Conference Agenda

The ever-increasing urbanization of the world meets us with pressing socio-environmental challenges. The sprawl of human settlements all over the planet leads to losses of arable land and biodiversity, and increases flood risk and climate change. Considering the developing will of limiting urban sprawl (see for example the No Net Land Take objective), we aim to understand the fundamental structure and dynamics of cities.

We analyze how the internal structure of cities, in particular the share of built-up land, unfolds radially, from center to periphery, revealing patterns that shape urban dynamics. Since cities present a wide variety of sizes, scaling laws provide a powerful framework for modeling such behavior, capturing how a system’s properties shift with its size. Viewing cities as systems and population as their defining scale, we study how cities sprawl as population grows, at the global scale.

We establish a robust radial scaling law which quantifies the connection between the distance to the city center and the share of built-up land, and how this relation scales with city size. This extends the homothetic scaling obtained in previous work to a global sample of cities and at different dates. We work on the 1860 cities of the world whose population is greater than 300,000 inhabitants in 2020. They present a large diversity of population size, topology, land use, urbanization policies and more. Nonetheless, the scaling law applies with surprising regularity. Looking at different points in time — from 1975 to 2020, with a 5 year step — allows us to analyze the evolution of this internal urban structure and scaling law of built-up land.

The dataset used in the study comes from the Global Human Settlement Layer (GHSL), produced by the Copernicus service of the European Commission. It provides high-resolution and high-quality, globally consistent distributions of built-up areas, which we combine with the World Urbanization Prospect database from the United Nations for population statistics. For each city of choice and each date, we analyze this GHS BUILT-S raster layer at 100 meters resolution and compute the average built-up land share in concentric rings of 200 meters width around the city center. To ensure the viable comparability between cities, we rescale for each city the distance to the center proportionally to the square root of its population, using the largest one, Tokyo (with population 37 millions in 2020) as a reference.

We analyze the evolution of the mean rescaled profile, and observe that built-up land increases over time all along the center-periphery profile, even when the size effect is controlled by the homothetic scaling law. In linear scale (measuring absolute land change), the change is especially visible near the center, while it appears more clearly in the periphery on a semilog graph (measuring relative land change). This result means that the built-up surface per capita increases over time globally. We link this urban sprawl phenomenon with economic development and further analyze its geographical variations at national scale on the planet. This clearly questions the sustainability of urban expansion.

Traditionally, urban development is studied in terms of land use in two-dimensions, largely ignoring the third dimension. However, building height is crucial because it enhances the availability of interior space of cities. Using a newly released global building height dataset of almost 3000 cities across 42 countries, we develop a scaling model to simultaneously examine the relationship between urban population size and both horizontal and vertical urban extents. Contrary to expectations, we find that the residents of most urban systems do not significantly benefit from vertical dimension, with population accommodation being primarily driven by horizontal extent. The associations with country-level external indicators demonstrate that the benefits of horizontal extent are more pronounced in urban systems with more extreme size distribution (most population concentrated in few cities). Moreover, building classification tests confirm the robustness of our findings across all building types. Our findings challenge the intuition that building height and high-rise development significantly contributes to accommodation, calling for targeted policies to improve its efficiency beyond land use.

The analogy with percolation is used to describe transitions between fragmented and connected landscapes, or vice versa. Examples include deforestation and the disintegration of forests into increasingly smaller plots. Here, we study settlements as captured by urban land cover. However, cities and settlements have an influence on space that goes beyond their boundaries. Therefore, we model this influence with a function that decreases as a power law with distance and with an exponent controlling this influence. To be more precise, each urban pixel “emits” this influence, so that larger settlements exhibit greater influence their surroundings. In our analysis, we model this influence as a scalar field and calculate it. For a given exponent, we chose a threshold for this intensity and check whether it is in a connected phase. By varying the threshold, we identified the critical threshold. We repeated the procedure while varying the exponent. Applying this approach to European countries, we found an exponential relationship between the values of the exponent and the influence threshold. Higher values of the exponent are associated with lower critical thresholds. We extract the exponential relationship for several European countries. To understand the implications of the exponential relationship between the influence exponent and the field threshold require further research, theoretically formalizing the problem according to the level-set percolation theory.