At a Glance
Cities are engineered landscapes, and planning and development choices can significantly exacerbate or mitigate the impacts of climate and environmental change. The height of buildings and their spatial configuration influence the urban form and surface texture, which further affect the surface aerodynamic processes, energy use efficiency and emissions. A “smart” engineered urban landscape can reduce heat stress, and improve energy efficiency and air quality.
Heat stress associated with climate change is one of the most serious climate threats to human society. The impact is further amplified for urban populations because of the urban heat island effect—a common phenomenon in which surface temperatures are higher in urban areas than in surrounding rural areas. Cities are also hotspots for carbon dioxide emissions and strong sources of anthropogenic aerosols. Because more than 50% of the world’s population currently lives in cities, and that percentage is projected to increase to 70% by year 2050, there is a pressing need to find effective solutions to cope with the heat and environmental stress.
It is now recognized that in addition to the traditional emphasis on building up the city’s preparedness or resilience, urban planning and adaptation agendas should also include active modification to the urban landscape in the face of climate change1.
In 2016, STEP postdoctoral fellow Lei Zhao, with his advisor, Michael Oppenheimer, developed a novel methodology that combined government building footprint dataset, ultra-high resolution imagery from LiDAR, and satellite observations to investigate the impact of surface morphology and texture on urban climate and environment2. The results point to robust relationships between urban morphological properties and the efficiency of heat convection from the city’s surface to the lower atmosphere. Specifically, the height of buildings and their spatial configuration are strong determinants of surface aerodynamic roughness, which represents the urban convection efficiency (Figure 3.1.1.). Intensified convection helps not only to reduce temperatures but also to disperse air pollutants.
The study has three important implications. First, this work for the first time demonstrates that surface morphology can affect the urban local climate and environment. Further, it advances the scientific understanding of the impacts of complex surfaces on surface aerodynamic processes. In climate models, land surface is usually modeled as a grid of “tiles” that represent surfaces such as vegetated land, glacier, wetland, lake, and urban areas (Figure 3.1.2.). For urban tiles, however, surface geometry, representation, and parameterization are still highly simplified. Insights generated from this study will help improve the ability of climate models to accurately quantify the surface energy, momentum and mass transfer between land and the atmosphere over these urban tiles.
Second, the study provides actionable guidance to policy makers on future urban planning and development concerning urban heat mitigation, climate change adaptation and air pollution abatement. Cities are engineered landscapes, and planning and development choices can significantly exacerbate or mitigate the impacts of climate and environmental change. Results from this study point to the possibility of “smart” engineering urban landscapes to reduce heat stress, and improve energy efficiency and air quality.
Third, it bridges a disconnect in the global climate research agenda between large-scale carbon mitigation and local-scale urban engineering. Unlike planetary-scale mitigation strategies, urban engineering has impacts on a much smaller area of land. Cities are functional units of climate mitigation agendas. A reorientation of some of the discussion of climate change mitigation and adaptation from global-scale climate intervention to small-scale urban planning and engineering can motivate local actions by delivering environmental benefits directly and immediately.
1 Zhao, L., X. Lee, and N.M. Schultz, 2017. A wedge strategy for mitigation of urban warming in future climate scenarios. Atmos. Chem. Phys. Discuss., in review. doi:10.5194/acp-2016-1046.
2 Zhao, L., M. Oppenheimer, Q. Li, Q. Zhu, and N.M. Schultz, 2017. Designing built-up morphology for urban climate and environmental co-benefits, in preparation.