Those are the findings of a comprehensive new analysis by the First Street Foundation
, a nonprofit research and technology group that experts say has put together the fullest picture yet of the country's growing vulnerability to flooding.
Today, around 8.7 million properties are located in Special Flood Hazard Areas as determined by FEMA's flood maps, the legal standard used in the US to manage floodplains, determine insurance requirements and price policy premiums.
But as many as 14.6 million properties -- nearly 70% more than are in FEMA's Special Flood Hazard Areas -- may actually be at significant risk of flooding, according to First Street's modeling. The discrepancy between FEMA's maps and this new data means that some 6 million property owners could be unaware of their current flood risk, the group says.
"If you're a homeowner, renter or buyer in this country and you want to understand flood risk, the only data that's available to you are the FEMA flood maps," said Matthew Eby, First Street's
founder and executive director. "And the FEMA flood maps are made to determine flood insurance rates -- not necessarily to determine what your personal flood risk is."
As climate change raises sea levels
and leads to more extreme rainfall events
, even more homeowners will have to contend with the threat of flooding, well within the life of a 30-year mortgage signed today.
First Street found that by 2050, the number of properties at significant risk of flooding is expected to climb even further to 16.2 million.
The issue is already growing in urgency, as huge swaths of the country have been inundated by devastating floods in recent years.
Flooding is now the most common and costly natural disaster in the US
, causing some $155 billion in property damages in the last decade, according to Michael Grimm, the assistant administrator for risk management for the Federal Insurance and Mitigation Administration, who testified before the House Science Committee in February.
Last year, many parts of the Midwest and South were swamped by flood waters that lingered for months and caused $6.2 billion in damages and four deaths
, according to the National Oceanic and Atmospheric Administration (NOAA).
In 2017, Hurricane Harvey, a powerful Category 4 storm, dumped a record-breaking 51 inches of rain on parts of Texas
, putting large parts of Houston underwater. But about half of the homes in Houston affected by flooding were outside mapped floodplains, according to a National Academy of Sciences report on urban flooding.
Right now, First Street's flood models show that the states with the greatest proportion of properties currently at substantial risk are West Virginia, Louisiana, Florida, Idaho and Montana.
In the future, changing precipitation patterns will continue to cause problems for many inland towns and cities.
But as the climate changes, the combined threats of storm surge from hurricanes and sea level rise means flooding will be most dramatic in coastal areas, First Street found.
By 2050, nearly 98% of the properties in New Orleans could be at substantial risk of flooding
. South Florida will also see major impacts, with Miami and Tampa both projected to see more than half of all properties at risk of flooding by 2050, according to the First Street model.