Modeling the Effect of Trees on Energy Demand for Indoor Cooling and Dehumidification Across Cities and Climates

Increasing urban tree cover is a common strategy to lower urban temperatures and indirectly the building energy demand for air-conditioning (AC). However, urban vegetation leads to increasing humidity with potential negative effects on the AC dehumidification loads in hot-humid climates, an effect that has so far been unexplored. Here, we included a building energy model into the urban ecohydrological model Urban Tethys-Chloris (UT&C-BEM) to quantify the AC energy reduction effects of trees in seven hot cities with varying background humidity. A numerical experiment was performed simulating various urban densities and tree cover scenarios in the city-climates of Riyadh, Phoenix, Dubai, New Delhi, Singapore, Lagos, and Tokyo. The relative contribution of tree shade, air temperature reduction, and humidity increase on the AC energy reduction was further quantified. We found that well-watered trees provide the largest average summer AC energy reduction of -17% in the hot-dry climate (Riyadh, Phoenix). As tree shade is the dominant factor leading to the AC energy reduction in all city-climates, humid cities also show an average summer AC energy reduction ranging from -6% to -9%. However, increasing humidity is affecting AC dehumidification loads, especially under higher ventilation rates in humid climates and in these cities, AC energy reduction is most efficient with up to 40% tree cover. Additionally, we found that trees effectively reduce peak AC energy consumption due to higher shading effects in those hours. These results can inform urban planning strategies to maximize reduction in the AC energy demand using urban trees. Plain Language Summary Urban trees can provide multiple benefits, such as reducing temperature and potentially air-conditioning (AC) energy consumption, but they might increase humidity. During AC operation, air is not only cooled but also dehumidified, which requires energy, to prevent indoor mold formation and health problems. However, a quantification of the humidity effects of urban trees on the AC energy consumption in hot-humid cities has so far been lacking. Here, we quantify how urban trees influence the summer AC energy consumption in different climates (Riyadh, Phoenix, Dubai, New Delhi, Singapore, Lagos, and Tokyo). We found that well-watered trees lead to the largest average AC energy reduction of -17% in hot-dry cities. In all cities, tree shading is the dominant factor leading to reduced AC energy consumption. Because of this, we also simulated an average AC energy reduction in hot-humid cities of -6% to -9%. However, increasing humidity leads to raised energy consumption for dehumidification, especially when indoor-outdoor air exchange is high. In hot-humid cities, AC energy reduction due to trees is the most efficient with up to 40% tree cover. Trees also provide larger energy reduction during AC peak hours. These findings can inform urban planning strategies to maximize the ecosystem services provided by trees.