New analysis triples risk from rising sea levels


Regular flooding could become a reality for hundreds of millions of people.


Flooding in the coastal city of Cebu, Philippines.

J LLOA

By Natalie Parletta

In three decades from now more than 300 million people’s homes could face annual coastal flooding and new high tide levels could cover land currently inhabited by around 150 million – many of them in Asia.

These forecasts, published in the journal Nature, are three times greater than current prevailing estimates, and that’s assuming moderate cuts to global carbon emissions.

They are, therefore, best-case scenarios based on carbon emission targets set by the 2015 Paris Agreement being met.

High emission models, combined with possible early-onset Antarctic ice sheet melting, project that an extra 40 million people will face coastal flooding by 2050, rising to a total of 630 million by the end of the century.

“These assessments show the potential of climate change to reshape cities, economies, coastlines, and entire global regions within our lifetimes,” says lead author Scott Kulp from Climate Central in the US.

“As the tideline rises higher than the ground people call home, nations will increasingly confront questions about whether, how much, and how long coastal defences can protect them.”

Kulp and co-author Benjamin Strauss produced the updated estimates using a new digital elevation model, CoastalDEM, which corrects systematic errors in current forecasts produced from NASA’s Shuttle Radar Topography Mission (SRTM).

SRTM measures the highest detectable elevations, which include treetops and rooftops. This can result in overestimates ranging from more than two metres for coastal elevations – exceeding this century’s projected sea level rise – to four metres in high-density urban areas.

CoastalDEM uses a neural network model that integrates 23 variables including population and vegetation data, reducing these average errors to around 10 centimetres – a small fraction of previous estimates.

"For all of the critical research that’s been done on climate change and sea level projections,” says Strauss, “it turns out that for most of the global coast we didn’t know the height of the ground beneath our feet.”

Even optimistic predictions based on low carbon emissions and stable Antarctic ice sheets greatly exceed previous estimates using SRTM, the authors note.

Regardless of the model or emission scenarios, they report that more than two-thirds of people living on vulnerable land are in eight Asian countries: China, Bangladesh, India, Vietnam, Indonesia, Thailand, the Philippines, and Japan.

In China alone, 43 to 57 million people could live below the high tide line by the end of the century, depending on Antarctic ice stability.

Twenty other countries – all island nations – could find 10% or more of their population falling below high tide levels, even with the most positive emission cuts.

The US could also face mass migration this century as a result of sea-level rise, impacting population distribution and straining inland resources. The impact on less developed countries is uncertain.

“It is difficult to extrapolate such projections and their impacts to more resource-constrained developing nations,” the authors warn, “though historically, large-scale migration events have posed serious challenges to political stability, driving conflict.

“If our findings stand, coastal communities worldwide must prepare themselves for much more difficult futures than may be currently anticipated.”

People can explore threatened areas at a neighbourhood level using an interactive global map Climate Central has produced from its new elevation data.

But while these scenarios vastly improve current estimates, more work is needed to help improve the accuracy of predictions.

“Our data improve the picture,” Strauss says, “but there is still a great need for governments and aerospace companies to produce and release more accurate elevation data. Lives and livelihoods depend on it.”

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Natalie Parletta is a freelance science writer based in Adelaide and an adjunct senior research fellow with the University of South Australia.
  1. https://www.nature.com/articles/s41467-019-12808-z
  2. https://unfccc.int/process-and-meetings/the-paris-agreement/what-is-the-paris-agreement
  3. https://www.researchgate.net/publication/322137473_CoastalDEM_A_Global_Coastal_Digital_Elevation_Model_Improved_from_SRTM_Using_a_Neural_Network
  4. https://coastal.climatecentral.org/
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