Climate crisis: Relative sea levels rising ‘up to four times faster’ in coastal cities, due to widespread subsidence

The world’s first combined assessment of sinking land and rising water makes for chilling reading, writes Harry Cockburn

(Getty Images)

Amid the worsening climate crisis, sea level rise presents a rapidly growing risk to millions of people living in coastal areas around the world, but new research reveals sinking land in many of the most vulnerable cities is exacerbating the problem.

Around our planet, sea levels have risen by around 21 centimetres since 1900, but over recent decades the process has speeded up, and global average sea level rise is now about 2.6mm a year.

However, the new study reveals that due to subsidence, most coastal populations are exposed to relative sea level rise up to four times faster than the global average.

The study, which is the first to analyse global sea-level rise combined with measurements of sinking land, shows that coastal inhabitants are living with an average sea level rise of 7.8mm - 9.9mm per year over the past twenty years.

The research team said the impact of subsidence combined with sea-level rise has until now been considered a local issue rather than a global one, and warned the impacts are “far larger than the global numbers reported by the Intergovernmental Panel on Climate Change (IPCC)”.

Lead researcher Professor Robert Nicholls, director of the UK’s Tyndall Centre for Climate Change Research and chair of climate adaptation at the University of East Anglia, 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 global coastal population lives where land is uplifting.”

The research team assessed four components of relative sea-level change. These were: climate induced sea-level change, the effects of glacier weight removal - which can cause 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.

“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,” said Professor Nicholls.

“We found that coastal populations live with sea-level rise at three and four times the global average and that the impacts of sea-level rise being experienced today are much larger than the global numbers being reported by the IPCC.

“Addressing human-induced subsidence is important in the short term, as it is an essential coastal adaptation to protect people and economies,” he said.

The team found rates of relative sea-level rise are most urgent in south, south east and east Asia as the area has many subsiding deltas and coastal flood plains, as well as growing coastal megacities and more than 70 per cent of the world’s coastal population.

They also found that over the 20th Century, the city of Tokyo experienced net subsidence of 4 metres, while Shanghai, Bangkok, New Orleans, and Jakarta, have experienced between 2 metres and 3 metres subsidence.

In Tokyo, Shanghai and Bangkok the subsidence has been stopped or greatly reduced by reduced groundwater extraction, while in other cities there has been little direct response to reduce subsidence.

Professor Nicholls said: “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.

“We hope that our analysis improves the 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.”

The research is published in the journal Nature Climate Change.