This story, first published Wednesday, was updated Thursday.

Unrelenting and unprecedented, back-to-back Atlantic hurricane seasons have punished the Gulf and East coasts of the United States. An unsurpassed 50 named storms have formed over the warming Atlantic waters since the start of the 2020 season, with a record-setting 18 striking the Lower 48 states, including seven hurricanes.

Scientists say this record-setting stretch of storminess is connected to a period of heightened hurricane activity that began in 2017 but may become increasingly common as the planet warms.

Few coastal communities from Texas to Maine have been untouched by the onslaught of cyclones, and the Gulf Coast has been hit particularly hard.

Louisiana has become a magnet for these storms, with four hurricanes and two tropical storms striking its coast since the start of the 2020 season. Some areas, such as Lake Charles and Grand Isle, have been hit more than once and have yet to recover.

Remarkably, two of the strongest hurricanes in Louisiana state history, Laura and Ida, have occurred in the past two years. The high-end Category 4 storms, each of which roared ashore with 150-mph winds, are only matched by the Last Island Hurricane of 1856. Along with their destructive winds, the duo pushed ashore a devastating storm surge, or rise in ocean water above normally dry land. Laura was blamed for 30 deaths in Louisiana, while Ida was responsible for about the same number of fatalities.

The wrath of many of these storms extended far inland because of their swaths of copious rainfall. Disastrous flooding unfolded from Ida’s remnants in the Northeast, causing more than 50 deaths.

The price tag of these storms is staggering and still mounting. Seven of last year’s landfalling tropical storms and hurricanes were deemed billion-dollar weather disasters by the National Oceanic and Atmospheric Administration, with total damage of over $42 billion.

While the costs of this year’s hurricane season are still being tallied, damage from Hurricane Ida on its own is sure to balloon into the tens of billions of dollars.

Meanwhile the 2021 hurricane season still has two months to go.

Why are so many storms striking land?

Decades of record-keeping show U.S. hurricane seasons have alternated between extremely busy and eerily quiet periods. The barrage of storms in the past two years is an extension of a very active period that began in 2017.

Before 2017, the nation recorded a 10-year landfall drought. It was the longest interval on record without a hurricane rated Category 3 or stronger making landfall in the United States. From 2009 to 2016, in a course of eight years, 13 named storms came ashore.

“We know the reasons for the hurricane drought,” said Suzana Camargo, a climate scientist at Lamont-Doherty Earth Observatory at Columbia University. “For instance, there were some patterns of winds that led to many hurricanes to avoid the continental U.S. for a few years. And now it seems that we are in the reverse pattern.”

The landfall drought ended when the Category 4 Hurricane Harvey slammed into Texas in 2017. Then came Category 4 storms Irma and Maria that same year, which hit the Florida Keys and Puerto Rico.

Since 2017, five storms rated Category 4 or stronger have hit the Gulf Coast in as many years, a record number over such a time interval.

Phil Klotzbach, a hurricane researcher at Colorado State University, linked the increase in landfalling storms to a persistent zone of high pressure off the U.S. East Coast over the past five hurricane seasons. The clockwise circulation around the high has frequently pushed storms in the western tropical Atlantic toward the Caribbean and Gulf of Mexico.

Climate models show that this subtropical high-pressure zone may intensify and extend farther west in a warming world, which could push storm tracks westward toward North America.

Scientists also connect the uptick in landfalling storms to periodic changes in vertical wind-shear patterns along the East Coast. Vertical wind shear describes changing wind speed and direction with altitude. High amounts of shear can weaken a hurricane or prevent it from forming, while weaker shear is conducive to hurricane organization and strengthening.

“If the wind shear is strong, then those hurricanes will weaken very rapidly so that you won’t get strong hurricanes landing on the East Coast,” said Mingfang Ting, a climate scientist at the Lamont-Doherty Earth Observatory.

During the resurgence of landfalling storms since 2017, shear has been relatively low in the western Atlantic, Klotzbach wrote. Climate models show that unchecked greenhouse-gas emissions could result in reduced shear along the East Coast, which could help boost hurricane intensification in the coming decades.

2021 hurricane season is far from over

More storms are probable this year, although it remains to be seen whether they will become landfall threats.

Atlantic hurricane season is not over until Nov. 30, which means forecasters will busily track storms for weeks. As 20 storms have already formed this year, only one remains on the list of names before forecasters will have to resort to a supplementary list. That has happened only twice before — last year and in 2005.

Klotzbach does not foresee this season slowing down soon.

“Unfortunately, the odds of landfalling hurricanes look elevated for the rest of the year, given the likelihood that La Niña will be re-emerging shortly,” he wrote. La Niña refers to a cyclical cooling of ocean waters in the tropical Pacific, which gives rise to warming water and reduced shear in the tropical Atlantic.

Klotzbach noted that El Niño conditions, which tend to increase shear and suppress Atlantic storms, haven’t been present during hurricane season since 2015. “At some point, we’re going to have to get another significant El Niño, and if that occurs, the odds of significant landfalls do go down,” he wrote.

Ting warned, though, that lulls in landfalls may decrease in the long term with increased global warming.

“You won’t have a flipping sign that sometimes is a drought, sometimes … a lot of hurricanes,” Ting said. “The general trend is going to have a lot more hurricanes intensify along the coastal area.”

Climate-change connections

Although the 2020 and 2021 hurricane seasons seem exceptional in our lifetimes and models portend an ominous future, researchers cannot attribute the increased activity to climate change yet, given the short time span when elevated landfalls have occurred. Active hurricane seasons date to the earliest record-keeping. Most notably, six hurricanes made landfall in the United States in 2020, 1985 and 1886, tied for the most U.S. landfalls.

“You see this big variability coming from periods that have lots of landfalls,” Camargo said. It’s hard to blame recent activity to human-induced climate change, Camargo said, “because there is the natural variability that can lead to this.”

However, researchers have uncovered three clear connections between hurricane behavior and climate change.

First, Camargo said, there is a higher proportion of intense storms. While the number of total storms per season does not appear to be increasing because of global warming, a greater percentage of storms are strengthening into powerful hurricanes. The intensification can be partly attributed to rising ocean temperatures, which fuel hurricanes.

Such intensification is particularly dangerous if it occurs quickly and leads to landfall, leaving coastal communities less time to prepare for a high-end storm.

In 2020, 10 of the 13 hurricanes that formed rapidly intensified or saw their peak winds leap at least 35 mph in 24 hours; several, including Laura, did this right before landfall. In 2021, six of the seven hurricanes have rapidly intensified, including Ida on its approach.

Second, hurricanes are stalling more and dropping more rain. In a warmer climate, the atmosphere can hold more water vapor and thus produce more-extreme rainfall. Slower-moving hurricanes also drop rain in a concentrated area, as seen along the North Carolina coast from Hurricane Florence in 2018 and along the Texas coast by Hurricane Harvey in 2017.

Rising sea levels are also worsening storm surges, which can cause significant flooding and damage, Klotzbach said. If coastal waters are already at an elevated height, a storm surge can bring water farther inland for more dangerous and widespread damage.

“We are going to get more and more of damaging hurricanes making landfall. Coupled with sea level, you’re going to see a lot more flooding in the coastal areas,” Ting said. “We need to work on mitigating these kind of effects, not only in terms of reducing emissions, but also [protective] adaptation on the coast.”