The influence of urban warming on large-scale air surface temperature trends is actively debated. Analyzes of temperature minimum at stations worldwide indicate near equal trends on windy and calm nights for 1950-2000. Assuming that the strongest heat islands develop at night under calm conditions, it has been implied that cities do not affect large-scale warming. However, most urban weather stations used in the study are located in parks or airports away from signal contamination by the built environment, consequently missing heat islands effects. The similarity of minimum temperature trends on windy and calm nights might simply reflect near similar heat fluxes regimes at rural and urban weather stations. In most cities, the weather station network is too sparse to accurately record the temperature variations associated with heat islands. Those are best resolved by satellite sensing, as seen in the thermal images series over London and Paris in August 2003. Both cities experienced significant heat islands up to 8 - 10ºC, centered in downtown at nighttime, and scattered over industrial and densely built suburbs at daytime. In Paris, contrary to prevailing concepts, the magnitudes of temperature anomalies were larger in daytime than at nighttime. The influence of urban warming on large scale air surface temperature has yet to be demonstrated. However, urban contribution to regional climate change has already been observed using satellite remote sensing techniques, and twenty five years of archived data are available to study the climate trends of urbanization, and its impact.

Keywords: Urbanization, Climate trends, Regional climate change, Urban Heat Island, Satellite remote sensing, London, Paris.