Implications For Heat Stress in One of America’s Most Rapidly Urbanizing Cities: Houston, We Have a Problem

The city of Houston, Texas has been experiencing rapid urbanization on a nearly unprecedented level over the past two decades. In this study, we fused NASA satellite data products along with in situ observations and socio-economic datasets to: 1) characterize land use-land cover change that has occurred over the Houston Metropolitan Area during the last two decades, 2) investigate the resultant changes in the urban heat island and air quality, and 3) determine how the socioeconomic layout of the Houston Metropolitan Area affects the human exposure to these changes. We utilized the National Land Cover Database derived from Landsat satellite imagery and coherent land products derived from NASA Moderate Resolution Imaging Spectroradiometer to analyze the urban growth of Houston. Our analysis shows an increase in area of urban and built-up land cover in excess of 20% during this time period. We also constructed decadal averages of land surface temperature (LST), the normalized difference vegetation index (NDVI), and white-sky albedo for the time periods 2000-2009 and 2010-2019.

The Daytime LST distribution shows a clear spatial expansion of warmer values, with the spatial extent of Daytime LST greater than 302 K increasing from 1,759 km 2 to 3,445 km 2 , and the spatial extent of Daytime LST greater than 305 K increasing from 3 km 2 to 26 km 2 . The Nighttime LST distribution shows a similar increase in the coverage of higher LST values, with the spatial extent of Nighttime LST greater than 290 K increasing from 4,814 km 2 to 6,712 km 2 , and the spatial extent of Nighttime LST greater than 293 K increasing from 29 km 2 to 106 km 2 . The distinct ring of urban expansion along the border of Harris County is quite evident in the NDVI difference between the two decades, with a net decrease being reported from the 2000-2009 decade to the 2010-2019 decade. Finally, the spatial patterns of AOD show a less impressive pattern, but the region in Western Harris County saw the highest increase in decadal-averaged AOD over the past 20 years. We found that the mean Daytime LST across the Houston Metropolitan Area (HMA) increased by 0.537 K, with Nighttime LST experiencing a slightly lower increase of 0.328 K. NDVI experienced a minor decrease of -0.004, and AOD saw nearly no change, reporting an increase of 0.002 across the HMA.

Additionally, we also found that site-averaged monthly maximum and minimum surface temperature as measured by 10 ASOS sites across the HMA has
increased by 0.15 and 0.30 K, respectively. Monthly minimum surface temperature values increasing by double to that of maximum surface temperature values across the two-decade period of our study has drastic heat stress implications in the HMA. Thus, the increase in both LST and surface temperature measurements has serious public health implications for the Houston area.

Additionally, we examined the implications of heat stress by socioeconomic level across both Harris County and the entire HMA by utilizing the Center for Disease Control (CDC)/Agency for Toxic Substances and Disease Registry (ATSDR) social vulnerability index (SVI). In this section of our study, we examined census tract-level data, binned by the percentage of a census tract’s population that met the 150% poverty threshold. We find that the >20% poverty regions of the HMA experienced the highest Day LST means in both decades, as well as the highest Day LST increase of 0.725 K decade -1 . However, we also find that the highest increase in Night LST, AOD, and highest decrease in NDVI of 0.364 K decade -1 , 0.00385 decade -1, and -0.009 decade -1 , respectively, all occur in the most socioeconomically affluent areas of the HMA. These regions of the HMA seeing the most
substantial changes of LST Night, AOD, and NDVI are most certainly tied to the rapid urbanization they have been experiencing, compared to the socioeconomically disadvantaged regions, which have seen little to no change in its urbanized LULC over the past 20 years. This analysis shows that economically disadvantaged populations are subject to higher exposure to urbanization-enhanced heat stress, and potentially, less access to green spaces.