(CNN) – Man-made climate change is making hurricanes more dangerous. They are producing more precipitation, moving more slowly once they make landfall, and generating larger storm surges along the coast. Hurricane Ida was a prime example of those changes, and scientists say storms like this will become more common as the planet warms.
Fueled by hotter-than-normal water in the Gulf of Mexico, Hurricane Ida destroyed homes, uprooted trees and cut off power for more than 1 million residents. It hit Mississippi and Louisiana, a state already ravaged by storms, and officials say they expect the death toll to rise in the coming days.
Hurricane Ida was the last of consecutive storms to hit Louisiana in recent years. But Katharine Hayhoe, chief scientist at the Nature Conservancy and a professor at Texas Tech University, said an important distinction is not the frequency of storms but their severity.
“We’ve always had hurricanes, we’ve always had heat waves, we’ve always had floods and droughts, but what climate change is doing is loading the weather data against us,” Hayhoe told CNN. “It’s creeping in when we’re not looking, changing the numbers as they roll and asking what is this, how could this happen.”
“The answer to that is climate change,” he said.
Hurricanes are getting stronger
Hurricanes, also called tropical cyclones or typhoons off North America, are intensifying due to the warming ocean, which studies say has absorbed about 90% of the planet’s excess heat trapped by greenhouse gases emitted by humans. And a recent study found that the planet is trapping about twice as much heat as it was almost 15 years ago.
As the planet warms, the storms get stronger.
The recent United Nations report on the state of the science on climate change concluded that the global proportion of cyclones ranging from category 3 to 5, the most intense storms, has increased in the last four decades due to rapid warming. of ocean temperatures. For each additional degree of heat, scientists say that not only will the proportion of intense cyclones continue to rise, but extreme rainfall events are also forecast to intensify by about 7%.
“We know that, in general, hurricanes are intensifying faster,” Hayhoe said. “They are bigger and stronger than they would be otherwise; they have much more associated rainfall and rising sea levels exacerbate storm surge.”
Before making landfall on Sunday, Hurricane Ida went through a remarkable period of strengthening: its maximum sustained winds increased by 105 km / h in just 24 hours.
Scientists define rapid escalation, a process that has historically been rare, as a 56 km / h surge in 24 hours or less. Hurricane Ida nearly doubled that definition.
For rapid intensification to occur, warm ocean water must extend far below the surface, hundreds of meters deep, to provide enough motor for the hurricane to strengthen. This has led scientists to believe that storms are more likely to intensify rapidly as a result of warmer oceans.
“If you increase the speed limit, you leave more room for storms to strengthen, so they can intensify more quickly,” Jim Kossin, a senior scientist at the Climate Service, an organization that provides climate risk analysis to governments and governments, told CNN. Business.
Ida made landfall in Louisiana as a Category 4, and is now tied in the record books with Laura in 2020 as the strongest hurricane to make landfall there.
Hurricanes are wetter and move slower
The storms are also generating more rainfall. For every degree of warming, the atmosphere can contain 7% more water vapor than could fall as rain. After flooding from Hurricane Harvey devastated the Houston area in 2017, scientists found that human-caused climate change overloaded the storm’s rains by about 15%.
Hurricane Ida has dumped more than 380 millimeters of rain in parts of Louisiana, according to the National Weather Service. As the storm’s debris spreads from the south to New England, another 75 to 200 millimeters of rain is expected in at least 12 states, including areas already affected by severe flooding this month, such as Tennessee. and New Jersey.
Paul Miller, a coastal meteorology scientist at Louisiana State University, said southeastern Louisiana, including Baton Rouge and New Orleans, experienced one of its wettest seasons on record. According to Miller, it rained 91% of the days between June 18 and July 19 at the New Orleans airport, compared to the region’s average of 40-50%.
“It has been a historically wet year in southeastern Louisiana even before Ida,” he told CNN. “This really exacerbated concerns about the flooding during Ida because the soil had already absorbed a lot of water this year.”
Satellite data also shows that storms are slowing down, particularly after they make landfall, according to Hayhoe. Larger and slower storms mean they can cover a larger area, stay on top, and dump more rain during that time period.
And, while experts say the research is still developing, a 2020 study published in the journal Nature found that storms remain stronger inland than five decades ago. Since hurricanes are powered by warm ocean water, they tend to weaken over land, but in recent years they have been raging longer after making landfall. The study concludes that warmer sea surface temperatures are causing “slower decomposition” by increasing the amount of moisture that a hurricane can carry.
The storm surge is rising
Storm surge, the flooding that occurs when hurricane-force winds push ocean water over land, is also getting worse due to rising sea levels. Hayhoe said storm surge can flood a larger area now than it would a hurricane of the same intensity 50 to 100 years ago.
As water accumulates along the shoreline, rivers and streams that empty into the ocean can also become blocked, forcing water levels to rise upstream. Storm surge and high winds from Hurricane Ida reversed the flow of the Mississippi River near New Orleans on Sunday, something the US Geological Survey says is “extremely rare.”
Scott Perrien, a hydrologist with the USGS, told CNN that the river level on Sunday rose about 2 meters due to storm surge at the gauge located in Belle Chasse, about 30 km south of New Orleans. At the same time, the river’s flow “decreased from about 60 cm per second to about 30 cm per second in the other direction,” Perrien said.
“I remember, all of a sudden, there was a reversal of the flow of the Mississippi River during Hurricane Katrina,” Perrien said, “but it is extremely rare.”
Tom Knutson, a senior scientist for the National Oceanic and Atmospheric Administration, said that, combined with wind and rain, storm surge is “one of the most dangerous aspects of hurricanes.”
Almost half of all tropical cyclone deaths come from storm surges.
Louisiana’s natural storm surge buffers – its coastal wetlands – are also disappearing due to rising sea levels and persistent severe hurricanes. “Seeing each additional hurricane, like Ida, hit the state is a reminder of the importance of preserving and restoring these environments,” Miller said.
As the ocean warms from fossil fuel emissions, the Gulf Coast is likely to experience more hurricanes that are as severe as Ida. Hayhoe said the only option left for humans is to quickly reduce greenhouse gas emissions or face the most catastrophic impacts of the climate crisis.
“The bottom line is that climate change is not just an environmental problem or a coastal problem, climate change is a problem of everything, a human problem,” Hayhoe said. “To care about climate impacts and advocate for climate action, we only have to be one thing, and that one thing is a human being living on planet Earth.”
CNN’s Brandon Miller contributed to this report.