Mitigating the Urban Heat Island under Climate Change through Urban Management

Abstract:

The urban heat island (UHI) represents ubiquitous urban warmth compared to surrounding rural areas. This phenomenon, when compounded with future climate warming, will exacerbate heat stress on urban residents who will comprise 70% of the world’s population by 2070. At the same time, urban climate adaptation plans have shown great potential for reducing the impacts of global change. In this study, we assess three mitigation strategies, including reflective roofs, green roofs, and street trees, to ameliorate the warming under climate change through both “online” and “offline” methods. The “online” method compares modeling results from a modified urban roof albedo configuration (ALB-MOD) where the roof albedo is raised to a high reflective value to the modeling results from the default configuration (CTRL), both using the Community Earth System Model (CESM). Three pairs of simulations under current climate forcing and two future scenarios (RCP4.5 and RCP8.5) are conducted. The “offline” method uses a surface temperature attribution solution derived previously for partitioning the UHI intensity to assess the efficacy of the mitigation strategies. The “offline” method supplements the “online” method in assessing green roof and street tree strategies, because the current design of CESM does not have explicit vegetation in the urban canopy configuration. The excellent agreement between the “online” and “offline” results confirms the validity of the offline scheme, supporting that the “offline” method can be used to predict the impacts of various urban adaptation strategies for development planning. Results show that albedo management is the most effective and viable way to mitigate UHIs, whereas although green roof and street trees strategies have evaporative cooling effects, the cooling is compensated by vegetation’s lower albedo, showing much less effectiveness on UHI mitigation. Although convection efficiency associated with the surface roughness is an important contributor to UHIs, both street trees and green roof bring minimal increase to the citywide surface roughness. Our results also show that the urban mitigation strategies can even completely offset the impacts of global warming on the local urban climate.