It all started with Hurricane Hanna, which swirled in from the Gulf of Mexico to Padre Island, Tex., on July 25. Originally forecast to move into South Texas as a tropical storm, with the biggest threat being heavy rainfall, the storm instead intensified quickly, becoming a strong Category 1 hurricane just before landfall.

Forecasters didn’t know it at the time, but Hanna offered a preview of what was to come during the record-setting and destructive 2020 Atlantic hurricane season, as storm after storm underwent a process known to meteorologists as “rapid intensification,” in which storms gain strength extremely fast. This is an especially dangerous process when it occurs close to land, coming too suddenly for coastal residents to escape an exponentially potent storm.

The technical definition of rapid intensification is when a storm’s maximum sustained winds increase by at least 35 mph in 24 hours. This season, storm after storm outperformed this baseline. This includes Hurricane Iota, which intensified at the astonishing rate of 80 mph in 24 hours, before slamming into the coast of northeastern Nicaragua late Monday night.

Rapid intensification typically occurs in high-end hurricanes that reach Category 3 or above. Scientists now say this is happening more frequently, as storms are given a turbo boost from rising ocean temperatures. During the 2020 hurricane season, the waters of the Atlantic were unusually mild, the result of human-caused global warming superimposed atop natural climate cycles.

The increase in rapidly intensifying storms is “not surprising,” said Suzana Camargo, a hurricane researcher at the Lamont-Doherty Earth Observatory at Columbia University, because human-caused climate change is expected to increase the occurrence of these particularly dangerous storms, most prone to explosive development.

According to Kerry Emanuel, a hurricane scientist at MIT, the 2020 Atlantic season provides a warning: The increasing tendency for hurricanes to rapidly intensify is a better gauge for how climate change is influencing them rather than how strong they ultimately get.

After Hanna slammed Padre Island, a staggering nine other 2020 storms would go on to exhibit similar or greater bursts of strengthening, often close to landfall and in places where it hadn’t happened before.

The 2020 season will go down in history for the dominance of the fast-blooming storms that prowled the Atlantic, raising the question of whether this is the new normal.

Two of the storms that intensified at the fastest rates occurred in the past two weeks, which is unusually late in the hurricane season. Hurricanes Eta and Iota each intensified by 80 mph in 24 hours, a rate that had been achieved just eight times before this year, and never after October, said research meteorologist Sam Lillo of the University of Colorado at Boulder.

Based on recent peer-reviewed studies he co-wrote, meteorologist Jim Kossin of the National Oceanic and Atmospheric Administration (NOAA), said it’s clear that the odds of a storm rapidly intensifying have increased compared with what they were just a few decades ago.

“In the early 1980s, the chance of having a hurricane intensification event of 35 mph or more in a 24-hour period was about 1 in 100,” he said in an email. “Thirty years later, the chance has gone up by a factor of five to about 1 in 20. I suspect that the chances would go up even further if we included the past few years in the analysis.”

Kossin concluded: “The warmer than normal waters have certainly contributed to the rapid and explosive intensification events this year, and it’s very likely that part of that warming is due to human activities.”

Plummeting air pressure and stronger winds mean a higher potential for disaster

It’s one thing to read about these trends and another to see them play out in the real world over and over again.

There were several storms this season that went through nearly continuous periods of rapid intensification lasting longer than a day, Lillo said. For example, Hurricane Delta, which struck Louisiana in early October, and Hurricane Eta, the first of two unprecedented back-to-back Category 4 landfalls in Nicaragua, both had winds increase by 105 mph in 36 hours.

Before 2020, Lillo found only four other storms since 1851 that achieved such a feat of rapid strengthening, including storms that are notorious to hurricane history buffs and those living along the Gulf Coast and in Florida: The Labor Day Hurricane of 1935, Andrew in 1992, Wilma in 2005 and Felix in 2007.

This year also had three storms that intensified by 100 mph in 36 hours, something Lillo says previously happened with only eight storms.

There’s another definition that captures how quickly a storm is growing more fierce, this one concerning the minimum air pressure reading in the storm. In general, the lower the pressure, the stronger the storm. Rapid deepening is defined as precipitous falls in atmospheric pressure of 42 millibars in 24 hours, and this has occurred with only 23 Atlantic storms since 1979, when reliable air pressure records began.

Yet 2020 had the most storms of any year to meet or beat the threshold, including Hurricanes Laura, Delta, Eta and Iota, Lillo said.

Before 2020, only three other Atlantic storms had an observed air pressure drop by 59 millibars or more in 24 hours: Gilbert in 1988 and Rita and Wilma in 2005. “Then Eta dropped 59mb and Iota dropped 65mb; two weeks apart, a few miles apart, and both [were] the latest in the year to do so,” Lillo said.

Gabe Vecchi, a climate scientist at Princeton University, said that while these statistics may come across as “kind of abstract,” they convey a troubling message. “These are the types of events … that are hard to predict on a weather time scale and leave people with little time to react … which can be extremely dangerous,” he wrote in an email.