Coastal US cities are sinking as sea levels continue to rise, new research shows

Several coastal cities around the United States are "disappearing" into the ground, according to new research, which could further exacerbate complications of sea level rise in the near future.

A considerable amount of land in 32 U.S. coastal cities could be at risk of flooding by 2050 due to subsidence, the gradual caving in or sinking of an area of land, according to a paper published Wednesday in Nature.

The continuous loss of land will affect most coastal cities, Leonard Ohenhen, a Ph.D. candidate at Virginia Tech University specializing in coastal vulnerability and large-scale land subsidence, told ABC News.

Large cities surrounded by water -- such as Boston, New Orleans and San Francisco -- will be among the regions that could experience flooding in the near future due to land elevation changes combined with sea level rise -- about 4 millimeters per year, said Ohenhen, who authored the paper.

Up to 273,000 people and 171,000 properties in coastal regions around the U.S. could be impacted, according to the paper's findings.

Coastal areas with higher elevation levels and lower rates of subsidence, such as the Pacific Coast, are found to have a lower flood threat overall, the study found.

For example, many parts of the mountainous terrain in San Francisco will not have flooding concerns, but the San Francisco International Airport and other parts of the city on reclaimed land are sinking into the surrounding bay, Manoochehr Shirzaei, director of Virginia Tech's Earth Observation and Innovation Lab, told ABC News.

Conversely, areas with low elevation levels and higher rates of subsidence, such as New Orleans and other areas along the Gulf Coast, were found to have a higher risk of flooding in the future.

New Orleans, for example, is built on low-lying land within the sediments of the Mississippi River, causing the entire city to subside at a rapid rate, said Shirzaei, who supervised the research.

Several locations surveyed on the Gulf Coast show significant levels of sinking -- at rates that are equal to or greater than the current rate of global sea level rise, Ohenhen said.

Biloxi, Mississippi, for example, is sinking at about 5 millimeters per year, Ohenhen said.

The main driver of the subsidence is groundwater extraction and compaction of the sediments, Shirzaei said.

Coastal subsidence is often underrepresented in flooding models, the authors said. The inundation coastal regions will experience due to rising sea levels may actually be worse than previously thought when factoring in how rapidly the land is sinking, according to the study.

However, land subsidence will be the key player in elevating flooding hazards in the next three decades, Shirzaei said.

"What happens on the land really, really affects us," Ohenhen said. "And so if you have a land sinking on one side and see rising on the other side, you're going to have areas that would be inundated in the future."

Nearly 40% of the U.S. population lives near the coast, according to the National Oceanic and Atmospheric Administration.

The sinking is expected to cause structural damage to most existing properties, Ohenhen said, adding the paper does not even address the worst-case scenario for many regions.

"As climate change continues to worsen, we are going to see even more worsened effects in most coastal cities," he said.

People living in coastal regions are already grasping with significant changes, Ohenhen said. In addition, disadvantaged populations, many of which are already disproportionally struggling with the inequalities of climate change, will especially face challenges due to subsidence and sea level rise, the authors said.

Parts of low-lying Florida, such as Miami, are already dealing with "sunny day flooding" that happens as a result of high tide. Miami showed the greatest share of exposure to flooding, with up to 122,000 people and up to 81,000 properties that could be at risk of flooding by 2050, according to the paper.

Local management policies will need to be put in place to fortify coastal infrastructure, as current hazard mitigation efforts are "inadequate," the researchers said. Among the solutions include installing sea walls, raising existing properties above the flood level and replenishing groundwater supplies, the experts said.

"Land subsidence can be mitigated fairly rapidly using engineering and nature-based solutions that we already have in our toolbox," Shirzaei said.