Interactions of Global-Warming and Urban Heat Islands in Different Climate-Zones

Abstract

IPCC results show that global-warming over the last 35 years has not been spatially uniform over the globe on both the seasonal and diurnal time-scales. Urban areas likewise produce their own climates, e.g., cities cool less rapidly than their rural surroundings at night, thus forming nocturnal urban heat island (UHIs). As UHIs interact with regional global climate-changes, this paper investigated interactions between these two phenomena in different climate zones around the world. It first focused on the spatial distribution of 2-m summer maximum-temperature trends for the period of 1970–2005 in the highly populated Southern California Air Basin, which exhibited a complex pattern of cooling in low-elevation coastal-areas and warming at inland areas. The coastal cooling resulted as global warming of inland areas produced enhanced cool-air sea breeze intrusions, i.e., a “reverse reaction” to global warming. To investigate interactions between global warming and UHI-growth, pairs of sites were identified near cooling-warming boundaries. The faster each urban area grew, the faster its UHI grew. To determine where UHI-growth and global climate-change either are additive or in opposition, requires understanding of the global distribution of climate types and of the diurnal and ­seasonal patterns of UHI-formation in each climate type. UHI formation is a ­function of the thermal inertia (TI) of adjacent rural surfaces. Coastal-cooling is most likely in west-coast marine-Mediterranean climates. Urban cool islands counter global-warming and reduces thermal-stress events in dry rural-soil climates, while heat-stress events are most-likely in wet rural-soil climates.