A new estimate triples the number of people in the path of rising seas

Trees and buildings created big margins of error in previous coastal elevation estimates

A new method for assessing coastal elevation suggests sea level rise could threaten areas now home to up to 480 million people by 2100. Most of those facing flood risks live in Asia, including in the Indonesian capital of Jakarta (shown).

Seika/Flickr (CC BY 2.0)

Sea level rise this century could flood coastal areas that are now home to 340 million to 480 million people, researchers from Climate Central, a research and advocacy group, report. That’s roughly triple the number of people estimated to be at risk using previous coastal elevation data.

The new estimate, published October 29 in Nature Communications, comes from efforts to refine NASA satellite elevation data, and it illustrates the implications of elevation data having been overestimated in some places by up to 5 to 10 meters. The results are presented in terms of how many people, by today’s population numbers, could be affected, but don’t predict how many people will actually be living in those coastal areas in 2100.

“The global threat from sea level rise and coastal flooding is far greater than what we thought it was,” says Benjamin Strauss, who heads Climate Central in Princeton, N.J.

While the research highlights an increased threat to people currently living in coastal areas, it does not estimate how much more land area will fall below flood projection lines, and whether that area includes a handful of coastal megacities or mostly large swaths of less populated land. So it’s unclear how many people in future cities might be at risk of inundation, which could limit the usefulness of the findings to city managers. The researchers say those details fell outside the scope of this study.

Still, the new estimate attempts to correct a large margin of error found in previous estimates of global coastal elevations. Those estimates are based on NASA’s Shuttle Radar Topography Mission, or SRTM, which created a global topographic map from satellite images and radar data. SRTM measures elevation by bouncing radar signals off Earth’s surface — whether that’s a tree, a building or the land itself. So the method can overestimate elevation levels, especially in forests and cities.

“If we’re overestimating elevations [in coastal cities], we’re getting a very over-optimistic view of the impact of flooding,” says Ashton Shortridge, a geographer at Michigan State University in East Lansing who was not involved in the study.

In the new work, Strauss and computational scientist Scott Kulp fine-tuned SRTM’s elevation estimates using an artificial neural network, a computer algorithm meant to mimic how the brain processes information. The algorithm was programed to account for trees, bridges and buildings to recalculate land elevations as if Earth were naked to produce more accurate elevation estimates.

Using the new elevation estimates, the researchers used 2010 population data to calculate how many people live on land that could be affected by sea level rise driven by climate change. If global warming can be kept to 2 degrees Celsius above preindustrial levels (SN: 7/10/18), rising sea levels, projected by the Intergovernmental Panel on Climate Change in 2013, would swamp areas that are currently home to 360 million people, the researchers conclude.

In the worst-case scenario — emissions left unchecked and rapidly crumbling Antarctic ice cliffs dramatically increasing estimates of sea level rise (SN: 2/6/19) — 480 million people by today’s demographics could be living on land at risk from sea rise by 2100, the study says. No matter what the scenario, over 70 percent of the people newly estimated to be vulnerable live in eight Asian countries, with the biggest chunk in China.

That’s not to say coastal population trends won’t change. Humans may migrate in the face of flood risk (SN: 8/15/18), or cities could redesign infrastructure to better handle higher tides and frequent floods (SN: 8/6/19).

Still, the new projections for affected population by 2100 represent a considerable jump from estimates using SRTM data, Kulp says. Those ranged from 95 million to 170 million people.

“We knew it was going to be big, but the fact that there are — when we look at a global scale — three times as many people potentially vulnerable … it’s still quite shocking,” Kulp says.

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