Unequal Rise of Population Exposure on Steep Terrain

Landslides may be mainly controlled by terrain steepness, but population exposure on hazardous slopes is shaped by human settlement decisions. Yet, despite growing recognition of landslide risk under a changing climate, we still lack a clear global picture of who is settling in steep terrain, how fast this exposure is changing, and where new hotspots are emerging. In this study, we take a global perspective on this challenge by combining terrain, population, and settlement datasets to explore how population exposure on steep hillslopes (≥10°) has evolved over the past five decades.
We find that the number of people living on steep hillslopes has increased by nearly 350 million since 1975. This growth is highly variable in terms of geography, socio-economic status and settlement types. The largest increases are seen in parts of South and Southeast Asia, tropical East Africa, and Central America. Our results show that low- and lower-middle-income countries account for almost 60% of the population living on steep hillslopes. Much of the growth appears to be driven by peri-urban and expanding urban settlements that are pushing outward into more marginal terrain. Broader structural drivers such as rapid population growth, land scarcity, and institutional fragility also seem to play a role. For example, three-quarters of the top 20 hotspots (by absolute exposure growth) fall in countries ranking lowest on the World Bank defined ‘Political Stability and Absence of Violence’ index.
These patterns suggest that the geography of landslide exposure is not just a function of physical terrain but is being actively reshaped by human dynamics, particularly at the urban fringe. As population pressure continues to rise in regions with limited flat land, and extreme rainfall events become more frequent, landslide risk on steep terrain is likely to keep growing. We identify hotspots of rapid growth and the potential drivers. These results provide a global baseline for more targeted landslide risk assessment and urban resilience planning.