2023-07-19 12:21:48
On July 18 and 19, exceptional heat was recorded in the south-east of France, mainly in altitude reference stations in the Alps, the Pyrenees and Corsica. Earlier this month, it was the average temperature of the globe which had reached an unprecedented level, exceeding 17°C for the first time since these data are recorded. On May 23, three records were broken simultaneously: highest air and ocean temperatures, lowest sea ice surface.
It is difficult to ignore the extent of the climatic and meteorological upheavals underway. I’World Meteorological Organization (WMO) has warned : “Global temperatures are expected to break records over the next five years, driven by greenhouse gases, which trap heat in the atmosphere, and the occurrence of the natural phenomenon El Niño”.
Local, global, daily or monthly record temperature? Absolute record or average measurement? What are we talking about ?
How are all-time temperature records defined?
Meteorologists measure the temperature of the air under cover, at a height between 1.5 and 2 meters above the ground. It is also necessary to take into account parameters that can alter the data: artificial surfaces (such as concrete, which stores heat), shadows cast (by a vertical shelter for example), the absence or insufficiency of ventilation, etc. Once the measurement has been correctly taken, a record is determined according to the zone chosen (a region, a country, a continent, the planet, etc.) and the period chosen (a day, a week, a season, since the start of the compilation of the data…).
Last step, verification. When a station measures an extreme temperature, a validation process is set up: ultimately, it is the WMO, dependent on the United Nations (UN), which is responsible for certifying these records, ensuring in particular the consistency of the reading with the history of the area and the equipment used to measure the temperature.
Thus, the absolute temperature record (56.7°C, recorded just one hundred and ten years ago, on July 10, 1913, in Death Valley, California, USA) is still authoritative, but it is much debated by scientists. This temperature could be the result of a sandstorm that occurred at the time, which would have contributed to increasing the temperature under the shelter where the measurements were taken.
In the same area, more recent readings, in August 2020 and July 2021, indicate a temperature of 54.4°C, but they are still being verified by the WMO. If confirmed, this would make them the highest temperatures recorded since the 55°C measured in Kébili (Tunisia) in 1931, the latter record also being controversial, like the majority of measurements dating from before 1950.
Among modern certified and reliable data, the highest temperature once again goes to Death Valley, but also to Kuwait and Pakistan. The WMO has indeed validated the temperature of 54°C reached in 2016, in these three areas.
In Europe, the WMO considers that the maximum was reached in Sicily (Italy) in 2021, with 48.8°C, exceeding the 48°C recorded in 1977 in Athens.
Old records challenged
Initially manual, temperature readings were automated in the middle of the 20th century with meteorological stations. In the early 1960s, satellite observations made their debut, to represent today “between 75 and 95% of the observation data”, according to Météo-France. More reliable and more numerous, modern readings have called into question the reliability of old temperature records.
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For example, the absolute record of 58°C which would have been reached in 1922 in Libya is no longer one. He was invalidated in 2012. The causes are multiple: the reading was taken on asphalt, the instrument used was unreliable and, above all, this measurement proved to be inconsistent with those taken in the surrounding area.
If the same investigation were launched to verify the case of the 1913 record in Death Valley, it could demonstrate an error of observation: according to two American researchers, Chris Burt and William Reid, the record in this area of the Mojave Desert , in California, East “incoherent” with the data of the time.
The WMO also recognizes the weaknesses of the old measures. THE Tunisian record from Kébili, at 55°C, in 1931 is subject to caution, she admits. “Between 1920 and 1933, Kébili recorded maximum temperatures of 50 to 55°C almost every summer. Since 2000, its absolute maximum has been only 48.5°C. » But, due to the lack of data available for this time, which does not allow either to validate or invalidate a measurement, the WMO accepts this record as being the highest temperature recorded in Africa.
How are average records estimated?
From the beginning of July, the planet sees the hottest days ever recorded follow one another. The average temperature (an indicator calculated from all the temperatures of the globe) reached 17.23°C on July 6, according to the American University of Maine calculationswhich is based on public data from the United States Agency for Oceanic and Atmospheric Observations (NOAA).
The figures that the European Copernicus Institute provided to the Monde show a slightly lower record, at 17.08°C. The difference between the two measures is that they are estimates produced by so-called “reanalysis” tools. The latter take measurements of observed temperatures and ask a computer model to interpret them. To each model its record, to sum up.
Climatologists focus more on records on an annual and monthly scale rather than on single days, whose temperatures also depend on natural variability. But, “when human influence increases the imbalance of the Earth’s energy balance (…)it is expected to observe new global surface temperature records”recalls the paleoclimatologist Valerie Masson-Delmotte on Twitter.
Over a longer period, the seven days from July 3 to 9 correspond to the hottest week ever seen, confirm Copernicus. The month of June was crowned the hottest month of June both by Copernicus and by the United States Oceanic and Atmospheric Observation Agency.
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