How scientists are working on attributing extreme events to climate change

As extreme events multiply, the same question keeps coming back: is climate change responsible? While there is no doubt that climate change is entirely due to human activities, scientists have until recently avoided linking an individual event to global warming. But, in recent years, researchers have developed methods to calculate how much the occurrence and intensity of heat waves, floods, storms or droughts have been influenced by the climate crisis. This is called the science of attribution.

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“It is a recent discipline, remained confidential until twelve years ago, and has skyrocketed for six or eight years”, says Aurélien Ribes, researcher at the National Center for Meteorological Research, attached to the CNRS and Météo-France, who is preparing to generalize attribution studies for events affecting France. The interest is to better characterize extreme events, but also to “raise awareness of climate change among the general public and political decision-makers”, he judges, since these phenomena are studied practically at the time they occur. These works are also increasingly used in legal disputes. “At the beginning, it caused a lot of debate because it disrupts the scientific work a bit: usually, you only publish a study when it is peer-reviewed, which takes about a year”explains Robert Vautard, climatologist and director of the Pierre-Simon-Laplace Institute.

More than four hundred studies of attribution of extreme events have been carried out since the first of its kind, published in 2004. It concerned the heat wave that occurred in the summer of 2003 in Western Europe, which claimed 70,000 victims. The methods differ somewhat, but the scientists generally proceed in the same way, according to a precise protocol: using digital models, they simulate thousands of times the current climate – warmed by human activities – and the pre-industrial climate, in order to count the number of times the extreme event under study occurs and to calculate its intensity. A synthesis is carried out, also using meteorological observations of the present and the past, which makes it possible to determine the part due to human influence on the climate. These models also make it possible to quantify the probability and intensity of a phenomenon in the future, depending on the warming scenarios.

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