Simulating the CO2 reduction caused by decreasing the air conditioning load in an urban area

•Energy conservation and CO2 reductions by heat-island mitigation were evaluated.•Interactions between air-conditioning loads and the outside atmosphere were simulated.•Planting greenery achieved the largest reductions in temperature of up to 0.6–1.0 °C.•A large-scale assessment of Kawasaki city during typical summer days was performed.•Increasing albedo achieved the highest CO2 reductions of up to 400 t-CO2/day.

In this paper, measures such as planting urban greenery and using high-albedo paint to mitigate the urban heat-island effect, conserve energy, and reduce CO2 emissions were assessed. As a typical energy-saving method for buildings, reducing the internal heat sources and increasing the insulation are also assessed. We used a coupled urban canopy and building energy model to predict the heat loads of buildings in city districts, the effects of air-conditioning on energy consumption, and air temperature changes. In this model, a vertical one-dimensional local atmospheric model is coupled with an air-conditioning load calculation model for buildings, making it possible to assess the interaction between anthropogenic heat release due to air-conditioning usage and the outside thermal environment. In this study, we selected a target study region in the city of Kawasaki, Japan. When typical city districts were assessed, planting greenery or increasing albedo achieved temperature reductions of 0.6–1.0 °C and 0.1–0.5 °C, respectively, and energy savings of 40–80 and 70–90 kJ/m2/day (per unit floor area) on a typical summer day. The results from the large-scale assessment show that urban greening or albedo increases achieved the highest energy savings, of up to 400 t-CO2/day, in the entire target study region.