure that makes us much less vulnerable to a shortage of drinking water today. Despite record heat in Europe, the 2003 heat wave claimed ten times fewer victims (70,000 deaths, according to Inserm) than the less intense heat waves of 1718-1719.

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