Why are the world’s cities sinking?

{"text":[[{"start":null,"text":""}],[{"start":6.33,"text":"The writer is a science commentator"}],[{"start":8.395,"text":"Houston, as the saying nearly goes, has a problem. The city is one of 25 American metropolises to be officially sinking, according to a new study using measurements from space."}],[{"start":21.2,"text":"The issue of subsidence affects not only coastal regions, where sea levels are rising due to climate change, but inland settlements too. Groundwater extraction hollows out the sediment beneath the surface, while urban development piles on the weight above. The net effect is a slow slump into the ground."}],[{"start":43.2,"text":"That slippage can weaken vital infrastructure, including buildings, bridges and sewers; reduce the capacity of underlying aquifers to hold water; increase flood risk; cause sinkholes. The new findings add to the need to take subsidence more seriously, a risk compounded by climate change and urban population growth. Just because the sinking is happening slowly does not mean it should be ignored."}],[{"start":71.939,"text":"Radar satellites can be used to measure ground elevation, by transmitting microwave pulses towards Earth and measuring how long it takes for the echo to return. Leonard Ohenhen, from Columbia University’s Lamont-Doherty Earth Observatory, teamed up with researchers mostly at Virginia Tech to look at data from the Sentinel-1 satellite collected between 2015 and 2021. By comparing echoes taken at different times over the country’s 28 most populous cities, they were able to calculate how much the ground had moved either up (uplift) or down (subsidence)."}],[{"start":112.37,"text":"As they reported last week in the journal Nature Cities, 25 cities showed subsidence, on average, rather than uplift. Of those, the Texan cities of Houston, Fort Worth and Dallas fared worst, showing average subsidence of more than 4mm per year (oil and gas drilling also empties underlying sediment). Some parts of Houston are sinking by more than 10mm a year."}],[{"start":142.216,"text":"That seems paltry by the standards of other notoriously sinking cities, such as Jakarta and Tehran, but those totemic examples almost demand their own scale. Indonesia has created a new capital, Nusantara, partly because of Jakarta’s subsidence of up to 15cm a year. About half of Jakarta — home to 11mn people — now lies below sea level. Thanks to drought and poor water management, parts of Tehran are sinking by up to 31cm a year; cracks are appearing in roads, world heritage sites and the airport. The current Iranian president has floated the idea of moving the capital, which faces continuing chronic water shortages."}],[{"start":190.542,"text":"But even a modest average rate of sinking, such as those seen in US metropolises, can mask variation, with some subsidence hotspots occurring in regions with an overall uplift. “From an urban risk perspective, the cities with the greatest spatial variability may experience the greatest hazard to urban infrastructure,” the authors write, pointing out that structures can silently weaken as neighbouring areas move and twist under differential subsidence."}],[{"start":223.067,"text":"Natural factors matter too, such as seismic hazards, soil composition and “glacial isostatic adjustment”, a kind of rebound sinking associated with the historic melting of nearby glaciers (as in New York). But the problem of human-induced subsidence is global, urgent and spreading, particularly in fast-growing cities."}],[{"start":248,"text":"China is a slumping hotspot, with nearly half of its cities, including Beijing, heading downwards. Mexico City is another capital on the slide. One 2024 paper estimated that nearly 2bn people globally were living in subsidence-affected areas and called it “the sinking crisis”."}],[{"start":268,"text":"Current warming aggravates the risk: thawing permafrost is causing subsidence in Alaska; rising seas combine with falling land to make flooding more frequent; climate-induced drought stokes demand for ever more water extraction, causing further destabilisation."}],[{"start":288,"text":"Ohenhen cites Lagos as another megacity under siege. Nigeria’s low-lying coastal metropolis, built on water-saturated sediment, now boasts tilted buildings, cracked pillars and walls, and even collapsed buildings. When structures fail, he points out, subsidence is rarely if ever considered; investigations focus on building codes, engineering problems or human error."}],[{"start":317.532,"text":"But, Ohenhen says, “the compounding effect of subsidence may push parts of the city towards sudden tipping points . . . cities can and should act”. Mitigations include capturing surface water to reduce the need to extract groundwater; recharging aquifers; limiting development in subsidence-prone areas."}],[{"start":341.05,"text":"It can work: stricter water management introduced from the 1950s put the brakes on subsidence in Tokyo and Osaka. We may be in a global slouch — but we can yet rise."}],[{"start":null,"text":""}],[{"start":360.15,"text":""}]],"url":"https://audio.ftmailbox.cn/album/a_1747625848_8929.mp3"}