As historic heat records follow one another, the leader of the La France insoumise deputies says the world has had its hottest week in more than 100,000 years. A statement most likely true.
July 2023 is “on track to become the hottest July ever measured”, according to the European observatory Copernicus. To warn about climate change, Mathilde Panot, the president of the La France Insoumise group in the Assembly, gave another dizzying figure on Wednesday July 19 in the 8:30 a.m. of franceinfo: “The world had the seven hottest days in July in 100,000 years.”
A statement mocked by Internet users on Twitter: “There were already thermometers 100,000 years ago?” “You clown, how can you know that? The records are only a century and a half old.”
However, this statement is most likely correct. There is no scientific publication that writes it “as is”, since the facts date from the beginning of July and it is too early for a study to be validated by the scientific committee of a journal, for example. But several data suggest that it is “very probable”, climatologist physicist François-Marie Bréon told franceinfo.
The hottest week since the start of weather records
In early July, the world experienced its hottest week, according to preliminary data from the World Meteorological Organization (WMO), which compiles daily temperatures recorded around the world. For example, it averaged 17.4° on July 7, a record. This is therefore the hottest week since WMO records began in the 1950s and “since probably the climate of 1850-1900 where we have observation data”says paleoclimatologist Valérie Masson-Delmotte.
But the thermometer has not always existed, and to go back in time we must also rely on estimates. They do not say the daily weather, but take into account ranges of several decades. The IPCC (page 6), the group of climate experts, says our decade was even warmer than the hottest ranges 6,500 years ago.
Between -10,000 and -100,000 years ago, the Earth experienced an ice age. To find temperatures as high as today, we must therefore go back before, more than 100,000 years ago.
The ice caps, the memory of the climate
To draw a picture of the climate before the weather readings, scientists use several methods. One of them consists in analyzing the rings that we see on the trunks of trees. Depending on their thickness and their chemical composition, we can read the temperature variations. For example, the beams of the Château de Fontainebleau (Seine-et-Marne) and trees in the nearby forest have allowed “the reconstruction of temperatures over the last 700 years”explains this publication from the Laboratory of Climate and Environmental Sciences.
To go back even further in time, scientists also study marine sediments or ice caps, which contain very deep air bubbles. Their atmospheric composition also makes it possible to estimate what the weather was like over hundreds of thousands of years.