Torrential rains in Japan, record heat waves in Europe and recurring droughts in the western United States. For the second consecutive year, the beginning of summer in the northern hemisphere was marked by extreme weather conditions. To what extent is global warming responsible?
In France, a particularly intense heat wave should begin on July 11 and last more than a week with temperatures of more than 38°C over a large part of the territory. The exceptionally hot weather will also affect the Iberian Peninsula, with temperatures above 40°C in Spain, Portugal and the United Kingdom. It comes after France, Portugal and Spain experienced heat waves in June, and Spain recorded the hottest May weather since the turn of the century.
Firefighters in France have already raised concerns about the increased possibility of wildfires due to dry and hot weather.
At the end of June, Japan was also hit by an unprecedented heat wave. The mercury rose to 35°C for consecutive days in the capital, Tokyo, and up to 40°C in Isesaki in the center of the country. These consistently high temperatures broke records for the time of year and were quickly followed by torrential rains over the Japanese archipelago.
Across the Pacific Ocean, the western states of the United States have been hit by severe droughts that are becoming an annual event. This year, however, experts fear reservoir levels will drop so low that the Hoover Dam will no longer be able to generate electricity for hundreds of thousands of American homes.
>> A mega drought in the United States creates problems for the Hoover Dam reservoir
In early July, a state of emergency was declared in five regions of northern Italy where high temperature records were broken one after another. Rome spent several days cooking at 38°C. In Sicily, the maximum in the municipality of Floridia reached 46°C. Saturday July 2, for the first time, temperatures of 10°C were recorded at the top of the Marmolada glacier, in the Italian Alps.
Following the heat wave, part of the glacier broke off and fell, killing 11 people. The next day, a pine forest south of Rome was the starting point of a forest fire.
“Interconnected phenomena” intensified by climate change
How to explain these extreme weather events that occur almost at the same time in several places around the world? “It’s absolutely no coincidence,” says Pascal Yiou, a climatologist and researcher at the Laboratory of Climate and Environmental Sciences. “These meteorological phenomena are interconnected. A cyclone or heat wave in the United States has repercussions all over the planet contributing to monsoons in India, for example.
While it’s not unusual for such events to occur in tandem, it remains unclear why they occur with such intensity. Yiou says global warming is to blame. “It disrupts the whole dynamics of the atmosphere,” he says. “Rising temperatures at the poles are disrupting wind energy and, therefore, the alternation of cyclones and anticyclones.”
Global warming creates, for example, favorable conditions for air mass conflicts between the ground and high altitudes. These conflicts can cause phenomena such as “cold drops” when a bubble of cold air collides with warmer temperatures near the ground, causing heavy rain and thunderstorms. Conversely, when a bubble of warm air collides with cool air at ground level, it can cause heat waves.
There is then a domino effect. A heat wave can intensify a drought or cause forest fires. Heavy downpours can trigger floods or landslides.
Climatologists such as Yiou say this vicious cycle is alarming. UN climate experts announced a ‘code red for humanity’ in a 2021 report that said heat waves, floods and other extreme weather events would increase in ‘unprecedented’ ways, in terms of frequency, scale, areas affected and times of year when they may occur.
“The start of summer this year, just like last year, shows that the warnings are already a reality,” Yiou said.
Attribution science
Although the overall impact of warming on global weather cannot be denied, scientists have long been reluctant to identify climate change as the cause of individual events. But since 2015, an international group of scientists known as World Weather Attribution (WWA) has been developing a method to determine how closely the intensity of a weather event relates to the climate crisis. The practice is called the science of attribution.
“Meteorological phenomena always come in multiples,” explains Robert Vautard, meteorologist and climatologist from the climate science research center of the Pierre-Simon Laplace Institute, which contributes to WWA research, “But today we know that global warming can have an impact on the probability of certain events. The objective is now to determine [how it affects] The scale.”
They use the same methods to calculate the influence of global warming on a series of phenomena. “Using numerical models, we compare a planet A, representing the planet we live on, to a planet B, representing a planet without any human activity,” explains Vautard. “We run thousands of simulations and count how many times an event occurs on each planet and at what level of intensity.
The goal of attribution science is to understand how global warming presents itself worldwide in our daily lives, and evidence shows that it contributes significantly to extreme weather events.
The WWA found that the heat wave that hit India and Pakistan in March and April was 30 times more likely to have happened due to climatic imbalances. A heat wave in Canada in June 2021 was found to be 150 times more likely due to global warming. “Concretely, what we have shown is that this event could have happened without climate change, but it was much less likely,” explains Vautard.
Measuring human influence
The analysis can also show that the events are not related to global warming. This was the case with winter storms Eleanor and Friederike, which hit Europe in January 2018.
In some cases, social and economic factors also play a role. The researchers found that climate change was not the main cause of famine in Madagascar, although the UN claims otherwise. Instead, poverty, natural weather conditions and poor infrastructure were found to be the main causes.
“Also, if we’re talking about something like flooding, we have to weigh things carefully,” says Vautard. “In addition to rainfall, there is also the question of human management of waterways. Fires are often caused by human behavior. But the human element is often difficult to measure.
It can also be more difficult to draw clear links between global warming and certain types of weather, such as cyclones and tornadoes. Yet, says Vautard, “today the influence of climate change on heat and cold waves is undeniable”.
The WWA will soon begin an investigation into whether global warming contributed to the heat wave in Japan in June.
The analysis of previous extreme weather events has already clarified one thing. “Extreme weather events will now be the norm,” says Vautard. “The only way to prevent the situation from getting worse is to fight global warming as much as possible.”
This article has been adapted from the original in French.
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