Urban climate archipelagos: a new framework for urban impacts on climate

Abstract

Earth is increasingly an “urbanized” planet. The “World Population Clock” registered a Population of 7,175,309,538 at 8:30 pm (LST) on Oct. 6, 2013. Current and future trends suggest that this population will increasingly reside in cities. Currently, 52 percent of the world population is urban, which means we are a majority “urbanized” society. Figure 1 indicates this trend will continue, with particular growth in less developed regions. In terms of ecological impacts, urbanization is one of the more significant and long-lasting forms of land transformation, and its extent of increase is at least proportional to population growth and economic development. Urbanization exerts environmental and climatic pressures on surrounding lands that are not fully understood. Viewed from the perspective of the amount of space it currently occupies, urbanization appears to be a minor form of land transformation. It is estimated that urbanization in the U.S. occupies about 3 percent of the land surface (Imhoff et al. 2004). In terms of climate impacts, urbanization affects its environment through different physical mechanisms: 1) the reduction of the fraction of vegetation and the subsequent reduction in photosynthesis and plant’s water transpiration and interception, 2) the alteration of water infiltration and surface runoff and their impacts on soil moisture and the water table, 3) the alteration of surface albedo and its effect on the surface energy partitioning, and 4) the modification of the surface roughness and its implication for the turbulent exchanges of water, energy, and momentum fluxes, all of which affect climate. The urban heat island (UHI) is a fairly well-understood urban-climate interaction (Grimmond et al. 2010; Stone et al. 2010; Zhou and Shepherd 2010). Urban areas are known to create the so-called UHI effect. On the other hand, using satellite observations over 38 of the most populous cities in the continental U.S., Imhoff et al. (2010) have shown that on a yearly average, urban areas are substantially warmer than the non-urban fringe by 2.9 degrees Celsius (C), except for urban areas within arid climates which behave as an urban heat sink. They also show that the UHI amplitude is remarkably asymmetric with an average 4.3 degrees C in summer and only 1.3 degrees C in winter. Seto and Shepherd (2009), Mitra et al. (2011) and Shepherd (2013) described how the urban environment affects other major climate systems (e.g., the water cycle, biogeochemical cycles, and weather).

Citation

Shepherd, J. Marshall; Andersen, T.; Strother, Chris; Horst, A.; Bounoua, L.; Mitra, C. 2013. Urban climate archipelagos: a new framework for urban impacts on climate. Earthzine.