If climate change goes unchecked, sea levels in the United States could rise as much as 23 feet, affecting 18 million people in hundreds of cities along the nation’s coasts. According to the nonprofit climate research group Climate Central, there is a real chance that some of the largest urban areas could be almost entirely submerged by the rising ocean levels caused by global warming.

Climate Central’s report estimated rising sea levels for thousands of municipalities around the country. In New Orleans, it found that there is at least a 50% chance that water levels will rise five feet by 2030, submerging 90% of the area’s homes and displacing more than 300,000 residents. Based on Climate Central’s data, these are the seven cities at risk of rising seas.

Click here to see what cities are at risk

24/7 Wall St. considered the seven large metropolitan areas in which there is a chance that at least a fifth of all homes could be submerged in the next 40 years. These cities appear to be particularly at risk because they are largely in low-lying areas next to large bodies of war connected to the ocean.

In addition, large residential or commercial areas in these cities are five feet or less above sea level. St. Petersburg, Fla., for example, is located on the Tampa Bay peninsula in the Gulf Coast. Its highest point is only 61 feet above sea level. In Metairie, La., the highest point is less than 20 feet above sea level.

Several of these at-risk areas have neighborhoods located so close to — or even below — sea level that they are protected by flood levees, which can fail in poor conditions. These risks were made apparent in New Orleans during Hurricane Katrina. Stockton, Calif., located in the San Joaquin Valley, is also protected by levees.

The decade when these cities are most at risk varies widely. New Orleans and Metairie in Louisiana could see dramatic effects — with well more than half the residents being displaced — in the next few decades. For other areas — St. Petersburg and Hollywood in Florida or Stockton and Huntington Beach in California — extreme flooding isn’t a real possibility until closer to the end of the century.

In order to identify the cities that are at risk, 24/7 Wall St. reviewed sea level rise estimates provided by Climate Central for the 200 largest cities in the country by population. The report calculates the odds that by a particular decade, waters will at some point reach a certain height based on a combination of rising ocean levels, storm surge, and tides. To make our list, a city had at least a one-in-six chance of at least 20% of housing units threatened by rising waters by 2050. Estimates of current population and housing units at risk are based on 2010 data from the U.S. Census Bureau. Figures in bullets represent the number of homes and population at risk based on 1-in-6 odds according to the study.

These are seven cities at risk of rising seas.

1. Metairie, La.
> Population at risk: 138,399 (est. 2040)
> Homes at risk: 65,649 (est. 2040)
> Pct. land area at risk: 100% (est. 2040)

According to recent measurements and climate projections, which indicate an exponential growth in rising sea levels, the water levels in Metairie have at least one-in-two odds of rising three feet by 2020. This means nearly the entire current population in this region — 98% of 138,399 people — would be under water, along with 64,339 homes. Climate Central projects at least one-in-six odds for the water level to rise by 10 feet by 2040, which would leave the entire region submerged. The alarming projections for this city are not surprising given its extremely low elevation. Large parts of the city are below sea level. The city already has seen the effects of its low-lying population: the devastating effects of Hurricane Katrina are still fresh on the minds of Metairie residents.

Also Read: Nine Cities Running Out of Water

2. Hollywood, Fla.
> Population at risk: 41,709 (est. 2050)
> Homes at risk: 23,638 (est. 2050)
> Pct. land area at risk: 40% (est. 2050)

Research conducted by Climate Central suggests there is at least a 50% chance water levels will rise by one foot in Hollywood by 2020. In this event, 4% of the current population — about 5,000 people — would experience flooding. Longer term projections indicate at least one-in-six odds that by 2050 the sea level will increase by four feet, displacing about 30% of the city’s residents, or 41,709 people. With the presence of floodplains in Hollywood, as well as Florida’s below-average elevations, there are safeguards in place for residents. For instance, because of the high risk of floods and the importance of having flood insurance for area residents, the National Flood Insurance program offers a 10% reduction for all policyholders in Hollywood.

3. Huntington Beach, Calif.
> Population at risk: 34,551 (est. 2020)
> Homes at risk: 15,286 (est. 2020)
> Pct. land area at risk: 19% (est. 2020)

According to Climate Central, there are at least one-in-two odds that by 2020 34,551 people will be displaced as a result of flooding after the water level in Huntington Beach City rises by three feet. About 19% of land in this city is below that elevation, containing more than 15,000 housing units. The city is 28 feet above sea level on average, but to the southeast the elevation is below five feet in large areas. In some parts of Orange county there are wetlands where sea level is measured at below zero feet. In addition, the city is located near to the Pacific Ocean, making it particularly vulnerable to flooding and rising sea levels.

4. Hialeah, Fla.
> Population at risk: 159,343 (est. 2050)
> Homes at risk: 52,379 (est. 2050)
> Pct. land area at risk: 72% (est. 2050)

Hialeah is one of three cities in Florida that rank among the most at risk in the nation if sea levels rise. However, the odds of sea levels rising four feet — the minimum amount needed to affect residents — is low in the next few decades. However, in the long run, the risks Hialeah faces could affect a sizable portion of the population. There are at least one-in-six odds that the water elevation in the city will rise four feet by 2050. If this were to happen, the area where 71% of the city’s population lives would be below water.

5. Stockton, Calif.
> Population at risk: 59,034 (est. 2030)
> Homes at risk: 21,567 (est. 2030)
> Pct. land area at risk: 30% (est. 2030)

Although Stockton is fairly far inland, the city remains one of the nation’s most at risk for flooding in the coming decades. The city is located on the Sacramento-San Joaquin River Delta, an important source of water for much of California. According to the American Planning Association, the city “lies so low that ocean tides influence a water channel in the middle of downtown.” In the same 2012 report on the delta, the association adds “there is a reason why the [area’s] land is cheap: The real estate is flood-prone and the weak levees are in need of expensive repairs.” According to Climate Central, there are at least one-in-two odds that Stockton will experience three feet of flooding by 2020. This would affect 17% of the population, or just over 48,000 people, as well as more than one-quarter of the city’s total land.

Also Read: State Where It Is Hardest To Find Full-Time Work

6. New Orleans, La.
> Population at risk: 342,179 (est. 2040)
> Homes at risk: 188,979 (est. 2040)
> Pct. land area at risk: 98% (est. 2040)

Flooding became a tragic reality in New Orleans in 2005, when Hurricane Katrina struck the city. Following the failure of nearby levees and floodwalls, parts of New Orleans were left under 20 feet of water. The city may face one of the most difficult futures of any in the nation in the coming years. If water levels rise by just one foot, 72% of the city’s population will be affected. If water levels rise by three feet, 83% of the city’s current population will be displaced. According to Climate Central, the odds are at least one-in-two that the water will rise three feet by 2020, and at least one-in-six that it will rise seven feet by then.

7. St. Petersburg, Fla.
> Population at risk: 91,451 (est. 2050)
> Homes at risk: 51,941 (est. 2050)
> Pct. land area at risk: 41% (est. 2050)

St. Petersburg is part of the greater Tampa Bay area on the Gulf Coast of Florida. The city is located at the end of the peninsula that helps make up the Tampa Bay metro area. According to Climate Central’s estimates, there is a 50% chance that the sea level surrounding the city will rise three feet by 2020. This would flood 15% of homes and displace more than 32,000 residents. The odds are at least one-in-six that waters will rise 10 feet by 2070, which would affect 44% of the population and cover nearly half of the city. Earlier this summer, the city joined dozens of others in a plea to President Barack Obama to enact meaningful policy on climate change. “St. Pete is frighteningly vulnerable to rising seas, stronger storms and other symptoms of climate chaos,” said Andy Bell, executive director of local nonprofit Suncoast Interfaith Power and Light.

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