Land subsidence can intensify the impact of rising sea levels in densely populated coastal regions across the world, regions already vulnerable to flooding, a study has found.
Researchers, including those from Germany’s Technical University of Munich and Tulane University in the US, found that people in densely populated coastal regions experience a relative sea-level rise of about six millimetres per year on average — nearly twice of climate-driven absolute sea-level rise of around 3.15 millimetres per year.
The amplified rise is caused by sinking land, a process known as subsidence, they said.
“We find that the average modern (1995-2020) global RSL (relative sea-level) rise experienced by coastal populations (six millimetres per year) is about twice the climate-driven absolute sea-level rise," the authors wrote in the study published in the journal Nature Communications.
“This reflects a strong tendency for higher rates of subsidence in densely populated areas, with 71 per cent of the global coastal population living in subsiding regions," they said.
The team added that intensive groundwater extraction, oil and gas production, compaction of young sediments in delta regions and structural loads in rapidly growing cities are among the most important factors that drive land subsidence.
“If we want to understand sea-level rise along coastlines and respond effectively, we must not only observe the ocean but also the land itself. Especially in densely populated coastal regions, human activities cause the land to subside more strongly — often due to excessive extraction of water and resources that previously stabilized the subsurface," lead author Julius Oelsmann, a researcher at the German Geodetic Research Institute, Technical University of Munich, said.
“The sheer weight of cities, along with long-term geological processes, can further intensify this subsidence. In doing so, we significantly amplify the effects of climate-driven sea-level rise," Oelsmann said.
Prominent hot spots of subsidence include Jakarta in Indonesia (-13.7 millimetres per year), Tianjin in China (-13.5 millimetres per year), Bangkok in Thailand (-8.5 millimetres per year), Lagos in Nigeria (-6.7 millimetres per year), and Alexandria in Egypt (-4 millimetres per year).
The researchers added that many Asian and African coastal cities as well deltas, where coastal populations are concentrated, are poorly instrumented or the data is not publicly available, such as in China or India.
Conversely, regions such as those in Sweden or Finland show a geological uplift leading to a relative decrease in sea level along coastlines, the land continues to rise as a result of post-glacial rebound following the last Ice Age — and does so faster than sea levels are increasing, the team said.
The researchers suggested groundwater management can counter subsidence.
“In many large coastal cities, groundwater extraction is a major driver of land subsidence. This means that local political and water management decisions can make a significant difference," author Florian Seitz, professor of geodetic geodynamics and director of the German Geodetic Research Institute, said.
“Improved groundwater management, stricter regulation of withdrawals, or targeted recharge of aquifers can at least slow subsidence rates and, in some cases, largely halt them," Seitz said.
