The sinking of America's coastal cities is posing a greater flood risk than initially expected due to sea level rise alone, according to recent research published in the journal Nature. This gradual sinking, also referred to as land subsidence, is projected to extend flooding to a larger number of communities than previously estimated. The study, which focused on 32 major cities along the US shoreline, examined the potential impact of climate change on future flooding. In a worst-case scenario without adequate flood defenses, it is anticipated that by 2050, up to one in 50 people and one in 35 properties in these cities could be susceptible to flooding during high tide.

The researchers emphasize the necessity for cities to prepare for this dual challenge of sinking land and rising seas. Given the twofold nature of the problem, solutions must address both aspects to effectively mitigate the risks.

"It's an imminent concern, and the potential risks we highlight are exceedingly realistic," says Leonard Ohenhen, the lead author of the study and a PhD candidate at Virginia Tech. "We genuinely hope that citizens and policymakers alike, encompassing every stakeholder in the community discussion, can initiate contemplation on how to respond to these impending changes."

Ohenhen and his co-authors employed radar satellite readings to chart land subsidence along US coasts. This methodology enabled them to observe the dynamic changes in the landscape over time, facilitating predictions of the extent to which cities may sink in the coming decades. Subsequently, they successfully modeled the probable scope of future flooding with greater precision compared to forecasts that overlook the impact of land subsidence.

"This represents a large new dataset that will hopefully be useful to these communities for planning their responses to this incoming sea level rise," says Eric Lindsey, an assistant professor at the University of New Mexico, who has studied land subsidence and sea level rise but was not involved in this new paper.

The flood forecast presented in the study is likely conservative. It considers high tides but overlooks flooding from groundwater as sinking land approaches the water table. "It's like sinking in the bottom of a bowl, and suddenly you start to get water coming up around your ankles," explains Kristina Hill, program director of the Institute of Urban and Regional Development at the University of California, Berkeley, who was not part of the study.

Some causes of land subsidence are natural, but others are human-made, implying that steps can be taken to mitigate the problem. For instance, the extraction of water, oil, and gas from the ground contributes to land sinking. Ceasing drilling and pumping activities can help slow down the sinking process.

Furthermore, it is possible to reverse some of the damage by replenishing aquifers. "The solutions to that problem of land subsidence are available in our toolbox and very effective in a short time frame," states Manoochehr Shirzaei, another author of the study and director of Virginia Tech’s Earth Observation and Innovation Lab.

As greenhouse gas emissions from fossil fuels drive climate change, leaving fossil fuels in the ground emerges as a solution to sea level rise. Relying solely on sea walls and levees won't be sufficient. Even with current defenses in place, an additional 1,389 square kilometers of land along US coasts could face threats from sea level rise by 2050.