45% of China’s urban land is sinking faster than 3 millimeters per year. 16% even more than 10 millimeters.
China has a huge problem that, in the long term, could compromise its economy and undermine its social fabric. Almost half of China’s major cities suffer from levels of subsidence ranging from moderate to severe. It means, in simple terms, that the earth is sinking under the feet of millions of people due to human activities, exposing the country’s coastal areas to an increasingly greater risk of adverse phenomena such as flooding and rising sea levels.
We can read it, in black and white, in a study published by Science, according to which 45% of the territory Chinese urban buildings are sinking faster than 3 millimeters per year and 16% more than 10, due to the decrease in aquifers but also due to the weight of surface built environment.
With an urban population already exceeding 900 million people, “even a small portion of subsided land in China could therefore result in a substantial threat to urban life,” said the study’s research team. led by Ao Zurui of South China Normal University.
Let us remember that subsidence - literally: a slow and progressive sinking of the bottom of a sea basin or a continental area - already costs China more than 7.5 billion yuan (1.04 billion dollars) in annual losses, and that, within the next century, nearly a quarter of coastal lands could be below sea level, placing hundreds of millions of people at even greater risk of flooding.
What’s happening in China
“China’s massive wave of urbanization could be threatened by land subsidence. Using a spatial synthetic aperture radar interferometry technique, we provided a systematic assessment of land subsidence in all major cities in China from 2015 to 2022,” explained the authors of the study published by Science.
The team’s predictions are not rosy at all: by 2120, 22 to 26 percent of China’s coastal lands will have a relative altitude lower than sea level, hosting 9 to 11 percent of the coastal population, due to the combined effect of city subsidence and sea level rise. There is a solution? Strengthen protective measures to mitigate potential damage resulting from subsidence.
“It really drives home the point that this is a national problem for China and not just a problem in one or two places. And it’s a microcosm of what’s happening in the rest of the world,” Robert Nicholls of the Tyndall Center for Climate Change Research at the University of East Anglia told CNN. Calculator in hand, 270 million people live on sinking land, while 67 million inhabit land that is sinking faster than 10 millimeters (0.4 inches) each year.
Rampant groundwater extraction in China is said to be a major driver of subsidence. Cities are pumping water from underground aquifers faster than it can be replenished, a situation exacerbated by drought fueled by climate change.
Excessive pumping lowers the water table and causes the ground above it to sink. However, the territory is also sinking due to the growing weight of the cities themselves; the soil can compact, naturally due to the weight of the sediments that accumulate over time and the heavy buildings that press on the ground, causing it to constantly sink.
Consequences and solutions
The northern city of Tianjin, home to more than 15 million people, has been identified as one of the hardest hit by this phenomenon. Last year, 3,000 residents were evacuated after a "sudden geological disaster" that investigators ultimately blamed on running out of water and the construction of geothermal wells.
Many of China’s old coal districts have suffered the same problem from excavation, with authorities often forced to inject cement into crumbling pits to strengthen the ground. As well as Shanghai, where dam systems several meters high have been built. The Chinese government is addressing the sinking in several ways, including implementing strict laws to control groundwater pumping.
Be careful though, because the sinking of the ground is not limited to China alone. Approximately 6.3 million square kilometers of land worldwide would be at risk. Among the most affected countries is Indonesia, with much of the capital Jakarta now below sea level. In the Netherlands, 25% of the land has sunk below sea level. And in Mexico City, arguably the fastest-sinking city in the world, the ground is sinking at a rate of 50 centimeters a year.
Nicholls says vulnerable cities could learn lessons from Tokyo, which sank about 5 meters until it banned groundwater extraction in the 1970s. “Mitigating subsidence should be taken very seriously, but you can’t stop it all, so we talk about adaptation and building dams,” he added. In any case, of the 44 main coastal cities affected by the problem, 30 are located in Asia.
Original article published on Money.it Italy 2024-04-26 07:02:00. Original title: "La Cina sprofonda": cosa svela l’ultimo studio choc di Science
