Urban heat islands (UHIs) exacerbate the risk of heat-related mortality associated with global climate change. The intensity of UHIs is known to vary with population size and mean annual precipitation but a unifying argument is missing, and geographically targeted guidelines for heat mitigation remain elusive. Here we analyze urban-rural surface temperature differences (∆T s) worldwide and find a nonlinear increase of ∆T s with precipitation that is controlled by water/energy limitations on evapotranspiration and that modulates the scaling of ∆T s with city size. We introduce a coarse-grained model linking population, background climate, and UHI intensity and we show that urban-rural changes in evapotranspiration and convection efficiency are the main determinants for warming. The direct implication of these nonlinearities is that mitigation strategies aimed at increasing green cover and albedo are more efficient in dry regions, while cooling tropical cities is a challenge that will require innovative solutions.
The size of cities is known to play a fundamental role in social and economic life. Yet, its relation to the structure of the underlying network of human interactions has not been investigated empirically in detail. In this paper, we map society-wide communication networks to the urban areas of two European countries. We show that both the total number of contacts and the total communication activity grow superlinearly with city population size, according to well-defined scaling relations and resulting from a multiplicative increase that affects most citizens. Perhaps surprisingly, however, the probability that an individual's contacts are also connected with each other remains largely unaffected. These empirical results predict a systematic and scale-invariant acceleration of interaction-based spreading phenomena as cities get bigger, which is numerically confirmed by applying epidemiological models to the studied networks. Our findings should provide a microscopic basis towards understanding the superlinear increase of different socioeconomic quantities with city size, that applies to almost all urban systems and includes, for instance, the creation of new inventions or the prevalence of certain contagious diseases.
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