Research indicates that land subsidence is often neglected as a danger in urban areas. Scientists utilized satellite data to provide accurate and thorough tracking of land movement throughout China.
According to researchers from the University of East Anglia (UEA) and Virginia Tech, land subsidence is often ignored as a danger in cities.
In an article published in the journal Science, Prof Robert Nicholls from the Tyndall Centre for Climate Change Research at UEA and Prof Manoochehr Shirzaei from Virginia Tech, along with the United Nations University for Water, Environment and Health, Ontario, underscore the significance of a new research paper that analyzes satellite data to systematically track land movement across China.
They commend the accomplishment of consistently measuring subsidence but argue that this is just the beginning of finding solutions. To predict future subsidence, models that account for all contributing factors, including human activity and climate change, along with their potential changes over time, are essential.
The accompanying research paper examines 82 cities with a combined population nearing 700 million. Findings indicate that 45% of the analyzed urban areas are sinking, with 16% experiencing subsidence at a rate of 10mm annually or higher.
It is estimated that about 270 million urban dwellers nationwide are impacted, with nearly 70 million facing rapid subsidence of 10mm or more each year. Cities like Beijing and Tianjin are identified as major hotspots.
Coastal cities, including Tianjin, are particularly vulnerable as sinking land exacerbates climate change and rising sea levels. The failure of sea defenses contributed significantly to the flooding and devastation experienced in New Orleans during Hurricane Katrina in 2005.
Shanghai, as the largest city in China, has subsided by nearly 3m over the past century, continuing to sink today. If subsidence continues alongside rising sea levels, the area in China that lies below sea level could rise by threefold by the year 2120, potentially affecting between 55 to 128 million people, which could have dire consequences unless there is a robust societal response.
“Subsidence threatens the structural stability of buildings and vital infrastructure, while also worsening the effects of climate change, especially in coastal cities where it intensifies rising sea levels,” stated Prof Nicholls, who although not involved in the study, specializes in sea-level rise, coastal erosion, flooding, and community adaptation to these changes.
The primary cause of subsidence in these urban environments is human activity. The extraction of groundwater, which lowers the water table, is identified as the most significant factor driving subsidence, along with geological characteristics and the weight of structures.
In Osaka and Tokyo, the cessation of groundwater extraction in the 1970s halted or significantly reduced city subsidence, demonstrating this mitigation strategy’s effectiveness. Local factors such as traffic vibrations and tunneling may also contribute to subsidence; for instance, Beijing experiences sinking of up to 45mm annually in areas close to subways and major roads. While natural land movement occurs, it is usually much less significant than changes caused by human actions.
Though human-induced subsidence was already known in China prior to this study, Profs Nicholls and Shirzaei emphasize that these new findings highlight the urgent need for a national response. This issue also arises in vulnerable cities outside of China and is a global concern.
They urge the research community to transition from merely measuring subsidence to understanding its implications and aiding in developing responses. These new detailed satellite measurements provide valuable subsidence data, but additional efforts are needed to collaborate with city planners to tackle these challenges. Coastal cities in China, among others, require focused attention.
“Numerous cities and regions worldwide are formulating strategies for tackling the risks associated with climate change and sea-level rise,” commented Prof Nicholls. “We must learn from these experiences to also confront the often-overlooked threat of subsidence.”
