As Chinese cities grow, they also sink.
An estimated 16% of the country's major cities are losing more than 10 millimeters of elevation per year and almost half are losing more than 3 millimeters per year, according to a new study published in the journal Science.
These amounts may seem small, but
they add up quickly
.
According to the study, within 100 years, a
quarter
of China's urban coastal land could be below sea level due to a combination of subsidence and sea level rise.
"It's a national problem," said Robert Nicholls, a climate scientist and civil engineer at the University of East Anglia, who reviewed the paper.
Nicholls added that, to his knowledge, this study is the first to measure subsidence in many urban areas at once using state-of-the-art
radar data
from satellites.
Sinking in these cities is due in part to the
weight of buildings
and infrastructure, according to the study.
Pumping water from cities' underground aquifers also plays a role, as does oil drilling and coal mining, all activities that leave empty spaces underground where soil and rock can compact or collapse.
Beijing
is one of the fastest sinking places in the country.
The same goes for nearby Tianjin, where thousands of residents were evacuated from high-rise apartment buildings last year after outside streets were suddenly separated.
Within these cities, the subsidence is uneven. When land next to each other sinks at different rates, anything built on top of that land is at risk of damage.
Other countries, including the United States, have
similar problems.
"Land subsidence is an overlooked problem that exists almost everywhere," said Manoochehr Shirzaei, a geophysicist at Virginia Tech who has studied subsidence in American coastal cities using similar methods.
Shirzaei also reviewed the new study on Chinese cities by Zurui Ao of South China Normal University, Xiaomei Hu and Shengli Tao of Peking University, and their colleagues.
"I think most of the adaptation strategies we have and resilience plans to combat climate change are inaccurate, simply because they didn't include land subsidence," he said.
"It hasn't been studied in the same way that, for example, sea level rise has been studied."
The new study was based on satellite radar measurements of how much the land surface increased or decreased in 82 major cities, representing three-quarters of China's urban population, between 2015 and 2022.
The researchers compared these measurements with data on possible contributing factors, such as the weight of buildings in these cities and changes in groundwater levels beneath them.
The researchers also combined their subsidence measurements with
sea level rise
projections to determine which cities could end up below sea level.
One caveat with these findings is that they assumed a constant rate of subsidence over the next 100 years, but these rates can change along with human activity.
About 6% of the territory of China's coastal cities currently has a relative elevation
below sea level.
If the global mean sea level rises by 0.87 meters (the greater of the two commonly used scenarios considered by researchers), that proportion could
increase to 26%
, according to this study.
Risks
Being below sea level does not mean that a city is automatically doomed to failure.
Much of the Netherlands is below sea level and sinking, but the country has been exhaustively designed to prevent flooding in some places and accommodate it in others.
The key to minimizing damage is to limit groundwater extraction, the researchers wrote.
Shanghai is already taking this approach and is sinking more slowly than other Chinese cities.
In Japan, groundwater management over the years has proven successful in stabilizing subsidence in Tokyo and Osaka.
Some places are even combating subsidence by injecting water into depleted aquifers in a process called managed recharge.
It is difficult to completely stop sinking, Nicholls said.
"You have to live with what's left."
Primarily, he said, this means adapting to sea level rise in coastal areas, not just sea level rise caused by climate change, but also the effects of land subsidence.
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