We asked 5 hurricane experts: Why were there so many hurricanes this season?

This is one of the most incredible images of 2017. It’s a satellite from the National Oceanic and Atmospheric Administration peering down on Hurricanes Katia and Irma and Tropical Storm Jose (from left to right) on September 8.


It’s an image that sums up what a punishing Atlantic hurricane season it has been. From late August to early October, it seemed that just as one storm was barreling west, another was spinning up right behind it.

Irma, the center storm in this image, broke a new record for hurricane intensity by sustaining 185 mph winds for 36 hours. In the days after this image was taken, Jose would obtain major hurricane status (Category 3), marking the first time on record where two Atlantic storms had 150 mph winds at the same time. Just a week later, Hurricane Maria would form and eventually knock out Puerto Rico’s power grid, setting off a terrible ongoing humanitarian disaster.

And the toll of these storms has been deadly and costly. We still don’t have an accurate death toll from Maria’s destructive path through Puerto Rico, a storm that could end up costing the cash-strapped island around $95 billion. Hurricanes Harvey and Irma are estimated to have caused $200 billion in damage. It’s been 12 years since storms of this intensity have made landfall in the US. Their return this year is, foremost, a reminder: We’re vulnerable.

Now that the Atlantic has quieted down and there are no new storms to track, I wanted to know: What the hell just happened?

Is it normal to have so many strong storms in a row? And how should we think about this season in relation to climate change? To take stock, I called up several climate and hurricane experts.

This season broke records

All the rain that Hurricane Harvey dumped on Texas and Louisiana, in one massive water drop. Javier Zarracina/ Vox

Technically, hurricane season ends on November 30, so more storms are possible in the coming weeks. Regardless, 2017 has been a very active year.

There are a lot of ways to measure the severity of a hurricane season: You can look at the number of storms, the number of days with at least one active hurricane, and a measure called “accumulated cyclone energy” (a measure of wind speeds accumulating over time). On all the measures, 2017 is trending near the top, explains Philip Klotzbach, an atmospheric scientist and hurricane expert at Colorado State University.

Since the beginning of the season, Klotzbach has been obsessively keeping track of the storms in a way, he says, that straddles the line of work and hobby. And he’s compiled a lot of records.

Here are a few, by his count, as of October 15:

  • 2017 ranks among the highest in terms of number of major hurricanes, with six achieving Category 3 (111-plus mph winds) or higher. The record since record-keeping began in 1851 is seven. (Though keep in mind we’ve only been tracking hurricanes with satellites since the 1970s, so the early records likely aren’t very comprehensive.)
  • 2017 had 19.25 days with a major named storm in the Atlantic, the sixth-highest number of such days.
  • 2017 is the seventh highest for accumulated cyclone energy in the Atlantic.
  • Here’s the entire record sheet compiled by Klotzbach.

Phllip Klotzbach

Why was this season so active? It’s complicated.

So what happened? Why did we see storm after storm after storm?

The experts I spoke to all said there’s no simple explanation for the number of hurricanes that can form in a given year. But “it’s a lot more nuanced than to say you warm the water and you get a lot more storms,” Klotzbach says.

Yes, warm waters are a key ingredient to fuel storms. But hurricanes are also influenced and steered by massive global trends in weather that are hard to predict: The warming or cooling of waters in the Pacific (El Niño and La Niña) and patterns like the Madden-Julian oscillation (an eastward-moving weather system that circles the globe every month or so and makes thunderstorms more likely) all play a role.

This year, the Atlantic was warmer than average — Klotzbach says August through October will likely rank third or fourth in terms of highest tropical Atlantic Ocean temperatures.

But that’s not enough to create an active year for storms. What also happened: The tropical Pacific was cooler than usual. When the Pacific is cool, that’s when “you tend to get a lot more activity in the Atlantic,” explains Princeton climate scientist Gabriel Vecchi. Those cooler Pacific waters help cool the upper atmosphere.

Big temperature differences between the surface and the upper atmosphere are another ingredient needed to form hurricanes. Temperature gradients make the atmosphere more unstable, and “a more unstable atmosphere is more conducive for deep thunderstorm formation, which is the building blocks of hurricanes,” Klotzbach says. In May, the NOAA predicted the 2017 hurricane season would be above normal, citing cooling waters in the Pacific and predicting two to four major hurricanes.

Finally, there was low wind shear throughout the season. Wind shear is the sudden changes in wind direction as you get higher and higher in the atmosphere. High wind shear tends to blow the tops off storms and saps them of their strength. “[Low wind shear] is the reason why storms like Irma were able to maintain 180 mph intensity for a day and a half,” Klotzbach says.

Naturally, there are years where hurricane activity is intense and years where it remits. In 2005 there were so many storms that forecasters had to dip into the Greek alphabet for names. But then wasn’t another major hurricane landfall in the United States for 12 years. (Hurricane Harvey broke the streak.)

But the “drought” we’ve experienced since 2005 is a bit of a misnomer. Hurricanes aren’t just an Atlantic phenomenon. Tropical cyclones — which are called typhoons, cyclones, or hurricanes depending on where you are — form in tropical regions the world over. Typhoon Haiyan, one of the strongest storms ever recorded on Earth, hit the Philippines in 2013, killing 6,300 people, during the so-called drought. And Cuba was hit by major hurricanes four times from 2006 to 2016, Klotzbach reminds.

Bottom line: Hurricanes are extremely complicated. No one variable causes them. They are the result of the impossibly complicated patterns of global weather. 2017 was a strong season due to warm Atlantic waters, a cooler upper atmosphere, and nothing in the weather to blow the storms apart or steer them away from our shores.

Was it climate change? It’s complicated.

Photo of the destruction of Hurricane Maria in Puerto Rico. Ricardo Arduengo/AFP/Getty Images

One of the thing that made the 2017 season feel like a sign of climate change was that Harvey, Irma, and Maria all made landfall as powerful Category 4 storms with winds exceeding 130 mph. And Harvey, in particular, dumped a truly staggering amount of rain over Houston. These are the types of storms climate scientists to expect to see more of in a warmer world.

It’s prompted several journalists to ask if the very powerful, deadly hurricanes this year represent a “new normal” in the age of global warming. Some even insisted that we call them “climate disasters.”

“Make no mistake: These storms weren’t natural,” meteorologist Eric Holthaus wrote for Grist in September. “A warmer, more violent atmosphere — heated up by our collective desire to ignore the fact that we live on a planet where such devastation is possible — juiced Harvey and Irma’s destruction.”

But the story linking climate change to any current storms is not so simple.

“When we get a particularly bad storm, people often try and attribute it to something larger,” Jennifer Collins, a hurricane researcher at the University of South Florida, says. “In the past, El Niño was often to blame. These days, more often people ask about a climate change connection. Attributing a single storm to anything, whether it is climate change or something else, is difficult.”

First off, yes: There’s consensus that the science of climate change predicts that in a warming world, hurricanes will become more intense, carry more rain, and cause worse coastal flooding linked in part to sea level rise.

But here the thing: We don’t yet currently know, conclusively, that the storms we saw were made larger and nastier by climate change. “At this point it’s really uncertain if there’s any detectable human influence on any hurricane or tropical cyclone metric,” Tom Knutson, an NOAA meteorologist who studies hurricanes, says. There’s just not enough data.

Vecchi admits, “We’re in an uncomfortable position, because it is not clear we should be able to see [increasing hurricane strength] in the data yet.”

Sure, the NOAA’s National Hurricane Center has airplanes that fly through storms to obtain hour-by-hour barometric pressure and wind speed data. But all this technology is relatively new. Meteorologists have only been tracking hurricanes with satellites since the 1970. It’s possible that historic hurricane records, which go back to the 1800s, are incomplete or have inaccurate information on wind speeds and size. Considering how hurricanes have been lashing against the Atlantic’s coasts for untold epochs, we just have a tiny slice of data to determine what’s “normal.”

We have much better — and more conclusive — evidence for climate change from more boring sources like global temperature averages, or the extent of global sea ice, or thousands of years’ worth of C02 levels stored frozen in ice cores. “The more volatile the phenomenon, the harder it is to detect trends,” Klotzbach says. And hurricanes are an extremely volatile phenomenon.

Hurricanes are certainly influenced by global temperatures. But it’s such a complicated system it’s hard to see that unfolding in real time. Instead, scientists will need months to complete in-depth “attribution studies” to tease out climate change’s influence on these storms.

That isn’t to say there aren’t some clues that climate change is currently making storms more intense. “Studies have also noted that in a warmer environment, we should see more storms which undergo rapid intensification,” Collins says. “We have seen such rapid intensifications with Hurricane Wilma, Rita, Katrina, Patricia, and Harvey.”

But those clues don’t add up to a slam-dunk case.

All the climate scientists I spoke to for this story agreed on this point. “We can’t say with 100 percent [certainty] yet that there was influence in these specific storms from climate change,” says Suzana Camargo, who studies extreme weather at Columbia University.

What they can and will do is run model simulations. Scientists will run a model of the storm but adjust for climate change–derived changes in CO2. From those models, they can make estimates about whether a storm’s wind or rain output was influenced by climate change.

“In the case of Hurricane Harvey, we had this huge amount of precipitation,” Camargo says. “When people do studies on Harvey, my expectation would be that part of it will be a slight contribution from climate change.”

Don’t think of this hurricane season as a “new normal.” Think of it as a new baseline.

Vecchi offers a different way to frame the story on climate change and hurricanes that I find very helpful.

Think of the storms we saw this year not as the “new normal” but as the baseline. “Our [current] vulnerability to these hurricanes really highlights to me the seriousness to which we should look at the predictions for the future,” he says.

If things can be so bad now, just think of what the future might hold.

Because even if the predictions don’t turn out to be correct — if hurricanes don’t grow larger and carry more rain — if there’s sea level rise due to climate change, these storms become more devastating. Storm surge — the mass of water hurricanes push onshore from the ocean — will have a higher bed of water that will rush farther inland and destroy more property.

“There’s also a really strong connection between temperature of air and its ability to hold water,” Vecchi says. “As the planet warms, we have a good basis to think that the extreme rainfall should increase.”

Hurricanes are one of the biggest, baddest, most in-your-face weather events we have. And so there are compelling reasons to use fierce ones to tell a story about climate change.

But I worry that using them to herald the coming of a new, more dangerous world will backfire on climate activists. Even in a steadily warming world, we’re likely to see many periods of hurricane “drought.” What happens to the warnings then?

“You want to keep your eye on the real ball,” Vecchi says. “When we have such strong evidence [for climate change], when there are things that we understand so well, why are we going to build a case around something that is a little more tenuous?”

What’s not complicated: more strong hurricanes are coming

Why Floridians should not be surprised they’re at risk for hurricanes, in one chart. NOAA

We don’t need the threat of climate change to know that hurricanes are a dangerous threat. They’ve inflicted grave damage on coastal communities for as long as we’ve had them.

On the 1940s and into the ’50s, South Florida was hit by five Category 4 or 5 hurricanes in six years. And yet since then, the South Florida coastline has only grown more densely populated, reaching a population of 6 million in 2016. And Tampa, on the Gulf side of Florida, is one of the fastest-growing cities in the country. The greatest threat is not from climate change but from the hubris to build so much in regions where we know hurricanes will continue to land.

“Islands in the Caribbean are very vulnerable to a strong hurricane, and Florida is vulnerable to a hurricane when one hits, and Houston is really vulnerable to flooding,” says Rebecca Morss, who studies the sociological aspects of extreme weather at the National Center for Atmospheric Research. “It’s just a matter of is the extreme rainfall going to happen now, or in 10 years? Hurricanes are going to come.” And that risk is hard for people to grapple with. Sure, in any given year, your personal risk of being hit by a hurricane is low. But if you’re living in a house on the coast for 30-plus years, you just might be impacted.

This year’s brutal hurricane season has given us a multitude of opportunities to reduce risks for the next inevitable brutal season, whenever it comes. We can rebuild Puerto Rico’s electrical system to be more resilient to extreme weather. Houston can grapple with its land use policies that have made it so flood-prone. South Florida can reconsider its trajectory to continue building more and more expensive real estate. We can talk more seriously about strategic retreat from our most vulnerable coastal areas. We can come up with a better long-term plan to pay for the damages from severe storms.

Here’s one positive note to end on. The climate and weather experts I spoke to all agreed: Hurricane forecasting keeps getting better year over year. Today, forecasts 120 hours out are more accurate than the 48-hour forecasts were in the 1970s. The data isn’t yet in for 2017, but it’s looking like the forecast tracks were pretty accurate.

This year, forecasters were able to warn days in advance that Hurricane Irma was going to turn toward Florida. This allowed emergency managers and leaders to make critical decisions around evacuation orders and readying supplies that undoubtedly saved many lives.

So more hurricanes are coming. They’ll likely grow bigger and more intense. And we’ll see them coming. But we need to start preparing now.