Two metropolisation gradients in the European system of cities revealed by scaling laws

Urban systems share with other complex systems constraints on their dynamics that are revealed by pervasive structural features, among which scaling laws. Scaling laws are relationships between cities’ attributes and their size (here measured by their population). When the relationship is non proportional with exponents larger than 1, scaling laws indicate the relative concentration of some urban functions at the higher levels of urban hierarchies. Superlinear scaling thus reveals the metropolisation trends that are produced in the urban system, according to our evolutionary theory perspective, by the hierarchical diffusion of innovation waves. Considering the current urban changes linked with the globalisation processes as an ‘innovation’ that is likely to diffuse hierarchically in urban systems, we analyse the relationships between 25 indicators expressive of their position in globalisation processes and the size of European cities (356 largest functional urban areas of the 28 European Union member states plus Switzerland and Norway). When summarised in a single metropolisation factor, we expected to find a unique superlinear scaling relationship that would reveal the hierarchical structure of the unifying European system of cities. We instead identify two distinct metropolisation gradients for each of the Western and Eastern subsystem that we interpret according to the delayed globalisation process in the latter. This provides a demonstration of the usefulness of scaling laws for summarising stages in the process of hierarchical diffusion of innovation in systems of cities.