Coastal populations are experiencing relative sea-level rise “up to four times faster than the global average”, which threatens many megacities located on deltas, according to research led by the University of East Anglia (UEA).
The research is the first to analyse global sea-level rise combined with measurements of sinking land. The impact of subsidence combined with sea-level rise caused by climate change “has until now been considered a local issue rather than a global one”, it concludes.
The study published on Monday in Nature Climate Change found coastal inhabitants are living with an average sea level rise of 7.8mm to 9.9mm per year over the past 20 years, compared with a global average rise of 2.6mm a year.
The impacts are far larger than the global indications previously reported by the Intergovernmental Panel on Climate Change (IPCC) – the UN body whose research is undertaken by leading climate scientists. This latest study involved collaboration with scientists in the UK, Germany and China.
Lead researcher Prof Robert Nicholls, director of the Tyndall Centre for Climate Change Research at UEA, said: “Climate induced sea-level rise is caused by melting glaciers and thermal expansion of water due to rising global temperatures. Rapid rates of subsidence in deltas and especially cities on deltas are also human-caused, mostly due to groundwater pumping, also oil and gas extraction, and sediment resupply prevented by upstream dams, flood defences, sand extraction or mining.”
About 58 per cent of the world’s coastal population lives on deltas where land is subsiding. Less than 1 per cent of the global coastal population lives where land is uplifting. “We wanted to look at the big picture globally, to better understand the impact of global sea-level rise combined with measurements of sinking land,” he added.
“Addressing human-induced subsidence is important in the short term, as it is an essential coastal adaptation to protect people and economies,” Prof Nicholls said.
The researchers assessed four components of relative sea-level change – climate-induced sea-level change; the effects of “glacier weight removal” causing land uplift or sinking, estimates of river delta subsidence, and subsidence in cities.
Sea-level measurements were taken from satellite data. The team then weighted their results by population to show their importance to people. They found high rates of relative sea-level rise were most urgent in Asia as it has many subsiding deltas and coastal flood plains, growing coastal megacities and more than 70 per cent of the world’s coastal population.
They also concluded that over the 20th century, Tokyo experienced net subsidence of 4 metres, while Shanghai, Bangkok, New Orleans, and Jakarta experienced between 2m and 3m subsidence.
In Tokyo, Shanghai and Bangkok, subsidence has been stopped or greatly reduced by reduced groundwater extraction, while in other cities there has been little direct response to reduce subsidence.
One of the main reasons that Jakarta, the capital city of Indonesia, is being moved to Borneo is because the city is sinking due to groundwater extraction from shallow wells, Prof Nicholl noted.
They hoped their analysis improves understanding of how sea-level rise and subsidence are hand-in-hand for science and coastal management policy worldwide. “Jakarta might be just the beginning,” he warned.