Shanghai, China. Credit: Claudia Seidensticker via Pexels
Coastal megacities in southeastern China face a future of accelerating sea level rise compounded by sinking land caused mostly by human activities, according to a new study in the journal Nature.
The research reconstructed changes in sea level along southeastern China’s coast since the beginning of the current geological epoch, the Holocene, which followed the end of the last ice age 11,700 years ago.
The analysis, which included data from geological records from fossilised coral and mangroves, as well as worldwide tide gauges, found that global sea levels have risen at an average of 1.5mm a year since 1900.
“The global mean sea level rise rate since 1900 is the fastest rate over at least the last 4 millennia,” says first author Yucheng Lin, a climate scientist at Rutgers University in the US and Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO).
The authors warn that without effective city-level adaptive risk management strategies, this will pose critical challenges to the sustainability and resilience of China’s densely populated coastal communities, which are home to more than 100 million people.
While the study focused on China, its findings have implications for other major cities built on low-lying coastal plains, such as New York in the US, Indonesia’s Jakarta and Manila in the Philippines.
“Sea-level change, a direct indicator of climate change, poses substantial risks to coastal communities and will continue to do so in the coming century,” the authors write.
Lin says 2 major forces, thermal expansion and melting glaciers, are driving the acceleration of sea level rise.
As the ocean absorbs excess heat and warms, the water in it expands and takes up more volume. Global glaciers and ice sheets in Greenland and Antarctica are also melting and adding more water to the oceans.
The study’s data shows that modern, global sea level rise is happening faster than at any time in the past 4,000 years. Credit: Yucheng Lin
The study also examined modern data on subsistence – the gradual sinking of the land – across 20 of China’s main coastal cities. This was done to determine how sea level rise will adversely affect the country’s delta regions – flat and fertile areas close to water.
“Deltas are great places, good for farming, fishing, urban development and naturally draw civilisations to them,” Lin says. “But they are really flat yet prone to human-caused subsidence, so sustained sea level rise could submerge them really fast.
“Centimetres of sea level rise will greatly increase the risk of flooding in deltas. 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.”
Subsistence can occur naturally through geological process or through human activities such as groundwater extraction.
“Urban activities such as road construction, underground development and building projects can exacerbate natural sediment compaction,” write the authors.
In Shanghai, Lin says, parts of the city sank more than 1m during the 20th century because of excessive groundwater use.
“Shanghai now is not sinking that fast anymore,” he says. “They recognised the problem and started regulating their groundwater usage.”
Vulnerability maps included in the study will help governments and city planners identify hotspots for subsistence and prepare for future sea level rise.