As many of Southeast Asia’s megacities sink, understanding and addressing the problem is key to helping urban populations adapt to climate change.
As Southeast Asia’s megacities continue to grow, some of the region’s biggest population centers are facing a growing design problem that most governments have only just begun to confront.
By some estimates, Jakarta is the fastest sinking city in the world, with some areas seeing 1 centimeter per year and others seeing as much as 20 centimeters. Forty percent of the city is already below sea level.
In Bangkok, the city now sinks as much as 30 millimeters per year, according to a report from 2015, and Ho Chi Minh City is sinking as much as 80 millimeters per year. In both cities, some areas may be sinking much faster, depending on local geology and the weight of buildings.
“The reality of Bangkok metropolitan region is a city of 15 million people, living, working and commuting on top of shifting muddy river delta,” says Kotchakorn Voraakhom, a Thai landscape architect during her TED talk. “We could be below sea level by 2030.”
According to Kotchakorn, the Thai capital, like many of Southeast Asia’s cities, is “growing in every direction, shifting from porous, agricultural land—the land that can breathe and absorb water—to a concrete jungle.”
The situation is similar in Jakarta, a city of 13 rivers. An estimated 97% of the Jakarta metro area’s wetlands have been paved over.
Understanding and addressing the region’s sinking cities is becoming increasingly important as this will determine how urban populations are impacted by climate change, from rising seas to flooding from extreme weather—or regular monsoons. A sinking city makes any built adaptations to climate change more complicated to design and possibly less effective.
For coastal cities like Ho Chi Minh City and Bangkok, their sinking problem is known as subsidence, a process in which soil condenses and the land settles. In undeveloped river deltas, sediment flowing downstream into the delta can normally compensate for any natural sinking and the elevation doesn’t change much.
Robert Nicholls, head of the Tyndall Centre for Climate Change Research in the UK, is the lead author of a new study on subsidence and sea level rise. “Rapid rates of subsidence in deltas and especially cities on deltas are also human-caused, mostly due to groundwater pumping, also oil and gas extraction, and sediment resupply prevented by upstream dams, flood defenses, sand extraction or mining,” he said.
Supachai Tantikom, Bangkok’s chief resilience officer, says that underneath his city, “We don’t have any rock layers and there are a lot of aquifers.”
“In the past, we used a lot of underground water,” he told CNA, “so this has caused land subsidence.”
The sinking city also experiences effects of sea level rise much faster than it otherwise would. According to a study Nicholls and his colleagues published in Nature Climate Change in March, global average sea level rise has been 2.5 millimeters per year for the past two decades but coastal communities are experiencing four times faster—7.8 to 9.9 millimeters per year—because they live on land that is sinking.
But subsidence means that even in a best-case scenario for climate change, in which the world keeps sea level rise to an absolute minimum, cities will still have to design solutions for many of the same impacts.
City governments search for policies to save their land
Sinking cities in Southeast Asia are taking a variety of approaches to their predicament.
Bangkok took action early to attempt to slow the city’s sinking. In 1977, the government restricted use of groundwater through a “Groundwater Act” and in 1985, the city introduced groundwater use penalties. The measures largely succeeded, cutting groundwater extraction to only 10% of the city’s water use, though the city’s sinking continues.
For Ho Chi Minh City, similar steps have proved difficult as there’s little data on groundwater use and many people disagree about the extent of land subsidence or its causes.
“Thousands of [wells]—small ones, big ones, household or industrial-scale—have been built,” said Ho Long Phi, a vice managing director at enCity, a Vietnamese urban consulting firm. “At the highest level, we could observe more than a million cubic meters per day of total groundwater extraction during the past 20 years.”
In the past few years, the city government has increased access to municipal piped water, reportedly reducing groundwater extraction by over two thirds. Residents in Ho Chi Minh City have also begun taking steps on their own, paying to raise their homes and other buildings when they can.
In Bangkok, Kotchakorn and others are pushing innovative design solutions that aim to help the city cope with increased impacts of flooding and rising seas.
“We’re going to live with water again, we used to live with water, right?” Kotchakorn said. “The floods used to be the source of food, the source of fertilization.”
While cities can work to slow the subsidence, governments and communities will need to invest in adaptations as well. One place to start is by changing the patterns of development that exacerbate flooding—protect and restore urban canals and green space.
Some of those working to help Jakarta adapt are pushing similar solutions. “Jakarta has no green spaces left,” said Dicky Edwin Hindarto, coordinator of the Indonesia Low Emission Network, in an interview with Mongabay. “We need to increase areas of parkland and mangroves instead of surfaces covered in concrete.”
Coastal cities are more vulnerable than previously thought
Cities like Bangkok and Ho Chi Minh City may also be much more vulnerable than previously thought.
A study published in late 2019 by Climate Central in the journal Nature Communications found that by 2050, the homes of at least 300 million people may be submerged by regular flooding due to rising sea levels. The study used machine learning to produce a new, more accurate global elevation dataset, replacing one that may have significantly overestimated the elevations of coastal areas.
Based on the updated model, the authors estimated that one billion people currently live less than 10 meters above today’s high tide lines. But the paper also noted that new adaptive measures could offer a partial solution, as they found that over 100 million people live on land that is already below high tide lines, meaning they’re protected in some way by levees or other built solutions.
In Thailand, the new research showed that 12 million people are living on land that may be inundated by annual floods by 2050, up from a previous estimate of 1 million. In Vietnam, the study found that 31 million people may see their land flooded, compared to a previous estimate of 9 million.
The actual numbers may be much higher, as the paper uses climate and sea level rise models based on the hope that the world will “moderately reduce warming emissions roughly in line with the 2015 Paris climate agreement”. The reality may be more severe.