The Urban Heat Island effect refers to an urban area that experiences higher average temperatures than surrounding areas. This is primarily due to the replacement of vegetation by urban development and urban structures. As such, our model explores how the development of a city over 20 years contributes to the development of an urban heat island.
For the model, we modelled temperature anomaly by using the equation that relates temperature to heat storage and heat capacity: ∆T = ∆Heat Storage/Heat Capacity. Heat capacity was coded as a variable that was a weighted average of the average heat capacities of vegetation and urban development. Change in heat storage was simplified to be equal to the inflows of solar radiation (represented as 1-albedo) plus the anthropogenic heat emission, minus the outflows of evapotranspiration and air flow. Albedo and air flow were presented as weighted averages of vegetation and urban development contributions, while evapotranspiration depends solely on vegetation. Anthropogenic heat emissions depends on urban development and air conditioner use, and air conditioner use depends on temperature, with data from anthropogenic heat emissions calculated using Munck et al’s (2013) study of air conditioners in Paris.
Our model refers to a city similar to Vancouver, but where urban development initially only covers 40% of land and construction is rapidly occurring to intensify land use and decrease vegetation. This thus changes the previously mentioned parameters of albedo, air flow, heat capacity and evapotranspiration. Albedo refers to the fraction of incoming solar radiation that a surface reflects back. Urban development lowers albedo and thereby increases incoming solar radiation. Similarly, urban development increases heat capacity and reduces air flow, which increases temperature. Conversely, vegetation removes heat via evapotranspiration, and thereby has a net cooling effect.
Finally, we added a health component to the model, as increased urban temperatures have been correlated with an increase of heat-related illnesses and heat-related mortality.
The model demonstrates that there is a positive feedback loop in the system: as temperatures increase, so does air conditioner use, which ironically contributes significantly to anthropogenic heat emission and thus has a net warming effect on the urban environment. As such, while the temperature anomaly initially grows linearly, the temperature anomaly accelerates exponentially as urban development continues to grow and air conditioner use increases. Since temperature is related to heat-related illness and death, these metrics also increase exponentially.
In light of these results, we invite you to reflect on whose bodies are placed at risk due to this phenomenon. This model idea was originally inspired by a podcast that focussed on analyzing climate change as a political issue of systemic injustice, and described urban heat islands as one of the many factors affecting differential climate vulnerability: urban heat islands are concentrated in the downtown core, which is often a poorer area of cities, where folks have difficulty affording additional costs of air conditioning. As global warming increases, so do urban heat islands as well as heat-related illnesses, which is reflected in this model. As such, our model provides a starting point for understanding urban heat islands as a phenomenon associated with climate justice, and invites us to think about the consequences of how humans are currently reshaping the environment.