Redlining and Urban Heat Islands: An analysis of historic housing discrimination and heat exposure

Abstract

Urban heat islands (UHI) are a phenomenon observed in built environments due to differences in terrestrial albedo that result in higher temperatures in urban areas. Variations in both environmental factors and physical infrastructure cause a wide range in temperature among neighborhoods within the same city. In the 1930’s, the Home Owners’ Loan Corporation (HOLC) used risk assessment maps (now commonly referred to as redlining maps) to further reinforce racist housing and lending policies that were commonplace in that era. This study assesses the environmental legacy of these policies on current heat islands. The analysis is based on land surface temperature (LST) estimates derived from Landsat 8 (OLI/TIRS) imagery and calculated using open-source code in Google Earth Engine. Differences in LST are calculated for 8,865 unique HOLC neighborhoods for 202 cities across the United States. A linear mixed effects model is used to analyze if urban areas have varying temperatures in the present day based on their historic HOLC ranking (scale of A-D, where neighborhoods ranked A were typically limited to upper-class white residents, and neighborhoods ranked D were typically composed of Black residents regardless of class or income). The model found that all grades were significantly different from each other (p<0.001), with zones ranked D an estimated 2.3°C hotter than zones ranked A. These findings indicate that despite the cessation of overt racebased housing policies, the effects of such policies are socially and environmentally measurable in the present day.