When extreme events are very rare, it is often difficult to prove that their probability of occurrence has changed due to climate change. Thus a new way of doing things has appeared to study the link between climate change and extreme events. This no longer attempts to compare frequencies of occurrence, but rather to understand how climate change influences the climatic parameters at the origin of extreme phenomena. This line of research seeks to establish the mechanisms that produce these extreme events. In this way, researchers can explain how global warming modifies the power, the duration or even the geographical extension of natural disasters.
This new current of research of the ESTIMR team, coordinated by Davide Faranda, is presented in the article “A climate-change attribution retrospective of some impactful weather extremes of 2021” which has just been published in the journal Weather and climate dynamics. the ESTIMR team has indeed proposed a new method for attributing extreme events to climate change without climate models, using only current and past meteorological observations.
For this, the ESTIMR team used the two-step method. First step: establish, using a map of atmospheric pressures, what were the large air masses and the high and low pressure systems at the time of the storm. Second step, determine if a similar situation had already occurred before climate change began to weigh on the atmosphere. For this, Faranda and his team used ERA 5, an European database that provides meteorological parameters for the atmosphere, land surface and sea since 1950.
They looked in this archive for configurations similar to that of the extreme events analyzed over a period between 1950 and 1979, that is to say, when the effects of climate change were still weak, even imperceptible, and in the period 1992-2021 that is in the actual climate, affected by climate change. The method also evaluates the role of internal variability of the climate system such as El Nino or the Atlantic Meridional overturning circulation.
The LSCE team used this method on a series of climate events that occurred in 2021. Among them, Cyclone Ida in the United States, the April cold wave in France, the tornadoes over the Po Valley in Italy, the winter storm Filomena in Spain and the Mediterranean cyclone Apollo. In most cases, they found a very clear link to climate change. However, Filomena and Apollo appear as peculiar events. Indeed, researchers have found no equivalent in historical records to the atmospheric configuration at the time of these events. "As we have not found a similar configuration, we are unable to determine the role of climate change in their occurrence", regrets Davide Faranda, in an interview for the CNRS journal. "However, this is not necesseraly a negative feature of the methodology that, on the contrary, is able to highlight unprecedented events", continue Faranda.
This method has been already applied to determine the role of climate change in enhnacing the impacts of the Corsica thunderstorm occured the 18 August 2022. The report of this study can be found here. This method has already received the attention of the attribution community, as the journal Science dedicated a perspective article to this technique, among others.
Faranda, D., Bourdin, S., Ginesta, M., Krouma, M., Noyelle, R., Pons, F., Yiou, P., and Messori, G.: A climate-change attribution retrospective of some impactful weather extremes of 2021, Weather Clim. Dynam., 3, 1311–1340, https://doi.org/10.5194/wcd-3-1311-2022, 2022.