Monday, in the footsteps of the hurricane Helenewe were still looking for hundreds of missing people, we were stuck on roads swallowed by floods, we mourned more than 230 dead, we got by without electricity, we slept in makeshift shelters. And in the Gulf of Mexico, the next cyclone was already honing its weapons.
In just 24 hours, between Sunday and Monday, the moderate hurricane Milton turned into a Homeric storm. On the Saffir-Simpson scale, which measures the intensity of hurricanes, it jumped from the first to the last rung, category 5. In meteorological jargon, we speak of “rapid intensification”. Tropical storms that are initially discreet and break loose shortly before making landfall are among the most dangerous. They leave little time for the authorities to sound the alert, and for residents to evacuate.
“Rapid intensification is difficult to predict,” explains Raphaël Rousseau-Rizzi, a researcher at Hydro-Québec who studied hurricanes during a doctorate in atmospheric sciences at MIT (2016-2021). “In recent years, the prediction of intensification has improved much less than that of trajectories. »
In our disrupted climate, the most powerful hurricanes become even more powerful. Tropical storms move more slowly — and therefore dump more rain in their path. And they are intensifying more and more quickly.
Meteorologists define “rapid intensification” as the acceleration of at least 30 knots (56 km/h) in 24 hours of a tropical storm’s sustained winds. An “extremely rapid intensification” corresponds to an acceleration of at least 50 knots (93 km/h) in 24 hours.
If Helenetwo weeks ago, fell into the first category, Milton is now at the extreme end: the sustained winds of this hurricane gained 145 km/h in 24 hours — one of the fastest intensifications ever measured.
More powerful, faster
Since the beginning of the 20th centurye century, tropical storms are gaining strength more and more quickly. The number of Category 1 cyclones to become a major hurricane (Category 3, 4 or 5) in less than 36 hours has doubled compared to the 1970s and 1980s, according to a recent study.
Last year, another hurricane starred in the same movie.
On October 23, 2023, Otis was quietly approaching Acapulco, Mexico. While forecasters expected a gradual intensification, Otis concentrated its forces at a frenetic pace before becoming, the following night, the most powerful hurricane ever recorded to hit the Pacific coast of America. Stunned, the meteorologists spoke of a “nightmare scenario”. The city of a million inhabitants was gutted; dozens of people lost their lives.
Hurricanes find their fuel on the surface of the oceans. They appear over warm seas, where a moist air mass can convect towards the sky. When this water vapor condenses at altitude, it releases its latent energy, which strengthens the cyclone.
Climate warming caused by greenhouse gases will create more intense hurricanes. The reason is quite simple: a warmer ocean can transmit more energy to a cyclone. And more powerful cyclones intensify more quickly.
Other factors?
This is the future. But when we look to the past, questions remain unanswered. Mr. Rousseau-Rizzi explains that scientists are not certain that it is exclusively global warming that has increased the number of rapidly intensifying hurricanes over the past 40 years.
Two other phenomena can come into play by modifying the temperature of the atmosphere and the ocean, he says: first, the natural variability of the climate; two, the decrease in aerosol emissions over the Atlantic caused by the reduction of air pollution in the 1970s and 1980s. “At the moment, there is no consensus on the relative contribution of these three causes. This is still a subject of active research,” says the man who worked with the world authority in the field, Professor Kerry Emanuel.
Since spring 2023, ocean warming has accelerated globally. In the Gulf of Mexico — where Helene And Milton wandered around before hitting land — we set new records this summer. What’s more, this region has suffered a “marine heat wave” in recent weeks which has increased its potential to produce powerful hurricanes.
These occasional phenomena play an important role in the formation of tropical cyclones, specifies Mr. Rousseau-Rizzi. Because it is not exactly hot water that causes powerful hurricanes, but rather the temperature difference between the water and the atmosphere. The stronger the contrast, the more the hurricane gets carried away.
One thing is certain, at 1.3°C of global warming, the cocktail is already explosive.
In 2017, Professor Emanuel noted in an article that an intensification of a tropical storm of 100 knots (185 km/h) in 24 hours was a phenomenon “essentially non-existent in the climate of the end of the 20th centurye century “. Last year, Otis has reached this threshold.
If the intensification of hurricanes surprises us in this first quarter of a century, we must prepare for other painful surprises. “Just think of an event that occurred once every 5,000 years and which, with a warming of 1.5°C, could become an event that occurs every 50 years,” climatologist Stefan wrote last year Rahmstorf of the Potsdam Institute for Research on the Effects of Climate Change.