You might think that stormy weather and increasing sea levels are the biggest threat to the survival of coastal cities.
But in many waterfront megacities the ground is now dropping up to 10 times quicker than the sea level is increasing, experts have warned.
The sinking of cities such as Jakarta and New Orleans is largely caused by humans pumping for groundwater and action must be taken for the places to survive.
Megacities on the coast have already sunk below sea level and more are set to follow suit, resulting in increased damage from flooding.
Scientists at Deltares Research Institute in Utrecht studied subsidence in five coastal ‘megacities’: Jakarta, Ho Chi Minh City, New Orleans, Dhaka and Bangkok, which they said will sink significantly unless action is taken.
They found that in north Jakarta, the city has sunk 4 metres in the last 35 years – a fall of 10 to 20cm per year.
Tokyo sank by almost 7ft (2 metres) until the practice of extracting groundwater from beneath parts of the city was halted and Venice also experienced major subsidence in the last century for the same reason.
Geologist Gilles Erken at the university, told BBC News: ‘Land subsidence and sea level rise are both happening, and they are both contributing to the same problem – larger and longer floods, and bigger inundation depth of floods.
At risk: Experts have warned that subsidence is a greater risk than rising sea levels for megacities on the coast. The cities have already sunk below sea level and more are set to follow suit, by sinking between six and 100millimetres per year (pictured)
‘The most rigorous solution and the best one is to stop pumping groundwater for drinking water, but then of course you need a new source of drinking water for these cities. But Tokyo did that and subsidence more or less stopped, and in Venice, too, they have done that.’
The total damage due to subsidence worldwide is estimated at billions of dollars a year and is set to increase. However, the problem is largely underestimated by governments.
Dr Erkens, who presented the study to the European Geosciences Union, explained that the consequences of floods increase due to subsidence, as areas remain deeper under water for longer.
The risk to humans is elevated as well as the economic cost of cleaning up the mess. Movements in the ground also lead to expensive repairs for roads, railways, water pipes and buildings.
He believes that the extraction of water to quench the thirst of rising populations in the vast cities is the primary cause of serious subsidence, while the five cities studied also naturally experience subsidence as they are built upon weaker soil.
Venice under water: Tokyo sunk by two metres until the practice of extracting groundwater from beneath parts of the city was halted and Venice also experienced major subsidence in the last century for the same reason. The Italian city is experiencing less flooding (pictured) since stopping the extraction of ground water
The study shows that subsidence only occurs in the places where the groundwater is being extracted so the solution is to source clean water from elsewhere to stop structures sinking.
‘Government bodies and inhabitants are often not aware that subsidence is an urgent problem,’ the report says.
While the natural reaction to flooding is often to build a barrier, if flooding is caused by subsidence, barriers will be of little help as the cause of the problem – the extraction of groundwater – has not been addressed, it says.
The scientists recommend that subsidence should be included along with water safety in planning to keep coastal cities functioning in the long term.
‘In their future scenarios, many governments only target the consequences of sea level rises, but subsidence is an equally urgent issue.
The sea is rising by 3 to 10mm a year; the ground may sink up to 100cm a year,’ they said.