Bad news for coastal communities like ours

Worst Impacts of Sea Level Rise Will Hit Earlier Than Expected

By AMERICAN GEOPHYSICAL UNION 

Updated land-elevation models reveal many coastal regions are lower than we thought, according to a new study in Earth’s Future. The first 1 to 2 meters of sea level rise will therefore inundate about twice as much land as earlier elevation models suggested.

According to current models, the most significant effects of sea level rise are expected to take place once it reaches several meters. However, a recent study has uncovered that the largest increases in flooding will occur after the first 2 meters (6.6 feet) of sea level rise, affecting an area of land that is more than double what was previously predicted by older elevation models.

The study utilized high-precision land elevation measurements from NASA’s ICESat-2 lidar satellite, launched in 2018, to enhance models of sea level rise and flooding. Prior evaluations were typically based on less accurate radar-based data.

“Radar is unable to fully penetrate vegetation and therefore overestimates surface elevation,” said Ronald Vernimmen, a researcher at the Dutch research firm Data for Sustainability. Many coastal areas are lower than scientists thought they were.

The study was published in the American Geophysical Union’s Earth’s Future, which publishes interdisciplinary research on the past, present, and future of our planet and its inhabitants.

The underestimates of land elevation mean coastal communities have less time to prepare for sea level rise than expected, with the biggest impacts of rising seas occurring earlier than previously thought. After those first few meters of sea level rise, the rate at which land area falls below mean sea level decreases.

Vernimmen, who works on flood protection and spatial planning advisory projects, started using these more accurate measurements of land elevation when he realized that existing land elevation estimates were not suitable for quantifying coastal flooding risk.

Using the new measurements of land elevation, Vernimmen and co-author Aljosja Hooijer found coastal areas lie much lower than older radar data had suggested. Analyses of the new lidar-based elevation model revealed 2 meters of sea-level rise would cover up to 2.4 times the land area as observed by radar-based elevation models.

For example, the lidar data suggest a 2-meter (6.6 feet) increase in sea level could put most of Bangkok and its 10 million residents below sea level, while older data suggested that Bangkok would still be largely above the mean sea level under that same amount of sea level rise. 

In total, after 2 meters (6.6 feet) of sea level rise, Vernimmen and Hooijer estimate that 240 million more people will live below the mean sea level. After 3 and 4 meters (9.8 and 13 feet) of sea level rise, that number increases by 140 million and by another 116 million, respectively.

Cities below future sea levels may not necessarily be submerged because levees, dikes, and pumping stations can protect some areas from rising seas; Amsterdam and New Orleans are modern examples of this. 

However, such protection measures can be expensive and take decades to implement. If vulnerable communities want to mitigate the most damage, they need to act before the sea rises those first few meters, according to Vernimmen.

Reference: “New LiDAR-Based Elevation Model Shows Greatest Increase in Global Coastal Exposure to Flooding to Be Caused by Early-Stage Sea-Level Rise” by Ronald Vernimmen and Aljosja Hooijer, 2 January 2023, Earth’s Future.
DOI: 10.1029/2022EF002880