C'est une question qui suit toute catastrophe naturelle, en particulier les ouragans monstrueux comme Ian :cela a-t-il été causé par le changement climatique ?

Lorsqu'on leur demande, des scientifiques comme Kevin Reed repoussent généralement. La plupart des chercheurs conviennent qu'il n'est pas valable de désigner une seule tempête et de dire qu'elle a été "causée" par le réchauffement de la planète. Trop de variables.

"C'est une question à laquelle il est vraiment difficile de répondre. Il n'y a pas de 'à quoi aurait ressemblé septembre 2022 sans le changement climatique ?' Nous n'avons pas cela", a déclaré Reed, professeur associé à l'école des sciences marines et atmosphériques de l'université de Stony Brook.

Mais il y a un consensus croissant sur le fait que l'élévation du niveau de la mer et les températures plus élevées au cours des cent dernières années ont déjà eu un impact sur les tempêtes comme Ian, qui a ratissé la Floride la semaine dernière, et pourraient continuer à le faire à l'avenir. À tout le moins, un temps plus chaud signifie des océans plus chauds, ce qui alimente la force des ouragans.

"Nous vivons dans un monde qui est plus chaud de plus d'un degré (Celsius), il ne fait aucun doute que les ouragans ont changé à certains égards à cause du changement climatique", a-t-il déclaré.

Tom Knutson, scientifique principal à la NOAA qui étudie le climat et les ouragans, a déclaré que les scientifiques sont plus confiants pour remarquer ce qui a changé au sujet des tempêtes dans le monde moderne. Mais moins sur la possibilité de relier ces changements au changement climatique. Prédire ce que l'avenir pourrait nous réserver est encore plus difficile.

C'est parce que les ouragans et les conditions météorologiques mondiales sont diablement compliqués. Certains des changements dans l'atmosphère et les tempêtes, par exemple, peuvent être attribués aux niveaux élevés de pollution physique de l'air, au-delà des gaz à effet de serre, rejetés dans l'atmosphère.

La technologie de suivi des tempêtes s'est également considérablement améliorée au cours des derniers siècles. Les scientifiques s'accordent à dire que nous attrapons maintenant beaucoup plus de "orages indésirables" - des cyclones faibles et de courte durée - que jamais. Cela peut fausser les données, c'est pourquoi la plupart des scientifiques sont prudents lorsqu'ils tracent une ligne directe entre un globe plus chaud et le nombre d'ouragans.

"C'est une image assez complexe et en ajoutant à la complexité, nous déduisons beaucoup de ces choses à partir de modèles climatiques qui eux-mêmes ont une incertitude", a déclaré Knutson. "Nous devons être prudents."

Voici où en est la science sur plusieurs fronts :

Onde de tempête plus élevée probable

Le niveau de la mer près du sud de la Floride a déjà augmenté d'environ huit pouces depuis 1950, selon les données des marégraphes de la NOAA. Cela signifie que l'eau commence à partir d'un niveau de base plus élevé, permettant à l'ouragan d'atteindre quelques centimètres plus haut et de couvrir plus de terrain.

Researchers expect sea level rise to accelerate as hotter temperatures cause polar ice sheets to melt faster. Over the next century, South Florida can expect to see more than three feet of sea level rise, according to estimates from the UN Intergovernmental Panel on Climate Change (IPCC). That will push storm surge higher when hurricanes roll through coastal communities.

One 2020 study modeled what 21 hurricanes that struck between 2000 and 2013 might look like under the climate conditions expected in 2100. The researchers estimated that, on average, floods would have been about 30% worse and covered about a quarter more land.

More extreme rainfall

One of the most straightforward connections between climate change and hurricanes is rain. Like anyone who's experienced humidity knows, warmer air holds more moisture.

For every 2 degrees Fahrenheit of warming, there is about 8% more water in the atmosphere, and the world has warmed at least 2 degrees Fahrenheit since pre-industrial times.

So perhaps it's no surprise that one study of the 2020 hurricane season found an average 8% increase in 3-day rainfall totals for hurricanes, and a 5% increase for tropical storms.

The lead author, Stony Brook's Reed, said they also found that a hotter planet increased the rate of rainfall. In the 2020 season, three-hour rainfall rates would increase 10% or higher for tropical storms and hurricanes.

"If you experience a similar storm in the future it's going to rain more because of climate change," he said. "If you had 30 inches of rain, you could say over 2.5 inches of that rain was due to climate change, meaning it would not have rained as much if we hadn't heated the planet."

Reed and his colleagues also produced a rapid study of Hurricane Ian on Thursday that suggested a 10% increase in extreme rain rates due to human-caused climate change.

This body of research, known as attribution science, looks to answer the question of how climate change influenced a particular storm. In Reed's case, he and his colleagues load up a powerful supercomputer weather model with the exact track and data from a modern-day storm, then re-set the clock back to the temperature and atmospheric conditions of the 1850s.

The clearest change they see is the storms are far less wet when set in the past.

Getting stronger faster

One of the most dangerous features of hurricanes is rapid intensification, which is when a storm's top wind speeds increase 35 mph or more in a single day. It's also difficult to predict so when a storm suddenly strengthens near the shore, coastal communities have little time to prepare or evacuate.

Early studies suggest climate change has already made rapid intensification more common. A 2021 IPCC report found that "the global frequency of [tropical cyclone] rapid intensification events has likely increased over the past four decades" and added that researchers have "medium confidence" that "none of these changes can be explained by natural variability alone."

Researchers can say with much more certainty that the conditions that lead to rapid intensification are becoming more common. Sea surface temperatures are rising at a rate of 0.14 degrees Fahrenheit per decade, according to NOAA, and atmospheric moisture is increasing between one and two percent per decade, according to the IPCC. Both of these factors may give future hurricanes more fuel to turbocharge their growth.

Meanwhile, NOAA and Columbia University researchers predict that climate change will weaken vertical wind shear, an atmospheric feature that can pump the brakes on rapid intensification.

"We've seen multiple studies that show the conditions in the North Atlantic basin are providing more opportunities for storms to intensify," said Kieran Bhatia, a former climate researcher at Princeton University who is now a vice president for the climate change perils advisory team at the insurance broker Guy Carpenter.

Fewer hurricanes, but stronger

Climate change might make hurricanes more intense but less frequent.

Reliable global records of hurricane intensity only go back about four decades, when weather satellites began scientists to accurately estimate the strength of storms. In the years since, hurricanes appear to be getting stronger, according to a 2020 paper from researchers at NOAA and the University of Wisconsin. They found that the likelihood that a cyclone will reach Category 3 wind speeds—the threshold to be designated a "major hurricane"—has risen about 25% since 1979, as extra heat in the oceans and atmosphere gives storms more fuel to grow.

But even as climate change makes storms stronger, scientists believe it is weakening the ocean currents that help cyclones form in the first place—particularly the Atlantic meridional overturning circulation (AMOC), which pulls warm surface waters from the tropics across the Atlantic Ocean. The IPCC's 2021 report says the AMOC, which also powers the Gulf Stream, is "very likely" to weaken over the 21st century.

As a result, hurricanes may become less frequent. A July paper published in Nature Climate Change estimated that tropical cyclones formed 13% less often in the 20th century than they did between 1850 and 1900. Although hurricane data before the satellite era is spotty, the international team of researchers combined real world observations with simulations from climate models to fill in the gaps and estimate the number of hurricanes that may have formed from 1850 to 2012.

"A string of consecutive seasons with Category 5s, that's something more consistent with what we'd expect in a warmer climate ... as opposed to a higher number of storms," Brian Soden, a professor of atmospheric sciences at the University of Miami's Rosenstiel School of Marine and Atmospheric Science, told the Miami Herald in 2020.

Slower and wetter storms

Jim Kosin, a climate scientist at the NOAA Cooperative Institute for Meteorological Satellite Studies in Madison, Wisconsin, published a paper in 2018 that suggested that tropical storms and hurricanes around the world had slowed down about 10% between 1949 and 2016, and hit the brakes even harder over land.

It was met with some criticism in the scientific community, but he followed it up with another research paper with NASA's Timothy Hall in 2019 that narrowed in on the slowdown of storms near the North American coast since 1950.

They found that hurricane forward speed has decreased since 1900, which can lead to even more rainfall and flooding as a storm stalls over land.

"More study is needed to determine how much more slowing will occur with continued warming. Still, it's entirely plausible that local rainfall increases could actually be dominated by this slowdown rather than the expected rain-rate increases due to global warming," Kossin told a NOAA blog.

Knutson, with NOAA, called it "the most convincing evidence of a trend I've seen so far."

But he cautioned that just because the trend was observed doesn't automatically mean climate change is to blame. "That's an open research question."

More landfalls may be in future

In the past, many storms at sea went undetected unless reported by vessels unlucky enough to encounter them. But good records go back a century or more on ones that make shore.

New research by Knutson, based on running past data into a computer model, projects what might happen to hurricane paths in the future if global warming continues unchecked. He found that while the number of storms making landfall hasn't really changed that much in the last century, an increasing fraction could in the future could.

Combined with other research suggesting that fewer but more powerful storms could form in the future, Knutson said his findings suggest that cities are in for fewer, but increasingly intense hits.

"It's kind of several effects cutting in different directions," he said.

Ever the careful scientist, Knutson also cautioned that his work was only a prediction.

"That's in the model. We'll see what happens in the real world," he said. + Explorer plus loin

Study finds that climate change added 10% to Ian's rainfall

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