Urban land subsidence

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The sinking of the ground under a city, most often because water is being pumped out of the sediments beneath it faster than it is replaced, compacting them irreversibly. It is the adaptation problem that is routinely mistaken for a sea-level problem: in many delta cities the ground is falling several times faster than the sea is rising, so relative sea level at the coast is dominated by the local term rather than the global one. Jakarta is the most-cited case, with recent assessments giving a citywide average around three and a half centimetres a year and considerably more in the north; Bangkok, Ho Chi Minh City, Manila, Semarang and parts of the Chinese coast show the same mechanism. The causal chain runs straight back through this Journey: where a piped network does not reach households and businesses, they sink boreholes; the aquifer draws down; the ground compacts; the flood defences have to be raised; and the cost of the missing water network is paid, with interest, as a coastal engineering bill.

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