Satellites confirm that the world’s coastal cities are sinking much faster than sea levels are rising, mainly due to the depletion of aquifers that supply residential and commercial areas.
Satellite data indicates land is sinking faster than sea levels are rising in many coastal cities around the world, according to a new study published in the journal Geophysical Research Letters.
If subsidence continues at this rate, these cities will face flooding much sooner than predicted by sea level rise models, the researchers add in their paper.
To reach this conclusion, the authors of this study, conducted by Matt Weifrom the University of Rhode Island in the United States, measured subsidence rates in 99 coastal cities around the world between 2015 and 2020, using satellite data.
For these measurements, the researchers analyzed a time series of satellite images of each city, taken at an interval of two months.
Because the satellites fly over the same part of the planet every 12 days, the researchers were able to compare each city’s land subsidence over time.
They found that subsidence rates are highly variable, both at different sites in each city and from city to city.
In most cities, there is at least one neighborhood that is sinking by more than two millimeters per year, while in 33 of the 99 cities, part of it is sinking by at least 10 millimeters per year. .
These data indicate that urban subsidence, in some cases, is five times greater than the global average sea level rise, set at 2 mm per year, the researchers point out.
The fastest shipwreck is occurring in South, Southeast and East Asia. Rapid sinking is also taking place in North America, Europe, Africa and Australia.
Human activity, mainly groundwater extraction, is probably the main cause of this sinking, the researchers point out.
They deduce that in the urban regions that sink the most, residential and commercial areas abound, a clear sign of a relationship between groundwater abstraction (for human consumption) and subsidence, since the land has tendency to settle as it sinks. aquifers.
They add that the causes vary from city to city, since the production of gas and oil can also contribute significantly to the sinking of cities, as is happening in the Gulf of Mexico. A previous study has established that the colossal weight of large cities also influences their collapse.
They also point out that even in cities that are generally stable, parts of them are sinking faster than sea levels are rising.
Many of these cities are so important economically and culturally that future coastal flooding will have a significant impact, both on their economies and on human life.
And they put some significant examples of this global phenomenon.
Istanbul, The capital of Turkey, with a population of 15 million, is the largest city in the country and the most populous in Europe. Although it seems mostly stable, a 5 × 20 km area on the western outskirts of Istanbul is sinking more than 2 mm per year.
some lakes, Nigeria’s capital, with a population of over 24 million, is the most populous metropolitan area in Africa. A 5 × 10 km area in the center is also sinking more than 2 mm per year.
Taipei, Taiwan’s capital is the largest city, with a population of 2.7 million. Most of it shows a subsidence of more than 2 mm per year.
BombayIndia’s second most populous city after Delhi, and the world’s seventh most populous city, with a population of around 20 million, is sinking by more than 2mm a year in much of it -this.
Auckland It is New Zealand’s largest city, with a population of 1.6 million. A significant part of it is sinking more than 2 mm per year.
the domain of tampa bay, which surrounds Tampa Bay on the west coast of Florida (USA), includes the cities of Tampa, Saint Petersburg and Clearwater. It is the 18th largest metropolitan area in the United States, with a population of over 3 million. A large area, located on the northwest side of the Tampa Bay area, including a 25 km long section along the coast, is sinking more than 2 mm per year.
something can be done
The researchers draw attention to a significant fact also verified in this study: cities like Shanghai and Jakarta in Indonesia were sinking more than 10 centimeters per year, but this sinking stopped, likely due to reduced groundwater extraction rates implemented as government regulationsthey write in their article.
In Jakarta, the Indonesian capital in particular, the effects of the sinking had been felt for years. Between 1982 and 2010 sinking rates of up to 280 millimeters per year were recorded in the city.
If this subsidence has slowed down, it has not prevented that a large part of the metropolis is already below sea level and in the grip of floods. As a result, earlier this year, Indonesia’s parliament voted to build a brand new capital and must flee the consequences, according to National Geographic.
However, the sinking stopped or slowed also in Houston in the 1970s, and in Silicon Valley in the 1960s, after corrective measures.
The authors note that these examples show that regulation can be an effective tool to stop subsidence in areas where groundwater extraction is the primary cause.
It will be a challenge to replicate this success in cities around the worldthey conclude in their article.
Subsidence in coastal cities around the world observed by InSAR. Pei-Chin Wu et al. Geophysical Research Letters, Volume 49, Number 7, April 16, 2022; e2022GL098477. DOI: https://doi.org/10.1029/2022GL098477