Subsidence, or the gradual sinking of land, could affect 19% of the world’s population by 2040, according to new research funded by Unesco.
If no action is taken, human activity, combined with drought and rising sea levels exacerbated by global heating, could put many of the world’s coastal cities at risk of severe flooding.
Jakarta has sunk more than 2.5 metres in the past 10 years, causing the Indonesian government to relocate the country’s capital to the island of Borneo.
In Europe, subsidence is responsible for placing 25% of the Netherlands below sea level. Flat coastal regions, as well as urban and agricultural centres in dry climates, are most at risk.
Gerardo Herrera-García, lead researcher on the project, who is attached to the Geological and Mining Institute of Spain, said: “Areas that are heavily populated or areas that need irrigation for agriculture because they are located in places that are dry for long periods of time, they need to pump the water from underground. When they pump the water, the natural recharge of the aquifer is smaller than the volume of water they are pumping out.”
This extraction of water from the ground causes the surface to sink. But lack of pumping regulations and rapidly increasing human populations are the most likely factors contributing to the rates of subsidence.
In Iran, the population has more than doubled in the past 50 years, while groundwater pumping has remained unregulated. The country’s cities are now among the fastest-sinking urban centres in the world, falling by up to 25cm each year.
While subsidence was a common issue throughout the 20th century, previously it was analysed only in a local context.
The new project, developed by an international team of scientists, sought to consolidate existing research. The scientists produced a universally applicable model to predict which areas were most at risk of subsidence.
The results showed that subsidence was a global issue, linked to global heating as well unsustainable farming practices. “The largest aquifers in the world are being depleted for agricultural purposes,” said Herrera-García.
According to Herrera-García, groundwater in the US, Mexico, China, and India, is being rapidly drained to meet global food demand. Continued subsidence in those areas will affect populations worldwide. Making global food production sustainable was possible, said Herrera-García, but the problem would have to be addressed soon.
Additionally, global warming is predicted to cause prolonged periods of drought, which will accelerate the rate of subsidence as more water is pumped from underground.
Meanwhile, sea levels are expected to rise by up to a metre in the next century. This means that more coastal cities will encounter the same problems as Jakarta, as more areas will become prone to flooding.
However, Herrera-García said that while subsidence was a big threat to global environment, it could be fixed far more easily than climate change. Technologies, such as satellites and radars, could quickly identify areas of subsidence, while “simple policies and tools” could be used by local authorities to efficiently combat the problem.
“In Tokyo they had a very big problem of subsidence in the first part of the last century. They implemented groundwater regulations and they solved the problem.”
Other solutions to subsidence include finding alternative water sources, practising efficient agriculture to use as little water as possible, and injecting water back into aquifers.
“These solutions are the same everywhere and can be applied to both large aquifers and smaller ones,” Herrera-García said. “I think we are on time. The solutions are there, and this is the time to implement them.”