Researchers from Dutch firm Data for Sustainability say their data shows that the land areas that would be inundated after the first one to two metres of sea level rise have been underestimated in the past.
The study used high-resolution measurements of land elevation from Nasa's ICESat-2 lidar satellite, launched in 2018, to improve upon models of sea level rise and inundation. Previous assessments typically relied on radar-based data, which are less precise.
"Radar is unable to fully penetrate vegetation and therefore overestimates surface elevation," said researcher Ronald Vernimmen. This had the impact of making many coastal areas appear to be higher than they actually were.
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 metres of sea level rise, the rate at which land area falls below mean sea level decreases.
Analyses of the new lidar-based elevation model revealed 2 two metres 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 two-metre 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 mean sea level.
In total, after two metres of sea level rise, the researchers estimate that 240 million more people will live below mean sea level.
After three and four metres of sea level rise, that number increases by 140 million and by another 116 million, respectively.
Cities below future sea level may not necessarily become 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 metres," Vernimmen said.
On a pathway with high greenhouse gas emissions and rapid ice sheet collapse, current models project that average sea level rise could reach 2.2 metres by 2100 and 3.9 metres by 2150.
