Urban Heat Islands:
Disproportionate Risk and Potential Solutions in San Francisco
Disproportionate Risk and Potential Solutions in San Francisco
Author: Nicholas Tan
ESPM 50 AC
This site aims to introduce the growing environmental concern of urban heat islands within the context of San Francisco and highlight how marginalized and historically disadvantaged groups are disproportionately at risk to urban heat islands. There will also be sections that explore potential solutions via urban design strategies that could mitigate the urban heat island effect.
Urban environments are grappling with the escalating consequences of the "urban heat island" effect, a phenomenon where urban environments prevent heat from naturally dissipating. Several factors, including less evapotranspiration from plants and soil and the vast spread of heat-retaining surfaces like concrete, large buildings that block horizontal cooling mechanisms like winds, and the continuous production of human-generated heat, exacerbate this condition.1
Studies show urban overheating results in thermal stress on urban dwellers, increased energy usage, and heightened air pollution levels. In Greece, urban heat islands were found to intensify heat waves by up to 3.5°C, increasing the risk of heat-related discomfort, mortalities, and morbidities.2 To combat rising temperatures, energy used to cool buildings will increase. In fact, cooling energy usage has been found to rise between 0.17 and 1.84 kWh/m2 due to 0.5 K of urban overheating.1 These increased energy usage needs combined with the compounding interactions between higher temperatures and atmospheric pollutants will also result in higher urban air pollution levels. One study in China found that the average haze contribution from urban heat islands is 0.7±0.3 K for semi-arid cities.3
As urban heat islands continue to intensify in cities, their effects on the environment will spread and materialize in new ways. For instance, it’s been shown that urbanization and urban heat islands increase stream temperatures, directly harming coldwater fish. Initial asphalt runoff temperatures were roughly 10°C warmer than sod runoff temperatures (35.0 vs. 25.5°C).4
The ramifications of this overheating are both immediate and looming. Recent studies predict that by 2050, the heat island effect could contribute to half of the warming induced by climate change. This means in scenarios where cities experience a 2-degree increase due to climate change, the heat island effect would add an additional degree of warming. 5
Su, Mi Aye, et al. “Empirical Evidence on the Impact of Urban Overheating on Building Cooling and Heating Energy Consumption.” iScience, vol. 24, no. 5, May 2021, p. 102495. DOI.org (Crossref), https://doi.org/10.1016/j.isci.2021.102495.
Founda, D., Santamouris, M. Synergies between Urban Heat Island and Heat Waves in Athens (Greece), during an extremely hot summer (2012). Sci Rep 7, 10973 (2017). https://doi.org/10.1038/s41598-017-11407-6
Cao, C., Lee, X., Liu, S. et al. Urban heat islands in China enhanced by haze pollution. Nat Commun 7, 12509 (2016). https://doi.org/10.1038/ncomms12509
US EPA, ORD. Urbanization - Temperature. 29 Dec. 2015, https://www.epa.gov/caddis-vol2/urbanization-temperature.
Huang, Kangning, et al. "Projecting global urban land expansion and heat island intensification through 2050." Environmental Research Letters 14.11 (2019): 114037.