Faster sea level rise threatens China’s coastal megacities

A team of scientists led by Rutgers researchers has uncovered evidence showing modern sea level rise is happening faster than at any time in the past 4,000 years, with China’s coastal cities especially at risk.

Reporting in Nature, their findings show that since 1900, global sea levels have risen at an average rate of 1.5 millimeters or about one-sixteenth of an inch a year, a pace that far exceeds any century-long period in the past four millennia.

“The global mean sea-level rise rate since 1900 is the fastest rate over these last four millennia,” said Yucheng Lin, who conducted the research as a postdoctoral associate at Rutgers and is a scientist at Australia’s national research agency, the Commonwealth Scientific and Industrial Research Organization in Hobart.

The scientists examined thousands of geological records drawn from a number of sources, including ancient coral reefs and mangroves, which serve as natural archives of past sea levels. They reconstructed sea level changes going back nearly 12,000 years, which marks the beginning of the current geological epoch, the Holocene, which followed the last major ice age.

Two major forces, thermal expansion and melting glaciers, are driving this acceleration, Lin said. As the planet warms because of climate change, oceans absorb heat and expand. At the same time, ice sheets in Greenland and Antarctica are melting, adding more water to the oceans.

“Getting warmer makes your ocean take up more volume,” Lin said. “And the glaciers respond faster because they are smaller than the ice sheets, which are often the size of continents. We are seeing more and more acceleration in Greenland now.”

While rising seas are a global issue, China faces a unique double threat, he said. Many of its largest and most economically important cities, including Shanghai, Shenzhen and Hong Kong, are in delta regions, which are naturally prone to sinking because they were built above thick and soft sediments.

“Deltas are great places, good for farming, fishing, urban development and naturally draw civilizations to them,” Lin said. “But they are really flat yet prone to human-caused subsidence, so sustained sea level rise could submerge them really fast.”

Although the researchers focused on China, lessons from the study apply globally, Lin said. Many major cities, such as New York, Jakarta and Manila, are built on low-lying coastal plains and face similar risks.

“We’ve been able to quantify the natural rate of sea level rise for this area,” Lin said. “But with human intervention, mostly groundwater extraction, we see subsidence even faster.” 

Subsidence refers to the gradual sinking or settling of the Earth’s surface. It can happen naturally because of geological processes, or it can be caused by human activities, such as groundwater extraction.

To determine how sea level rise will adversely affect China’s deltas, the team examined a combination of geological records, subsidence data and human activity impacts across coastal regions, especially in the Yangtze River Delta and Pearl River Delta. These areas are home to several megacities.

In Shanghai, parts of the city sank more than one meter (about three feet) during the 20th century because of excessive groundwater use, Lin said. That is orders of magnitude faster than the current global sea level rise rate.

“Centimeters of sea level rise will greatly increase the risk of flooding in deltas,” Lin said. “Not only are these areas important domestically, they’re also important internationally. If coastal risks happen there, the global supply chain will be vulnerable.”

Despite the findings, Lin’s research offers hope, he said. Cities such as Shanghai have already taken steps to reduce subsidence by regulating groundwater use and even reinjecting freshwater into underground aquifers. “Shanghai now is not sinking that fast anymore,” Lin said, referring to actions the city took in the late 20^th Century, and more recently. “They recognized the problem and started regulating their groundwater usage.”