A multi-city analysis of the natural and human drivers of the urban heat island

Abstract

The world's population is increasingly moving to cities, with a present day urban population of over 3.6 billion that is expected to nearly double by 2050. One of the key features of the urban environment is an increase in temperature relative to the surrounding rural areas, called the urban heat island, which can have negative impacts on the health and wellbeing of urban dwellers. This study uses a novel approach of analyzing a large number of cities from around the world to investigate the similarities and differences in urban environments among cities to explore the behavior and drivers of the urban heat island. This methodology reveals two new conditions that increase the magnitude of the heat island - low dewpoint temperature and high air temperature. Many of the cities show increases in the magnitude of the heat island during hot or dry periods of 1.0 °C or more during the daytime and 2.0 °C at night relative to cool or humid periods. The heat wave results are of particular note due to the added stress on urban residents during periods when the population is already at risk. For cities in temperate climate regimes, differences among cities in vegetative cover or impervious surface area leads to increases in urban temperatures of up to 1.0 °C during the summer, while cities with high pollution can see reductions in the heat island by 1.5 °C. Cities in tropical or Mediterranean climates have the strongest heat islands during the dry season indicating that urban infrastructure is the key driver in these cities. These results indicate that mitigation of the urban heat island is possible by altering the urban landscape through changes in the urban vegetation and the structure of the built environment.