Why Was This Year’s Hurricane Season So Intense?

The 2017 Atlantic hurricane season isn’t over until the end of November. But if you’ve even glanced at the headlines over the past few months, you know this year’s storms have already left an indelible mark.

Consider the following stretch, which began in late August: First there was Harvey, which dumped three or more feet of rain over Houston, causing 75 deaths. Then Irma leveled some of the Caribbean’s Leeward Islands and slashed up through Florida, killing 14 people in the Florida Keys and some 134 overall. If that wasn’t enough, Maria later hit Dominica and Puerto Rico, which is still struggling to recover.

Hurricane Irma strengthened to a Category 5 storm with sustained winds of 185 mph.

In addition to the loss of life, the damage left in the wake of these hurricanes will probably add up to hundreds of billions of dollars. For many Americans, this summer and fall was the worst hurricane season since 2005 when Katrina and Rita tore through the Gulf of Mexico.

At first or even second glance, this hurricane season seems like it was among the worst. But was it really? If so, why? And how does climate change fit in?

This year’s hurricane season really did stand out. But most of it came down to one incredible month.

“The season was kind of near-normal through August, and then it’s been near-normal in October,” says Phil Klotzbach, an atmospheric scientist at the Colorado State University. “September was off the charts.”

On Twitter and in the press, Klotzbach has spent the last few months calling attention to 2017’s hurricane superlatives. He also keeps a list of statistics, updated in real time, of the current season as a whole.

September was truly unique, he says. That month, not one but three unusually long-lived, strong hurricanes—Irma, Maria, and Jose—formed near the central Atlantic’s Cape Verde archipelago and moved northwards.

Hurricane Maria inflicted severe damage on Puerto Rico, seen here on September 23, 2017.

Scientists track a statistic called hurricane days, which a hurricane accrues for every day it lives. If three hurricanes coexist on the same day, they rack up three days. In this respect, September was the worst month in recorded history. It had more named storm days, more hurricane days, and more days with a Category 3 or higher hurricane than ever before.

Hurricane watchers also track a figure called accumulated cyclone energy, which is like the total kinetic energy of every storm this season put together. In the North Atlantic, that total wind energy—now, still with a month to go—is already more than twice the average measured between 1981 and 2010.

The 2017 season also broke a lucky streak for the continental U.S., which hadn’t been hit by a major hurricane since 2005. “Up to September of this year, a topic of popular discussion was what was going with the hurricane drought,” says Gabriel Vecchi, a geophysicist at Princeton. “There were papers being written on this.”

But even after that remarkable September, 2017 won’t take the overall crown as the worst-ever year unless this next month is particularly unlucky. “It’s a top-ten year,” Klotzbach says, but “there’s years that have been more active.”

While that qualifies this year’s hurricane season, it does not take away from it. This year’s storms did grow big and strong. We won’t know how exactly that happened until scientists have performed careful, quantitative analyses. But broadly speaking, two major factors are to blame.

One, the Atlantic hosted swaths of warm water this year, revving up the engine that fuels hurricanes.

Let’s rewind to an unlikely place. In 1824, as steam engines were transforming the world, French military engineer Nicolas Carnot figured out the math behind how they actually worked. That same math applies to hurricanes, according to MIT atmospheric scientist Kerry Emanuel.

What matters is the temperature difference between two reservoirs of thermal energy, called the thermal potential. In a hurricane, the contrast between the warm surface of the sea and the cool atmosphere does the work: it sucks wind gusts up the spiraling wall around the calm “eye” and then sends them curling outward.

Carnot’s equations show that the bigger this temperature contrast is, the faster a hurricane’s winds can get at their theoretical maximum.

This year, that difference was heightened by warmer-than-usual water. In the key month of September, the tropical Atlantic was the third warmest it has ever been, Klotzbach says. The only warmer years were the infamous 2005, which produced four separate Category 5 hurricanes, and 2010, which was also an intense hurricane year.

NOAA's GOES satellite shows, from left to right, Hurricane Katia in the Gulf of Mexico, Hurricane Irma in the Caribbean Sea, and Hurricane Jose in the Atlantic Ocean on September 8, 2017.

This could be because of a current called the Atlantic meridional overturning circulation. Since the 1990s, the Atlantic has been on a warm kick, fueled by the circulation of balmy tropical waters to higher latitudes. But there’s another possibility, too: At the same time that increased circulation was ramping up ocean temperatures, the Atlantic also started getting more heat from the sun than it did a few decades ago.

This theory holds that, after World War II, American and European industries produced sulfate pollutants that floated into the atmosphere above the Atlantic. Combined with dust whipped up from arid parts of Africa, this pollution kept the ocean shaded and cool. But by the 1990s, with pollution controls in place in Europe and North America, and droughts being mitigated in North Africa, this shade was removed, and the Atlantic warmed up again. “It’s a big scientific debate, what the relative contribution of these two things is,” Vecchi says.

Whatever the root cause, though, one fact is clear: the Atlantic was warm this year. That heat gave the engines of hurricanes their fuel. But to get powerful storms, we also need to cut their brakes.

The other major factor behind this unusual season was that, in 2017, the wind patterns that normally quench hurricanes didn’t appear, thanks to La Niña-like conditions.

It’s hard to tell from satellite pictures, but the vortexes at the cores of hurricanes can tower ten miles high. They reach from the bottom to the top of the troposphere, the slice of atmosphere in which we live.

This can be a major weakness, since the tops and bottoms of hurricanes are exposed to other winds unrelated to the hurricane that blow at different speeds and in different directions. This effect, called vertical wind shear, can kick the bottom out of a hurricane’s central engine, transforming it into an unsteady Jenga tower. It blocks storms from ever reaching their potential. And in 2017, vertical wind shear in the tropical Atlantic was weak.

Blame it on conditions related to La Niña, the atmospheric phenomenon that’s the cooler counterpart to El Niño, which occurs when the Pacific Ocean warms. La Niña, though centered on the Pacific, also helps hurricanes form in the Atlantic.

Though this year wasn’t an official La Niña, it was at least similar enough to cause a long chain of consequences, Vecchi says. The Pacific was cooler than usual, and that shifted rainfall patterns which in turn changed atmospheric currents over the Atlantic—which in turn lessened the difference between winds close to the surface and winds higher up. That gave hurricanes a more stable atmosphere in which to form, allowing them to power up and maintain that power over a longer period of time.

On top of this, the La Niña conditions also cooled the upper atmosphere above the Atlantic, further boosting the temperature gap between the warm ocean and the cold atmosphere.

Working together, researchers believe these factors primed the pump for an active season. But there’s another potential reason why year’s hurricane season was so intense, and that’s climate change.

Attributing storms and seasons to climate change is tough and ongoing. But this year’s storms match the kind of hurricanes scientists expect from a warmer world.

A few things about this hurricane season stood out to James Kossin, an atmospheric scientist at NOAA’s Center for Weather and Climate. One was the astronomical amount of rainfall Harvey released over Houston. Another was how Irma intensified rapidly and then stayed extraordinarily strong longer than any previous storm.

“They are consistent with the kind of changes that we expect under climate change,” Kossin says. “It’s pretty darn likely that we’re affecting these monsters.” But that’s pretty much as far as he—or the other scientists interviewed for this piece—will go at the moment.

Right now, the National Science Foundation has opened up a few quick grants to allow researchers to delve into the specifics of the recent storms. In the following months and years, we’ll see careful studies come out, Kossins says, which will help pinpoint the role of climate change.

Some are already underway. “We’ve actually done an analysis of Harvey,” says MIT’s Emanuel. According to his team’s ongoing work, hurricane-related rainstorms over Texas that should occur once in 100 years will now happen about once every 20 years, thanks to climate change. And their frequency might increase again to once every five or six years by 2100, he says.

Hurricane Harvey caused catastrophic flooding throughout southeast Texas, including Port Arthur.

In many respects, disentangling the current role of climate change in hurricanes is a tricky business. High quality hurricane data doesn’t go back many years, researchers say, making it difficult to separate climate change from the other patterns discussed above.

But there is clear, basic science: key aspects of climate change will almost certainly lead to worse hurricanes. For every 1.8˚ F (1˚ C) the air warms, it can hold 7% more water, leading to more intense rainfall. And higher seas will lead to more dangerous storm surges, which didn’t play a big role in this year’s storms but which often contribute a sizeable share of hurricane deaths and damages.

This year can provide an important lesson, says Vecchi, the Princeton geophysicist. “As we expect hurricanes to become more intense over the coming century, this should be something we’re concerned about.”

“Here we are in 2017, in the United States, a very technologically advanced civilization and society, and we are very vulnerable to hurricanes,” he says. “We saw it.”