US coastal cities face higher 2050 flood risk because they are sinking

Sea levels are rising faster than expected in coastal cities in the US, largely due to the land sinking as a result of the extraction of water and fossil fuels from underground. That means, by 2050, up to 518,000 extra people living in these areas could be at risk of significant flooding, if adequate defence measures aren’t taken.

Coastal cities often experience subsidence – the gradual sinking of the land over time. One of the biggest drivers of this is the extraction of resources from the ground, such as water and fossil fuels, which causes the earth to compact.

To investigate how subsidence and rising sea levels affects coastal communities, Leonard Ohenhen at Viriginia Tech and his colleagues created a model based on land elevation changes and projected sea level rise of 32 major cities on the shore, including Boston and San Francisco.

The researchers found that cities on the Atlantic and Gulf of Mexico coasts, such as New Orleans, had lower elevations and greater rates of subsidence – sinking at least 2 millimetres more per year than other regions, leading to a higher threat of flooding. Cities on the Pacific coast are more shielded from the threats of sea level rise, owing to their higher elevation.

“We were surprised to see Biloxi, Mississippi, experienced the greatest rate of subsidence,” says Ohenhen.

They also found that existing assessments of flood risk in the US, which don’t consider the compounding effect of subsidence, underestimate the threat. By 2050, they found that nearly 1400 more square kilometres of land is at risk of flooding than is stated in current estimates, putting 1 in 50 people and 1 in 35 homes in danger in the 32 cities.

This would be an extra 518,000 people at flood risk and 288,000 more homes.

These findings highlight the urgent need to strengthen US flood defences now, says team member Manoochehr Shirzaei, also at Virginia Tech. “Individual cities will need to take different adaptation measures. For New Orleans, flood defences should be upgraded for the entire city, while San Francisco probably needs to just defend important infrastructure.”

Other things could also be done to reduce the threat. “Where subsidence is driven by groundwater extraction, oil and gas exploitation, and other anthropogenic stresses, we should be doing all we can to reverse these trends before it is too late,” says Shimon Anisfeld at Yale University.

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