Some Coastal Cities Are Sinking Faster Than the Ocean Is Rising

Coastal communities are facing a hidden threat that is making rising seas far more dangerous: the land beneath them is sinking, ScienceDaily reported on August 2. New research shows that people in densely populated coastal regions experience an average relative sea-level rise of about 6 millimeters per year, nearly three times the global average.

The Hidden Threat

Most public discussions of sea-level rise focus on the ocean itself. But coastal land is also moving - down - in many places, driven by groundwater extraction, sediment compaction, and tectonic activity. When the two motions combine, the effective sea-level rise at the coast is much larger than the global average. The new study assembled satellite and tide-gauge data to produce the first global estimate of the combined effect.

"Subsidence is the silent partner of sea-level rise. You only notice it when the water reaches your doorstep," the lead author told ScienceDaily.

Where It's Worst

South and Southeast Asian cities - Jakarta, Manila, Bangkok, Ho Chi Minh City - are the most exposed, with effective sea-level rise in some neighborhoods exceeding 10 millimeters per year. Many US Gulf Coast and Middle Eastern cities face similar rates. The pattern correlates strongly with groundwater extraction, suggesting that policy choices - not just climate choices - are driving the worst outcomes.

What's Next

The team is sharing the dataset with urban planners and adaptation agencies. Several cities - including Jakarta, which has begun moving its capital - have already begun subsidence-mitigation programs, primarily by reducing groundwater extraction. The work was published in the journal Nature Climate Change.