The TLDR: First, thanks to a 2019 study, we discovered that rising sea levels—due to global warming—will wipe great parts of coastal cities around the world. Then in January, new research showed that cities are at risk of collapsing under their own increasing weight. Now, a new study shows that the most densely populated cities are sinking at a faster and dizzying rate—due to human activity, especially groundwater removal. Taken together, they spell a dismal future for a great many of us.

Study #1: Rising waters

The study: In October, 2019, Climate Central published alarming data which revealed that many cities will become unlivable by 2050. The reason: coastal flooding, where the sea water suddenly enters and swamps large swathes of land. 

The new methodology: Previous predictions were based on NASA’s satellite data that overestimated the altitude of coastal land. The reason: Satellites often mistake the top of a building or tree for the height of the land below. This study instead relied on artificial intelligence to figure out the error rate and correct for it. The vast disparity came as a shock. For example, on the left is what Vietnam was expected to look like at high tide in 2050. The image on the right shows the revised prediction.

The findings: The biggest changes in damage predictions were in Asia:

• 150 million people are now living on land that will be below the high-tide line by 2050. By 2100, land that is home to 200 million people could sink permanently below the high tide line—making it unlivable.
• The reason: Sea levels are expected to rise between two to seven feet (0.6 meters to 2.1 meters) over the next three decades.
• 70% of the people at risk are in eight Asian countries: China, Bangladesh, India, Vietnam, Indonesia, Thailand, the Philippines, and Japan.
• The cities expected to disappear or be mostly underwater include: Ho Chi Minh City, Bangkok, Alexandria, Basra, Shanghai and Hong Kong. 

Findings for India: 35 million Indians are at risk. Most of Mumbai—especially south and suburban parts—will be either under water or experience annual catastrophic flooding. This is what at-risk areas in Mumbai will look like in 2050:

And this will be Kolkata:

Study #2: Heavy weight of cities

The study: An American geophysicist Tom Parsons looked at the sheer weight of big cities—public buildings, parking garages, residential/commercial buildings, light and heavy industrial buildings, warehouses, transportation centres etc. And he focused just on one: San Francisco—but his findings will hold true of any of the ‘heavier’ major cities with a similar urban development profile. 

The findings: He concluded that the weight of San Francisco is around 1.6 trillion kilograms, or the equivalent of 250,000,000 elephants. And the city has sunk by 80 millimetres (3.1 inches) as it has grown substantially over time. 

Why the weight matters: “This heavy burden is enough to potentially bend the area’s lithosphere, which is the rigid outer part of the Earth consisting of the crust and upper mantle, causing it to sink”—which is especially true in parts of the city with lots of tall buildings. Example: downtown San Francisco, where there is a lot of downward pressure within a small area. Hence, the sorry state of the luxury 58-story Millennium Tower, which has sunk by more than 400 millimetres (15.7 inches) over the last decade, and is now tilting to one side.

Point to note: Parsons says his findings are especially important for developing countries—where vast numbers of people are migrating to cities, creating even greater pressure to amp up construction. Also: his data doesn’t include infrastructure like roads, bridges and other paved areas—and does not factor in the weight of humans.

Study #3: Our sinking cities

This latest study arrives at the most worrying conclusion: the effects of rising sea levels will be disproportionately felt by most densely populated cities in the world.

The study: An international team of researchers compiled data from four key sources to eliminate effects of rises in sea level. And they isolated natural ‘subsidence’ or sinking of land in 117 river deltas—to arrive at estimates of human-caused subsidence in 138 large coastal cities.

The findings: are as follows:

• The most populated cities are experiencing an average sea rise of 7.8mm to 9.9 mm per year—which is 3-4X the global average of  2.6 mm. 
• Why this matters: More than one in five people live along the coastline where the sea level is increasing at 10 mm (or 0.4 inches) or more per year. 
• But the area they live in only accounts for less than 1% of the world's coastline.
• Also this: Over the 20th century, parts of Jakarta, New Orleans, Shanghai, and Bangkok sank between 6 to 10 feet! Jakarta, for example, is sinking at 10 cm a year!
• The worst-hit: South, Southeast and East Asia—which collectively contain 71% of the global coastal population living below 10 metres in sea elevation.

Key takeaway: from one of the lead researchers: “We are talking about not a forecast; we are saying this is happening today.” And you can see how well or badly different regions are doing below. India, outlined in purple, is sinking at the rate of less than 5-10mm per year. But Southeast Asia in red is in a dire state at more than 20 mm/year.

One key cause: is location, as in many big cities tend to be located on river deltas: "Deltas are where rivers bring sediment to the sea. And the weight of the sediment plus the compression with the sediment causes consolidation.”

The really big problem: is, as always, human activity—specifically groundwater removal—“which causes the sediments in aquifers to compact together and the land above to sink.” Parts of Tokyo sank more than 13 feet during the 20th century due to rapid groundwater depletion. Other kinds of damaging human activity include oil and gas extraction, upstream dams, flood defenses, sand extraction or mining.

The bottomline: Rising seas and sinking cities represent a nightmarish double whammy. But the good news about catastrophes caused by human activity is that we can stop certain activities, and take up others. Example: We can build smart cities in the truest sense of the word.

Reading list

National Geographic and CNN have the most details on the most recent study on sinking cities. New York Times has an excellent interactive piece—with lots of eye-opening maps—on the rising sea levels. Bloomberg News and EuroNews have most on the weight of San Francisco—and its implications for other cities. Also in EuroNews: The sheer weight of the human-made stuff—which will soon weigh more than natural life on the planet. We didn’t want to dishearten you any further, but the Wired report on how human activity is triggering high tides is a must read.