The sudden crash of hail hitting Hector Dorantes’s balcony on the night of 12 June made him quickly look out his window in the Benito Juárez municipality, on the western side of Mexico City. Hundreds of leaves were falling from the trees to the ground, which was covered in what appeared to be a blanket of snow. Minutes after the downpour, the phone rang.
“Are you okay?” his neighbor asked. The hail had done more than carpet the ground: “The supermarket just collapsed.”
Dorantes left his home immediately, but as he took his first step onto the street, his foot sank into a 20-centimeter layer of marble-sized hail. As he approached the nearby shopping complex, the honks of stopped traffic and the sirens of ambulances and fire trucks kept blaring. When he arrived, the scene stunned him. “It was a disaster,” he said.
Approximately 20 metric tons of hail collapsed the 1,000-square-meter roof of the local supermarket. And although there were no injuries, more than a dozen other incidents were reported that night in the Álvaro Obregón, Benito Juárez, Iztapalapa, and Coyoacán municipalities.
Four trees, each between 10 and 20 meters tall, fell and damaged the power grid in several parts of the city. Nine water obstructions stopped traffic for up to 4 hours in Benito Juárez and Álvaro Obregón, the most affected municipalities. In addition to the supermarket, 12 structures and roofs of houses and businesses were damaged in both municipalities.
To deal with the damage on the night of 12 June and the early morning of 13 June, a total of 1,200 responders from the city’s Water System, Fire Department, Comprehensive Risk Management and Civil Protection, and the Secretariat of Works and Services were needed.
Although hail is frequent in Mexico City during the spring-summer transition, citizens were surprised by the sheer volume covering the highways and houses on 12 June.
Dorantes, who had never seen anything like that before, immediately thought of one thing. “It should be because of climate change,” he said.
He was not the only one who thought about it. In the days following the hailstorm, people from both civil society and the scientific community began to discuss whether or not climate change might have something to do with what happened.
For Paulina Ordóñez, an Earth physicist at the Center for Atmospheric Sciences and Climate Change at the National Autonomous University of Mexico, the only way to resolve the question would be to conduct an attribution study to understand whether the duration, impact, or probability of a meteorological phenomenon could be affected by human-induced climate change.
In the case of hailstorms, evidence is still scarce because of the difficulty of studying the phenomenon. The most recent research indicates that the recurrence of hailstorms will decrease with climate change, but their severity will increase over time.
“Climate change is undisputed, the increase in global temperature in proportion to the concentration of greenhouse gases is a fact.…It’s not that the hailstorm in the city is not related. We just don’t know it yet,” Ordoñez said.
Although the hail might be a consequence of the rainy season, the staff of the Early Warning System of the Comprehensive Risk Management and Civil Protection of Mexico City anticipated a high-pressure system over the west of the city produced by a number of factors, said Guillermo Ayala, director of the agency.
On 7 June—5 days before the hailstorm—the Megalopolis Environmental Commission registered a high concentration of ozone. This, combined with temperatures of up to 30°C and intense solar radiation, increased the region’s stationary winds, which in turn caused the contamination in the city not to dissipate, producing the ozone concentration.
The ozone produced a “large bubble of high pressure” that ended precisely on Sunday, 12 June, explained Ayala. When that happened, winds and moisture from the Pacific Ocean and the Gulf of Mexico interacted with a cold front coming from the north, generating a “temperature shock” that produced the hail, Ayala said.
Mexico City is not the only part of the country that has been recently affected by severe weather events. Just 1 day after the capital’s hailstorm, the government of the state of Jalisco confirmed that the city of Guadalajara had experienced its heaviest rainfall in 30 years. In a total of 40 flooding events, 92 houses had water levels above 50 centimeters, and 92 cars were stranded.
At the same time, the city of Monterrey is experiencing its worst drought since 1988. As a consequence, the state government of Nuevo León has been supplying only 6 hours of water a day to its inhabitants since the beginning of June.
José Martín Cortés, a meteorologist at Universidad Veracruzana, said that the severe weather in Jalisco and Nuevo León, which involved new record levels of precipitation and drought, is more attributable to anthropogenic climate change than the hail in Mexico City. As with Mexico City, however, Guadalajara and Monterrey are among the most urbanized areas in the country.
Still, for Jorge García, a climate physicist at Columbia University, the problem is that the question being asked (“Is this severe weather caused by climate change?”) is wrongly formulated from the start. Extreme weather events have always existed; none of them are specifically, solely “caused” by climate change.
But climate change has affected the atmosphere in which all these events happen, and that influences the severity of the events. Mexico is one of the fastest-warming countries in the world (a rate of 0.3°C per decade); this causes the atmosphere to be able to hold more moisture, which, when combined with the winds inside clouds, strengthens the already perfect conditions for strong hail storms to form, García explained.
So instead of asking whether anthropogenic climate change caused the hailstorm, García said that “what we can ask ourselves is how climate change affected the amount of hail and the probability of having this storm that turned the whole city white. Would this storm have been the same 100 years ago? Most likely not.”
—Humberto Basilio (@HumbertoBasilio), Science Writer