Urban Heat Islands

Urban climates have existed ever since the first human settlements altered the natural environment thousands of years ago. During the first century ce, Pliny the Elder recorded anecdotal accounts about the differences between the air in ancient Rome and the air in rural surroundings. Urban development accompanied global population growth over the following centuries. Today, well over half of the world’s population lives in cities and that proportion is increasing. All urban inhabitants experience a local climate that is somewhat different from that in surrounding rural areas. That, coupled with increasing challenges presented by large-scale climate change, necessitates research to understand how the unique climates caused by urbanization behave, how they impact the residents who live there, and what can and should be done to address ill-effects equitably. The first formal scientific assessment of urban climate was published by Luke Howard almost two hundred years ago in 1833. Over time, urban climatology and meteorology have blossomed into rich fields encompassing studies of a range of modified climate effects including energy balance, roughness length, turbulence and wind, atmospheric moisture, and air quality. The focus of this review is the urban heat island (UHI) effect. The UHI is generally defined as higher temperatures in urban areas compared to rural areas, which is a common feature in most cities worldwide. The first section of this review lists a select number of publications and websites that provide a sound foundation to help those new to urban climate studies become acclimated to the field. The next section delves deeper into the definition of exactly what the UHI is and how it is observed. In particular, a distinction is made between UHIs in the canopy layer (usually measured 2m above the surface) and surface UHIs (measured using thermal imagery). The second section explores the impacts on human and natural systems caused by the UHI. This includes subsections about urban effects on long-term temperature records and human comfort, health, and mortality. The next section takes a look at the various methods used to limit the adverse effects of the UHI. Ideas ranging from green and blue strategies to advanced building materials are discussed. The last part of this review concerns environmental justice. Many of the people who face the direst of unintended consequences caused by urbanization are those who are least well equipped to handle the additional pressures.