At the end of June, Japan was confronted with an unprecedented heat wave.

The mercury soared to 35°C for several consecutive days in Tokyo, the capital, and up to 40°C in Isesaki, in the center of the country.

Unheard of at this time of year.

After this stifling heat, it is now torrential rains that are falling on the archipelago. 

On the other side of the globe, a fortnight after the heat wave that hit France, a state of emergency was declared in five regions of northern Italy.

There too, the thermometer panics and the temperature records fall one after the other.

Rome has lived like this for several days under 38°C.

In Sicily, the town of Floridia has already reached the 46°C mark.

On Saturday July 2, for the first time, 10°C was reached at the top of the Marmolada glacier, in the Italian Alps.

Direct consequences of this heat wave: part of this glacier broke off causing the death of at least seven people.

The next day, a forest fire broke out in a pine forest south of Rome.

>> In pictures: droughts, fires and melting glaciers hit Italy

The American continent is not spared either.

This year again, the western United States suffered a major episode of drought, to the point of raising fears of the imminent shutdown of the Hoover dam, which produces electricity for hundreds of thousands of American homes. 

"Interconnected phenomena" intensified by global warming

How to explain, first of all, that these events occur almost at the same time, in several places on the globe?

"It's absolutely no coincidence," replies Pascal Yiou, climatologist, researcher at the Laboratory of Climate and Environmental Sciences.

"Weather phenomena are interconnected: a cyclone or a heat wave in the United States will have an impact on the whole planet, and therefore on the monsoon in India, for example."

If the concomitance of these events is therefore easily explained, the question of their intensity also arises.

And there, Pascal Yiou points the finger at a culprit: global warming.

"It disrupts all the atmospheric dynamics!", He sums up.

"By increasing the temperature at the poles, it disrupts the energy of the winds and, therefore, the alternation of cyclones and anticyclones." 

In short, global warming promotes, for example, air mass conflicts between ground and altitude.

This can cause, for example, the "cold drop" phenomenon, when a bubble of cold air collides with warmer temperatures on the ground, causing very heavy precipitation and thunderstorms.

Conversely, a "hot drop" can create a heat wave.

And then, it's the domino effect, a heat wave can intensify an episode of drought, causing forest fires or heavy rains causing floods and landslides.

Faced with this observation, climatologists like Pascal Yiou are constantly sounding the alarm.

Heat waves, floods and other extreme events will increase "unprecedentedly" in terms of magnitude, frequency, time of year when they will hit and affected areas, warned UN climate experts in August in the IPCC report.

"This beginning of summer, like last year, shows that all of this is already a reality", concludes Pascal Yiou.

A science of attribution

If the impact of global warming on global weather is well established, scientists have long been reluctant to link an individual event to climate change.

But since 2015, an international group of scientists, the World Weather Attribution (WAA), has developed a method to determine to what extent the occurrence and intensity of an event are related to the climate crisis.

This is called the science of attribution.

“Weather phenomena always have multiple causes,” explains Robert Vautard, meteorologist and climatologist at the Pierre-Simon Laplace Institute, which works within the WAA.

"But today, we know that global warming can modify the probability of certain events. The objective is to determine the magnitude." 

The method is always the same: "Thanks to numerical models, we compare a planet A, the one in which we live, and a planet B, which would be spared from all human activity", explains the researcher.

"We do thousands of simulations and count how many times an event would have happened in both cases, and at what intensity." 

The members of the WAA have thus shown that the heat wave which affected India and Pakistan in March and April was thirty times more likely to occur because of climate change.

For the heat wave that hit Canada in June 2021, the probability increased by 150. "In concrete terms, what is shown is that this event could have occurred without climate change, but with much less probability.

A human part still difficult to measure

Conversely, analyzes sometimes show that an event is not linked to global warming.

This is the case, for example, of the winter storms Eleanor and Friederike, which hit Europe in January 2018.

The group of researchers also takes into account economic and social criteria.

He thus decided that climate change was not the main factor of the famine in Madagascar, contrary to the assertions of the UN.

The main culprits are said to be poverty, natural weather conditions and poor infrastructure. 

"Today, the influence of climate change on heat or cold waves is no longer in doubt", summarizes Robert Vautard.

"For some phenomena, however, it remains difficult to establish a clear link with global warming. This is the case for cyclones or tornadoes."

"Similarly, if we are talking about floods, for example, we have to make a distinction between things. Because in addition to rainfall, there is also the question of human management of waterways. Same thing for fires: the departure of fire is often due to a human gesture", he continues.

"This human part is necessarily difficult to measure."

"Ultimately, the whole issue of the science of attribution is that it makes it possible to become aware of the way in which global warming manifests itself on a daily basis", concludes Robert Vautard.

"Extreme weather events will now be the norm. The only way to prevent the situation from getting worse is to fight global warming as much as possible."

Soon, the WAA will decide on the role of global warming on the heat wave in Japan.

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