The urban heat island effect (UHI) has been widely observed globally, causing climate,
health, and energy impacts in cities. The UHI intensities have been found to largely
depend on background climate and the properties of the urban fabric. Yet, a complete
mechanistic understanding of how UHIs develop at a global scale is still missing. Using
an urban ecohydrological and land-surface model (urban Tethys-Chloris) in
combination with multi-source remote sensing data, we performed simulations for 49
large urban clusters across the Northern Hemisphere in 2009-2019 and analysed how
surface and canopy air UHIs (SUHI and CUHI, respectively) develop during day and
night. Biophysical drivers triggering the development of SUHIs and CUHIs have similar
dependencies on background climate, but with different magnitudes. In humid regions
daytime UHIs can be largely explained by the urban-rural difference in
evapotranspiration, whereas heat convection and conduction are important in arid
areas. Plant irrigation can largely promote daytime urban evapotranspiration only in
arid and semi-arid climates. During night, heat conduction from the urban fabric to the
environment creates large UHIs mostly in warm arid regions. Overall, this study
presents a mechanistic quantification of how UHIs develop worldwide and proposes
viable solutions for sustainable climate-sensitive mitigation strategies.