The study used high-resolution measurements of land elevation. The measurements were taken from NASA's ICESat-2 lidar satellite, launched in 2018, to improve upon models of sea level rise and inundation. The radar-based data are, however, less precise, researchers have said.
"Radar is unable to fully penetrate vegetation and therefore overestimates surface elevation," said Ronald Vernimmen, a researcher at the research firm Data for Sustainability in The Netherlands.
Since coastal areas are lower, the land elevation in those areas means the communities who live by the coastal areas have less time to prepare in case of a sea level rise, with the biggest impacts of rising sea levels happening earlier than thought.
The researchers found that after those first few metres of sea level rise, the rate at which land area falls below the mean sea level decreases. They started using these more accurate measurements of land elevation when they realised that existing land elevation estimates were not suitable for quantifying coastal flooding risk.
With the new land elevation measurements, it was found that coastal areas lie much lower. Analyses of the new Lidar-based elevation model showed two metres of sea-level rise would cover up to 2.4 times the land area as observed by radar-based elevation models.
If there is a sea level rise of two metres, nearly 240 million people will live below the mean sea level. If there are three and four metres of rise, there will be an increase of 140 million and 116 million, respectively.
The researchers noted that cities below future sea level 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, they said. However, such protection measures can be expensive and take decades to implement, according to the researchers. If vulnerable communities want to mitigate the most damage, they need to act before the sea rises those first few meters, they added.
(With agency inputs)
