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Sinking Land More Than Doubles Sea Level Rise on the World's Most Crowded Coasts
Sinking land is more than doubling the rate of sea level rise experienced by people living along the world's most densely settled coastlines, according to research published on 2 August in Nature Communications. The study, carried out by the German Geodetic Research Institute at the Technical University of Munich and by Tulane University in New Orleans, finds that residents of heavily populated coastal regions face relative sea level rise averaging about six millimetres a year, close to three times the global average of 2.1 millimetres measured across all coastlines.
The difference between those two figures is the ground itself. Sea level rise attributable to a warming climate currently runs at roughly 3.15 millimetres a year. Where the land beneath a city is also subsiding, the two effects compound, and the water arrives considerably faster than ocean measurements alone would indicate. More than half a billion people live in low elevation coastal zones exposed to that combination.
National rates weighted by population put Thailand, Bangladesh, Nigeria, Egypt, China and Indonesia at the top, between roughly seven and ten millimetres a year. The United States, the Netherlands and Italy sit between four and five millimetres. Individual cities are starker. Jakarta averages 13.7 millimetres a year, Tianjin 13.5, Bangkok 8.5, Lagos 6.7 and Alexandria four. Conditions vary sharply within a single city, and the extremes are considerable.
The drivers are largely local and largely human. Groundwater pumping, oil and gas extraction, the compaction of young sediments in river deltas and the accumulated weight of urban construction all push the surface downward. Tectonic movement and the continuing adjustment of the crust following the retreat of ancient ice sheets add a slower background signal. In Sweden and Finland that rebound is strong enough to outpace the ocean, producing a relative fall in sea level along parts of the coast.
For asset owners, lenders and insurers, the finding reframes coastal exposure as a question about the land as much as the sea. Flood defences, drainage networks, port facilities and building foundations are engineered against expected water levels, and a design assumption drawn from global averages understates the hazard in precisely those places where population and property are most concentrated. Because vertical ground motion varies over short distances, assessments built on national or regional figures can conceal sharply localised risk within a portfolio.
Subsidence, unlike ocean warming, responds to local policy. Tokyo once sank by more than ten centimetres a year before regulation and alternative water supplies slowed it substantially, and Texas established the Harris Galveston Subsidence District in 1975 for much the same purpose. Both cases indicate that the sinking half of the problem remains tractable.