Compound environmental impact of urban mitigation strategies: Co-benefits, trade-offs, and unintended consequence

Urban areas confront a number of environmental issues including excessive thermal stress and concentrated emissions of greenhouse gases and pollutants. In past decades, many mitigation strategies have been designed and implemented to counteract these issues and ameliorating the environmental quality in cities, which can be broadly classified as white, green or blue infrastructure. The functioning and efficacy of urban mitigation strategies involve complex interactions between landscape dynamics, anthropogenic activities, and atmospheric transport, which leads to compound, rather than singular, environmental impacts. In this study, we conducted a critical review of the compound environmental impact of urban mitigation strategies, and evaluated, besides the targeted cooling effect, the resultant co-benefits, trade-offs, or unintended consequence, in terms of building energy saving, air quality improvement, carbon emission offset, and impact to human health. Furthermore, we proposed a novel mathematical framework that is capable of assessing the compound environmental impact in a unified way, together with some preliminary results as the proof-of-concept. A number of knowledge gaps are identified which calls for future transdisciplinary synergy among urban engineers, atmosphere and climate scientists, and epidemiologists.