“Subsidence is when land sinks relative to sea level, usually owing to extraction of subsurface water, rock or other resources.” It’s happening both inland and in coastal areas, exacerbating sea level rise.
Also, parts of the world, like the US East Coast, are sinking due to “glacial isostatic rebound”, see below.
One in ten residents of China’s coastal cities could be living below sea level within a century, as a result of land subsidence and climate change, according to a paper published in Science today1.
Some 16% of the mapped area of China’s major cities is sinking “rapidly” — faster than 10 millimetres every year. An even greater area, roughly 45%, is sinking at a “moderate” rate, the paper says, meaning a downward trajectory of greater than 3 mm annually. Affected cities include the capital Beijing, as well as regional capitals, including Fuzhou, Hefei and Xi’an.
The situation could see one-quarter of China’s coastal lands slip below sea level within a few decades, posing “serious threats” to the hundreds of millions of people who live on the coast, the paper notes.
“Subsidence is certainly not only a problem in China. Many other parts of the world share the same problem,” says Ding Xiaoli, a geodesist at the Hong Kong Polytechnic University.
Subsidence is when land sinks relative to sea level, usually owing to extraction of subsurface water, rock or other resources.
In the famously low-lying Netherlands, roughly one-quarter of the land has subsided to below sea level. And by 2040, almost one-fifth of the world’s population is projected to be living on sinking land.
Jakarta has become the most rapidly subsiding capital in the world, prompting Indonesia to propose a new capital city.
In the United States, more than 44,000 square kilometres of land across 45 states has been directly affected by subsidence, with more than 80% of the cases relating to groundwater extraction, often for agricultural purposes.
The paper links a range of natural and human factors to sinking, such as the depth of a city’s bedrock, groundwater depletion, the weight of buildings, the use of transport systems and underground mining.
Previous studies2 have found that excessive groundwater extraction is a key cause of severe land subsidence in cities across the world.
But there are also stories of successful mitigation. Tokyo slowed its sinking from a rapid 240 mm a year in the 1960s to about 10 mm a year in the early 2000s, after passing laws that limited groundwater pumping. Shanghai, China, which sank by a staggering 2.6 metres between 1921 and 1965, reduced its annual rate of sinking to about 5 mm after implementing a series of environmental regulations.
“The key to addressing China’s city subsidence could lie in the long-term, sustained control of groundwater extraction,” the paper says.
Ding says that, in cities such as Macao and Hong Kong where no groundwater is used, subsidence mainly comes from consolidation — downward movement as a result of soil being compressed — after land reclamation.
The authors also listed the weight of buildings as a factor. Contrary to expectations, heavier buildings, such as the skyscrapers in Shanghai, tend to sink slower than lighter structures do, possibly because those buildings are anchored on deeper rock, according to the paper.
Tehran, which sits 1,200 meters (3,900 feet) above sea level against the Alborz Mountains on a plateau, has rapidly grown over the last 100 years to a sprawling city of 13 million people in its metropolitan area.
All those people have put incredible pressure on water resources on a semi-arid plateau in a country that saw only 171 millimeters (6.7 inches) of rain last year. Over-reliance on ground aquifers has seen increasingly salty water pumped from below ground.
“Surface soil contains water and air. When you pump water from under the ground surface, you cause some empty space to be formed in the soil,” Arabi told The Associated Press. “Gradually, the pressure from above causes the soil particles to stick together and this leads to sinking of the ground and formation of cracks.”
Rain and snow to recharge the underground aquifers have been in short supply. Over the past decade, Iran has seen the most prolonged and severe drought in more than 30 years, according to the United Nations’ Food and Agriculture Organization. An estimated 97 percent of the country has faced some level of drought, Iran’s Meteorological Organization says.
That has caused the sinkholes and fissures now seen around Tehran.
Iranian authorities say they have measured up to 22 centimeters (8.6 inches) of annual subsidence near the capital, while the normal range would be only as high as 3 centimeters (1.1 inches) per year.
Even higher numbers have been measured in other parts of the country. Some sinkholes formed in western Iran are as deep as 60 meters (196 feet).
Virginia Institute of Marine Science:
Glacial Isostatic Adjustment
This process is causing most of our East Coast tide stations to subside (and thus relative sea level to rise). Exceptions are Eastport, Maine; Portland, Maine; and Boston, Massachusetts, which fall near the “zero line” between fore-bulge collapse and isostatic rebound. Jacksonville, Florida is uplifting slightly (causing relative sea level to fall), but not necessarily due to GIA (Sella et al. 2007). The maximum subsidence due to GIA (1.5 millimeters per year) is found around New Jersey (Karegar et al. 2017).
Ohenhen pointed out how the problem with the subsidence mapped isn’t just that land is sinking, but also that “hotspots” of subsidence are occurring in population hubs and around concentrations of infrastructure.
“For example, significant areas of critical infrastructure in New York, including JFK and LaGuardia airports and its runways, along with the railway systems, are affected by subsidence rates exceeding 2 mm per year,” he added. “The effects of these right now and into the future are potential damage to infrastructure and increased flood risks.”
