Subsidence in urban areas is an underestimated danger, according to scientists from the University of East Anglia (UEA) and Virginia Tech.

Professor Robert Nicholls from UEA’s Tyndall Center for Climate Change Research and Professor Manoochehr Shirzaei from Virginia Tech University and the UN University for Water, Environment and Health, Ontario, write in the journal Sciencehighlights the importance of new research analyzing satellite data. It accurately and consistently reflects the movement of the world in China.

While they say in their comments that measuring collapse consistently is a great achievement, they also say that this is just the beginning of finding a solution. Predicting future collapses requires models that take into account all factors, including human activities and climate change, and how these may change over time.

A research paper published in the same issue examines 82 cities with a total population of approximately 700 million. The results show that 45% of the urban areas analyzed are collapsing, with 16% subsiding at a rate of 10 mm or more per year.

Historical and future challenges

An estimated 270 million urban residents are affected in the country, with approximately 70 million experiencing subsidence at a rate of 10 mm or more per year. Hotspots include Beijing and Tianjin.

Coastal cities such as Tianjin are particularly affected as land subsidence exacerbates climate change and sea level rise. One of the reasons why the flooding caused by Hurricane Katrina brought so much destruction and death to New Orleans in 2005 was the inundation of sea defenses.

Shanghai, China’s largest city, sank 3 meters in the last century and continues to sink today. If subsidence is combined with sea level rise, China’s below-sea urban area could triple by 2120, affecting 55-128 million residents. Without a strong public response, it could be disastrous.

“Collapse threatens the structural integrity of buildings and critical infrastructure and exacerbates the effects of climate change in the form of flooding, particularly in coastal cities where sea level rise is driving the project,” said Professor Nicholls, who was not involved in the study but led the research. Research focuses on sea level rise, coastal erosion and flooding, and how communities can adapt to these changes.

Causes and reduction of subsidence

Subsidence is mainly caused by human activities in cities. Groundwater extraction, which lowers groundwater levels, is considered the most important factor in subsidence, along with the geology and the weight of buildings.

In Osaka and Tokyo, groundwater extraction stopped in the 1970s and urban subsidence stopped or was greatly reduced; this points to an effective mitigation strategy. Road vibration and tunneling are also potentially a local contributing factor; Beijing is sinking at a rate of 45 mm per year near subways and highways. Natural land movement up or down also occurs, but is usually much less than human-caused changes.

Although human-caused subsidence in China was known before this study, Professors Nicholls and Shirzai say these new findings reinforce the need for a national response. This problem occurs in sensitive cities outside of China and is a widespread problem worldwide.

They encourage the research community to move from measurement to understanding results and supporting responses. New satellite measurements provide new and detailed data on subsidence, but the ways in which they use this information need to be significantly improved by working with urban planners to solve these problems. Affected coastal cities in China and in general need special attention.

“Many cities and regions around the world are developing strategies to manage the risks of climate change and sea level rise,” Professor Nicholls said. “We must learn from this experience to also address the threat of land subsidence, which is more widespread than currently recognized.”