Scientists have found that global sea levels are climbing more quickly than at any time in the past 4,000 years, posing severe risks to China's major coastal cities. The rapid rise, driven by warming oceans and melting ice, combines with land subsidence from human activities like groundwater pumping. Cities such as Shanghai are implementing measures to stabilize the ground amid these threats.
A team led by researchers from Rutgers University has revealed that sea levels today are rising faster than during any period in the last 4,000 years, with China's coastal regions facing acute dangers. Published in Nature, the study analyzed thousands of geological records from ancient coral reefs and mangrove formations to reconstruct ocean changes over nearly 12,000 years, back to the start of the Holocene epoch after the last ice age.
Since 1900, global sea levels have increased at an average rate of 1.5 millimeters per year—about one-sixteenth of an inch—the quickest pace in at least four millennia. "The global mean sea level rise rate since 1900 is the fastest rate over at least the last four millennia," said Yucheng Lin, a postdoctoral associate at Rutgers and now a scientist at Australia's Commonwealth Scientific and Industrial Research Organization.
The acceleration stems from two key processes: thermal expansion, where warming oceans absorb heat and expand, and the melting of glaciers and ice sheets in Greenland and Antarctica, which add water to the seas. "Getting warmer makes your ocean take up more volume," Lin explained. "And the glaciers respond faster because they are smaller than the ice sheets."
China's megacities, including Shanghai, Shenzhen, and Hong Kong, lie in vulnerable delta regions like the Yangtze and Pearl River Deltas, built on soft sediments that naturally subside. Human groundwater extraction has worsened this, with parts of Shanghai sinking more than one meter—around three feet—during the 20th century, far exceeding the global sea level rise average. "We've been able to quantify the natural rate of sea level rise for this area," Lin said. "But human intervention, mostly groundwater extraction, makes it happen much faster."
Even small rises heighten flood risks in these flat, fertile areas critical for farming, transport, and global manufacturing. "Centimeters of sea level rise will greatly increase the risk of flooding in deltas," Lin noted. "These areas are not only important domestically, they're also international manufacturing hubs. If coastal risks happen there, the global supply chain will be vulnerable."
Progress is evident in places like Shanghai, where regulating groundwater use and reinjecting freshwater has slowed subsidence. "Shanghai now is not sinking that fast anymore," Lin said. "They recognized the problem and started regulating their groundwater usage." The study, supported by the National Science Foundation and NASA, used PaleoSTeHM software to model data and created vulnerability maps for planners. While focused on China, the findings warn other delta cities worldwide, such as New York and Jakarta.