Urban heat islands (UHIs) exacerbate the risk of heat-related mortality associated with global climate change. The intensity of UHIs varies with population size and mean annual precipitation, but a unifying explanation for this variation is lacking, and there are no geographically targeted guidelines for heat mitigation. Here we analyse summertime differences between urban and rural surface temperatures (ΔTs) worldwide and find a nonlinear increase in ΔTs with precipitation that is controlled by water or energy limitations on evapotranspiration and that modulates the scaling of ΔTs with city size. We introduce a coarse-grained model that links population, background climate, and UHI intensity, and show that urban–rural differences in evapotranspiration and convection efficiency are the main determinants of warming. The direct implication of these nonlinearities is that mitigation strategies aimed at increasing green cover and albedo are more efficient in dry regions, whereas the challenge of cooling tropical cities will require innovative solutions.
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G.M. was supported by the The Branco Weiss Fellowship—Society in Science administered by ETH Zurich. E.B.-Z. acknowledges support by the US National Science Foundation under grant no. ICER 1664091, the SRN under cooperative agreement no. 1444758, and the Army Research Office under contract W911NF-15-1-0003 (program manager J. Barzyk). M.S. was supported by the Future Cities Laboratory at the Singapore-ETH Centre, which was established collaboratively between ETH Zurich and Singapore’s National Research Foundation (FI 370074016), under its Campus for Research Excellence and Technological Enterprise programme. We thank P. Edwards, J. Carmeliet, C. Küffer, and D. Richards for help and discussions at the beginning of this research.
The authors declare no competing interests.
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Peer review information Nature thanks Lahouari Bounoua, Ben Crawford and Qihao Weng for their contribution to the peer review of this work.
Supplementary Methods, Supplementary Tables 1–6, Supplementary Figs 1–25 and Supplementary References.