Coastal Nations, Megacities Face 20 Feet of Sea Rise

By Benjamin Strauss

If, as suggested by a comprehensive new review in the journal Science, 2°C of global warming will lock in at least 20 feet (6 meters) of eventual sea level rise, what would 2°C of warming (3.6°F) mean for the future and heritage of global nations and cities? 

It would mean a world we don’t recognize, one losing more than 444,000 square miles of land, where more than 375 million people live today. China, the world’s largest emitter of greenhouse gases, is home to by far the largest share of those people, nearly a quarter. The U.S., second biggest emitter, ranks seventh on the exposure list — but is the most affected nation, by total population, outside of heavily threatened Asia. Many large and small nations face much more daunting challenges from the perspective of the percentage of their populations on land at risk.

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Among global megacities, more than a quarter of Shanghai, Hong Kong, Mumbai and Calcutta populations live below the 6-meter line. The top 20 affected cities by total population are all in Asia.

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These findings and the maps and tables on this page come from Climate Central’s own analysis, using essentially the same underlying data and methods behind our first global assessment of sea level rise exposure (New York Times; Climate Central). We improved on that earlier work by using a higher resolution global ocean model output from Mark Merrifield at the University of Hawaii, and we extended our analysis to assess megacities based on boundaries from Natural Earth 2.0.0.

For the brief study here, we used a water level of 20 feet (more precisely, 6 meters) above local high tide lines (Mean Higher High Water levels) globally. Actual sea level rise will almost certainly vary by several feet from region to region, depending upon multiple factors; but this level of detail is difficult to project and was not included in the Science paper that inspired this project.

Comparative assessment using high quality elevation and population data for the U.S. suggests that the coarser and less accurate global data available for this global analysis may actually underestimate global population exposure by roughly a factor of two. The global elevation dataset used here also does not extend past 60 degrees north latitude, meaning that national assessments did not include the sparsely populated northern reaches of Russia, Canada, Scandinavia and more.

Top 20 Megacities Below the Line
MEGACITYPOPULATION AFFECTED% OF CURRENT POPULATION1.  Shanghai, China
2.  Hong Kong, China
3.  Taizhou, China
4.  Mumbai, India
5.  Calcutta, India
6.  Tianjin, China
7.  Jakarta, Indonesia
8.  Nantong, China
9.  Ho Chi Minh City, Vietnam
10.  Osaka, Japan
11.  Chittagong, Bangladesh
12.  Tokya, Japan
13.  Hanoi, Vietnam
14.  Huaiyin, China
15.  Shantou, China
16.  Nam Dinh, Vietnam
17.  Jiagmen, China
18.  Khulna, Bangledesh
19.  Barisal, Bangladesh
20.  Lianyungang, China11,500,000
  6,200,000
  6,100,000
  5,900,000
  5,800,000
  5,100,000
  4,900,000
  4,700,000
  4,400,000
  4,100,000
  4,000,000
  3,800,000
  3,800,000
  3,400,000
  3,100,000
  3,100,000
  3,000,000
  2,900,000
  2,800,000
  2,800,00039%
28%
67%
27%
25%
12%
11%
72%
44%
25%
44%
15%
30%
43%
23%
74%
51%
22%
40%
92%

We have also posted a more detailed analysis and zip code-searchable interactive map of potential impacts in the U.S., where the higher quality data available make these easily possible.

Sea levels are already rising and posing challenges today. The twentieth foot of sea level rise could possibly arrive as soon as 2200; or it might take many more centuries for that total to accrue. Accordingly, the population exposure numbers in Climate Central’s analysis on this page do not represent fully materialized threats in the immediate moment. Under any circumstances, coastal populations and economies will reshape themselves over time. But the accumulating research on how sensitive sea level is to warming over the long run — and the amount of humanity in the restless ocean’s way — point to unrelenting centuries of defense, retreat, and a reimagining of life along our coasts.

The Climate Central research, mapping and data visualization associated with this page have received support from the Kresge Foundation, National Science Foundation grant ARC 1203415, and additional sources.