Coastlines have long attracted dense population and economic activity, and more than 40 percent of people worldwide now live within 100 kilometers of the sea, where they face sea-level rise, erosion, storm-driven flooding and tropical cyclones. Retreat from these zones is often discussed as an adaptation option, but until now its global scale and the factors that drive it had not been systematically quantified.
An international team led by researchers at Sichuan University, working with remote sensing specialists Alexander Prishchepov and Shengping Ding from the University of Copenhagen, mapped settlement movements in 1,071 coastal regions across 155 countries using nighttime light observations combined with socioeconomic datasets. They report that 56 percent of these regions experienced retreat away from the coast between 1992 and 2019, 16 percent-including the Copenhagen area in Denmark-shifted closer to the shoreline, and 28 percent showed little net change.
"More than half of the world's coastal regions are moving away from the shoreline, with Africa (67%) and Oceania (59%) leading the retreat. However, in parts of Asia and South America, many communities continue to expand toward the coast" explains co-author Dr. Shengping Ding, who just defended his PhD thesis on a topic relevant to the article at the Department of Geosciences and Natural Resource Management at the University of Copenhagen.
The authors describe a marked adaptation gap for low-income coastal regions, particularly in Africa and Asia, where many communities either remain in place or expand toward the coast. In these areas, pursuit of economic opportunity, dependence on coastal land and limited alternatives constrain relocation options, leaving large populations exposed to coastal flooding and erosion.
Retreat is most common in middle-income countries, which the team argues sit at a tipping point in terms of resources and capacity. These states often have enough institutional and financial capability to support relocation away from hazard zones but have not reached the income levels needed to rely mainly on expensive protective infrastructure.
In contrast, both low- and high-income regions are more likely to remain near the coast or advance toward it, but for different reasons. Poorer communities often extend shoreline settlements to gain access to jobs, infrastructure and markets, while wealthier regions may trust that existing and planned defenses, early warning systems and engineered protections will manage the growing hazard.
The pattern is visible in Denmark, where recent flood risk and damage have fueled political debate over delayed coastal protection policies. Despite these concerns, the study finds that Denmark, represented by the Copenhagen region, belongs to the minority of coastal areas that have expanded toward the shoreline over recent decades.
"Compared to poorer regions, Denmark has strong infrastructure and greater capacity to adapt to rising coastal risk. Danes also tend to trust that policymakers will do what is necessary to protect vulnerable coastlines. Places like Copenhagen and Aarhus historically served as major trade and industrial coastal cities. However, trust alone may not be enough, with documented erosion in parts of Denmark, proactive inland planning and resilience measures are becoming increasingly important" says Co-author Associate Professor Alexander V. Prishchepov, Department of Geosciences and Natural Resource Management, University of Copenhagen.
The research indicates that the frequency of past coastal disasters is not the main driver of retreat. Instead, the rate at which settlements move inland is more closely linked to current vulnerability, especially where protection infrastructure is limited and communities have constrained social, economic and political capacity to adapt in place.
"Our analysis shows that coastal retreat mostly happens a response to low protection and weak adaptive capacity in places where communities don't have the means to protect themselves. Such regions tend to experience faster retreat, not necessarily because of more hazards, but because they lack the capacity to stay," adds Shengping Ding.
To explore these relationships, the team applied mixed-effects modelling, a statistical approach that accounts for both fixed and varying influences across regions. The model suggests that a 1% improvement in adaptive capacity is associated with a 4.2% decrease in retreat speed, while a 1% increase in structural protection corresponds to a 6.4% reduction in the rate of retreat.
According to Prishchepov, the results have direct implications for coastal adaptation strategies in countries that continue to build along shorelines, including Denmark, where land reclamation and coastal development remain common.
"Our research shows that vulnerability, not just hazard exposure, determines whether communities adapt proactively or are forced to retreat reactively. For countries like Denmark, where coastal expansion continues, understanding these global dynamics is crucial for policymakers to avoid future maladaptation", says Prishchepov. "Globally, we must shift from reactive retreat to proactive planning, integrating social vulnerability into long-term coastal management."
He also calls for more work to refine the picture in regions where satellite nightlights may not fully capture settlement change. "Though this research offers valuable insight into global migration patterns, more study is needed to fully understand them. Particularly in socially-fragile regions such as African countries, night-time light data may not tell the whole story, as economic activity and settlements extent are not necessarily linked to luminous activity in regions with limited electrification," says Prishchepov.
Research Report:Global coastal human settlement retreat driven by vulnerability to coastal climate hazards
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