Counties along the U.S. shoreline that endured hurricane-strength winds from Florence in September experienced a surge in population from 1980 to 2017, with an increase of 95 people per square mile—more than double the density. Overall, Gulf and East Coast shoreline counties, those vulnerable to hurricane strikes, increased by 160 people per square mile, compared with 26 people per square mile in the rest of the mainland, over the same period.

“Coastal population and exposure growth is certainly the predominant driver of increased damage costs associated with hurricanes,” says Steve Bowen, director and meteorologist at consulting firm Aon’s Risk Solutions division.

Rising Storm Costs

The U.S. has had 40 hurricanes identified as billion-dollar disasters since 1980, using 2018 inflation-adjusted dollars, with a cumulative damage estimate of $862 billion, according to National Oceanic and Atmospheric Administration U.S. Billion-Dollar Weather and Climate Disasters analysis.

Damage by billion-dollar U.S. hurricanes Strength at U.S. landfall Category 5

Category 5 4

4 3

3 2

2 1

1 Other* $30 billion 10 1 *Includes tropical storms and Sandy

Notes: 2018 inflation-adjusted dollars; includes storms with less than $1 billion in damage at the time of the event but exceed $1 billion after adjusting for inflation

Source: NOAA National Centers for Environmental Information (NCEI) U.S. Billion-Dollar Weather and Climate Disasters (2018)

The trio of Harvey, Maria and Irma in 2017 combined to cost $268 billion, 31% of the damage since 1980, making it the most expensive year in 38 seasons. Based on Moody’s estimates, Florence caused less damage than those storms. At the upper end of the $38 billion to $50 billion damage estimate, it would rank among the biggest storms since 1980.

Location, Not Wind

Wind strength isn’t the only factor that determines how damaging a storm will be. Florence made landfall in the Carolinas as a Category 1 hurricane, due to wind strength. The devastating effect of Florence, like Harvey in 2017, was primarily related to flooding.

Florence, 2018 Category 1 at U.S. landfall People per square mile, 2017 100 50 25 0 10 200 73,000 Tropical storm- strength winds Hurricane- strength winds Storm path

Relatively weak storms are among the costliest. “Wind category does not fully capture the damage cost potential from historic hurricanes,” says Adam Smith of the National Oceanic and Atmospheric Administration. Many population centers and infrastructure exist in vulnerable areas like coasts and river floodplains, he says.

For storms like Harvey, changes in land use that come with urbanization, such as replacing permeable surfaces like grass with impermeable surfaces like concrete, can cause big changes in water runoff.

Sandy had downgraded from being a hurricane to what’s known as an “extratropical” storm by the time it made landfall in 2012, but it hit major population centers causing “extensive damage across several northeastern states,” making it the 4th most expensive storm to hit the U.S. since 1980 with $72.19 billion in damages according to NOAA’s analysis.

People per square mile, 2017 Sandy, 2012 Extratropical at U.S. landfall $72.19 billion in damage (No. 4 since 1980) 73,000 200 100 50 25 0 10 Tropical storm- strength winds Hurricane- strength winds Storm path Harvey, 2017 Category 4 at landfall $127.50 billion in damage (No. 2) Katrina, 2005 Category 3 at landfall $165.01 billion in damage (No. 1) Irma, 2017 Category 4 at landfall $50.97 billion in damage (No. 5) People per square mile, 2017 Sandy, 2012 Extratropical at U.S. landfall $72.19 billion in damage (No. 4 since 1980) 0 10 50 100 200 73,000 25 Tropical storm- strength winds Storm path Hurricane- strength winds Harvey, 2017 Category 4 $127.50 billion (No. 2) Irma, 2017 Category 4 $50.97 billion (No. 5) Katrina, 2005 Category 3 $165.01 billion (No. 1) People per square mile, 2017 Sandy, 2012 Extratropical at U.S. landfall $72.19 billion in damage (No. 4 since 1980) 0 200 25 100 50 73,000 10 Tropical storm- strength winds Hurricane- strength winds Storm path Harvey, 2017 Category 4 $127.50 billion (No. 2) Katrina, 2005 Category 3 $165.01 billion (No. 1) Irma, 2017 Category 4 $50.97 billion (No. 5) People per square mile, 2017 10 0 25 50 100 73,000 200 Sandy, 2012 Extratropical at landfall $72.19 billion in damage (No. 4 since 1980) Tropical storm- strength winds Storm path Hurricane- strength winds Katrina, 2005 Category 3 $165.01 billion (No. 1) Harvey, 2017 Category 4 $127.50 billion (No. 2) Irma, 2017 Category 4 $50.97 billion (No. 5)

Slower and Wetter

Another contributing factor to the severity of Florence was the fact that the storm lingered over the area so long, drenching eastern North Carolina.

The speed North Atlantic hurricanes cross over land slowed 17% from 1949 to 2016, says Jim Kossin, atmospheric scientist at NOAA. Mr. Kossin published a peer-reviewed paper on the subject in Nature in June. All things being equal, if you slow a storm’s forward speed by 17%, you will increase the rain in the area by 17%, resulting in substantially more freshwater flooding, he says.

Homes at Risk

“As we have more people living in hurricane-prone areas, the damage is likely to increase,” says Phil Klotzbach, a research scientist in the atmospheric science department at Colorado State University.

Along the Atlantic and Gulf coasts, 6.9 million homes are at risk for hurricane storm surge damage, with 67% located in 15 metropolitan areas, according to CoreLogic. The reconstruction cost value—the cost to completely rebuild at-risk homes in a 100% destruction scenario in 2018, including labor and materials—is $1.6 trillion.