Penn StateNASA



Like many Americans, I used to dream of owning coastal property. I have experienced hurricanes - I went to help friends recover from Andrew in Miami in 1992, and I lived through Hurricane Fran in NC in 1996, so I’ve seen the destruction they can cause. But I still maintained my dream. Every time I go to the beach, I pick up real estate brochures; on a cold, snowy weekend, I would look at coastal properties on Zillow. But my dream is being replaced by practicality. Coastal property is a lousy investment in the 21st century!

Miami skyline
Development in downtown Miami

Harvey, Irma, Maria, three massive hurricanes in three weeks! The big question is whether 2017 was just an unusually active year, or if these monster storms are a new normal, the grand result of a warming planet. Was the lull in massive hurricanes between 2005 and 2017 significant, or was it just luck?

Each of these storms had some incredible elements. The eye of Harvey roared ashore in south Texas more than a hundred miles south of the booming metropolis of Houston, the fourth largest city in the US with a population of close to 6 million people. The storm moved north towards the city and literally parked there for 4 or 5 days drawing moisture in from the warm Gulf of Mexico. By the time the storm moved on to the north, parts of Houston had received close to 52 inches of rain. Harvey dumped a grand total of 33 trillion gallons of water on Houston and points north, causing catastrophic flooding. Just for context, 33 trillion gallons would fill cube with sides of 3 miles, it’s a massive amount of water! Several small creeks north of the city were over 19 feet over their banks. The storm was the single largest rain event in the lower 48 states of the US ever! The total damage, still rising as I write this, is estimated to be about $150 billion, again one of the largest ever catastrophes. Harvey caused 82 deaths in the US.

At the outset, Houston is a very flood-prone city. Houston lies on a flat plain near the ocean. The natural landscape is grassland that is drained by creeks called bayou. The city lies very close to the Gulf of Mexico, which gets very warm in the late summer, close to 87 degrees F! This warmth is transferred to the air keeping coastal areas warm and humid. The Gulf is an enormous heat and water factory. A key fact to understand is that for every one degree of temperature increase, an air mass can hold 3 % more moisture, so as the Gulf has warmed over recent years it contributes more energy and rain to hurricanes making them most intense and much wetter.

Aerial view of Houston development
Aerial view of Houston development
Houston Development
Credit: Tim Bralower

Houston might have been able to absorb this change in its natural state, but the city has grown very rapidly over the last two decades as industry has surged and jobs have been plentiful. The city prides itself on being business-friendly and as a result, it has no zoning, meaning that there are few limits on construction. A shopping mall or factory can be located right next to a housing development or a city park. As a result, the city has a massive amount of concrete, roads, parking lots and rooftops with very little consideration paid to drainage. This contributed in a major way to flooding during Harvey as the photographs below testify. Harvey was a 1000-year event, meaning the levels of rainfall are only expected that rarely. But as it turns out, the storm was the third 500-year event in the last three years. So, it is clear that it is part of a new normal.

Satellite image Hurricane Harvey
Satellite view of Hurricane Harvey
Credit: ABI image captured by NOAA’s GOES-16 satellite (RAMMB/CIRA SLIDER) [Public domain], via Wikimedia Commons
Flooding in Houston during Hurricane Harvey
Flooding in Houston during Hurricane Harvey
Credit: SC National Guard. Flickr [Public domain], via Wikimedia Commons
Video: Hurricane Floods: A city under water Hurricane Harvey (19:22)
Please note: This video contains offensive language that we do not condone. 
Click for the video transcript.
Please note that the following transcript contains offensive language the we do not condone.




SPEAKER 1: So guys, this is what I came out here to see. It may not look like it, but this is Highway 288. At this point it's about six--

SPEAKER 2: Still here. Ain't going nowhere. Yeah.

SPEAKER 3: I think we're a whole [INAUDIBLE]

SPEAKER 1: First time in Houston and this is what I get. This is what's going on in Houston, Texas right now. Instagram. Instagram. Plane just sitting there. Can't go nowhere.

SPEAKER 4: Oh, sh*t!



SPEAKER 2: All right. This is Highway 288, Old Spanish Trail. Man, that sh*t crazy. Tropical storm Harvey making his mark. That's the interstate right there. And that got to be at least 15, 20 feet of water. 288 way down there.


It's coming back up now.


SPEAKER 6: What's that n****r sh*it, man. F**k.

SPEAKER 5: No, I got it where it ain't going nowhere. The boat can sit right here for a minute. Here, you all wanted the boat out. The boat is out.

SPEAKER 7: How you all doing?

SPEAKER 8: All right.


SPEAKER 7: This is insane. Never seen this bayou like this. We in some serious trouble if this thing go over. Snakes and stuff, probably all in these people yards now.

SPEAKER 10: Saint Augustine and Porter, man. Look at this hill. The truck can't even get through.


SPEAKER 5: This mo**erfu**er don't understand English.

SPEAKER 11: Ain't no way I have my [INAUDIBLE] in that mo**erfu**er.

SPEAKER 5: [INAUDIBLE] Fu** this sh*t, though.




SPEAKER 1: Don't know what it's like for you all, but right now, this is the current situation. I moved my car to the other side of the complex where there's less water. But yeah, the water is almost up to the doorstep. But, you know, I hope everybody else is doing all right.

SPEAKER 12: And this is a tornado warning until 10:30. If you are in the path, if you're getting alerts on your mobiles phone, this is the time to get inside and just stay put.



Oh my God!


SPEAKER 14: Girl, what the hell? I'm done.

SPEAKER 15: Here comes this truck.

SPEAKER 16: That's the SS Dumpster.

SPEAKER 17: There's the dumpster floating down the road.

SPEAKER 18: [INAUDIBLE] hard. The water's [INAUDIBLE]

SPEAKER 19: Snake out here. [INAUDIBLE] I can see my feet.


Irma arose rapidly in the tropical Atlantic and at one point it had sustained wind speeds of 185 mph. This storm intensified rapidly, which is a characteristic of a large hurricane, increasing by 45 mph in one day. hit the islands of Barbuda, Antigua, St Martin and St. Barthelemy and caused catastrophic damage in these locations. The island of Barbuda, in particular, was literally flattened. Irma next set her eyes on Cuba where it came ashore as a magnitude 5 storm with sustained winds of 160 mph and a massive storm surge. The storm led to collapsed buildings and flooding of coastal areas including the historic Malecón in Havana. Fortunately, Cuban authorities had evacuated close to a million people from low-lying areas.

Damage from Hurricane Irma on island of St Martin
Damage from Irma on the island of St Martin
Credit: Ministry of Defense, Netherlands [CC0 1.0], via Wikimedia Commons

A few kilometers makes a major difference in the history of a hurricane and the landfall in Cuba, which was not initially predicted, weakened the storm significantly. Initial forecasts were for Irma to come ashore near Miami with wind speeds near 155 mph, but the storm tracked a little further to the west and made landfall in the Florida Keys with maximum winds of 130 mph, then again in Marco Island on the west coast of Florida with winds of 115 mph. As it turns out, there have been far larger storms in Florida including Hurricane Andrew in 1992, which flattened the Miami suburbs. But Irma was yet another reminder of how vulnerable the state is to storms. Much of the southern half of Florida is a natural swamp or marshland that originally looked like the Everglades National Park. But as in Houston, commercial interests, and in the case of Florida, the desire of citizens to own a small piece of paradise, have led to massive construction in the last decade, runaway development with insufficient environmental regulation. So, instead of swampland that served to absorb moisture and drain it back towards the ocean, large expanses of concrete funnel it into walls of water in cities and suburbs. Mangroves forest that previously protected the coast has been flattened. In fact, the Florida environment was destroyed a long time before this as the Army Corps of Engineers modified the natural drainage to provide water for the sugar industry.

sawgrass prairie in the Everglades
Sawgrass prairie in the Everglades
Landsat image of everglades showing development in Miami and Fort Lauderdale
Landsat image of the Everglades showing development in Miami and Fort Lauderdale in upper right
Credit: USGS [Public domain], via Wikimedia Commons

As it turns out, Irma was less of a wind event than a storm surge event. As a hurricane moves towards land, it pushes water ahead of it, literally a wall of water. The result is storm surge. Hurricane Katrina in 2005 was also a storm surge event, with a surge of 28 feet measured at Pass Christian, Mississippi, just outside of New Orleans, the largest surge ever measured in a US hurricane. New Orleans is at or even below sea level and its levee system, upgraded after Katrina, is designed to deal with surge, but still, it remains highly vulnerable. The surge from Irma was about 10 feet in the Keys and Marco Island, enough to cause significant damage. Nevertheless, the storm was generally viewed by experts as another wake-up call to what will likely happen in the future if a major storm with winds over 160 miles an hour and a 20-foot storm surge hits Miami or Tampa which is extremely flood-prone. In all, Irma caused over 100 fatalities, most of which were in the Caribbean.

Damaged boats alongside the road
Boats across US 1 from their usual spots in a Big Pine Key marina.
Credit: Dan Chapman, USFWS, Wikimedia Commons (Public Domain)

Barely a week later, monster storm Maria developed as Irma had…this storm also developed by rapid intensification with an increase in wind speed of over 60 mph in one day! By the time it hit Dominica, Maria had sustained winds of 160 mph and it caused utter devastation and killed 15 people, then it took aim at Puerto Rico. The storm hit the east coast of the island with sustained winds of 155 mph and dumped up to 3 feet of rain in mountainous areas. The impact on Puerto Rico is like the combined effect of Harvey in Houston and Irma in Barbuda. Maria caused massive destruction on the island. The power grid was destroyed leaving all 3.4 million residents without electricity. Many people had no running water for days, and sewers and cell phone networks were also out. Dams were in danger of breaching. 60,000 homes lost most or all of their roofs and only 392 out of 5000 miles of roads remained open. The storm defoliated a large number of trees on the island and led to the loss of 80 percent of the agriculture. The total damage is estimated at $90 billion, but that does not include the misery the storm caused humans. Diseases spread due to the lack of clean drinking water. The water-borne bacterial infection leptospirosis was widespread. Overall, the storm directly or indirectly led to approximately 500 deaths.

Damage from Maria on Puerto Rico
Damage from Maria on Puerto Rico
Deforestation in Puerto Rico
Deforestation in Puerto Rico as a result of Hurricane Maria
Aerial view of Roseau, showing widespread damage to roofs
An aerial view of part of Roseau (capital of Dominica) revealing widespread damage to roofs. Flash floods clogged roads with debris - vegetative and structural - and mud.
A road in the Roseau area littered with debris
A road in the Roseau area is littered with structural debris, damaged vegetation and downed power poles and lines.

So, finally, we come back to the question of whether climate change is responsible for the surge in powerful hurricanes in 2017. At the outset, we must stress that this question cannot be answered unequivocally. However, there are several factors that make it safe to say that large storms will be more common in the future and that they will cause increasing amounts of damage. To develop, storms need warm temperatures (over 80 degrees), abundant moisture, and circulation as you can see in the video below.

Video: Fuel for the Storm (2:19)

Click for the video transcript.
We’ve all heard that hurricanes are one of the most powerful and destructive forces on Earth. But did you ever wonder where they get their strength? The formation of a hurricane is complicated, but basically, it depends on 3 factors: First, you need warm water, at least 80 degrees. The second ingredient is moist air. And finally, there need to be converging winds for a hurricane to form. The actual process begins with a cluster of thunderstorms moving across the surface of the ocean. When the surface water is warm, the storm sucks up heat energy from the water, just like a straw sucks up a liquid. This creates moisture in the air. If wind conditions are right, the storm becomes a hurricane. This heat energy is the fuel for the storm. And the warmer the water, the more moisture is in the air. And that could mean bigger and stronger hurricanes. Satellite data shows the heat and energy transfer in action. Notice how this hurricane leaves a trail of cooler water behind. Scientists use sea surface temperature data from satellites to help forecast the intensity of storms. Hurricane Katrina, which was the third largest to make landfall in the U.S., crossed over Gulf waters that had temperatures between two and three degrees higher than normal. This spawned sustained winds of over 140mph, extending 100 miles from the eye of the storm. And with greater intensity, there’s a higher chance of death and destruction. This is why warming ocean temperatures matter; it’s like adding fuel to a fire and taking the world, literally, by storm.

As we will see in the lab at the end of this module, the ocean has warmed by 1 to 2 degrees C (3-4 degrees F) over the last century, and this leads to a 12-16 percent increase in moisture (3% per degree). Thus there is a lot more fuel for hurricane development. The formation of hurricanes is also helped by weather disturbances off West Africa, but there is not yet a relationship between these events and climate change.

In the case of Harvey, the volume of water is clearly a result of an extra warm ocean; for Irma and Maria, the ferocity of the winds and the rapid intensification is also related to water temperature. So, climate change is adding fuel to the fire for large hurricane development and 2017 is a harbinger of things to come. There is one other factor to consider, perhaps one that will prove the most devastating in decades to come. Sea level rise. The ocean is now about a foot higher than it was in 1900. Projections are for a possible 6-foot rise in sea level by the end of this century if we don’t cut greenhouse gas levels significantly. We will discuss the issue of sea level rise in great detail in Module 10. Such a rise would mean that even a storm such as Irma with a moderate storm surge would be catastrophic.