PNNL

Abstract

Accurately capturing the impact of urban trees on temperature can help optimize urban heat mitigation strategies. Recently, there has been a widespread use of remotely sensed land surface temperature (Ts) to quantify the cooling efficiency (CE) of urban trees. However, remotely sensed Ts reflects heat radiation from the surface of an object seen from the point of view of the thermal sensor, which is not a good proxy for the air temperature (Ta) perceived by humans. The extent to 27 which the CEs derived from Ts reflect the true experiences of urban residents remains unclear. Therefore, this study systematically compared the Ts-based CE (CETs) with the Ta-based CE (CETa) in 392 European urban clusters. CETs and CETa were defined as the reduction in Ts and Ta, respectively, for every 1% increase in fractional tree cover (FTC). The results show that the increase in FTC has a substantial impact on reducing Ts and Ta in most cities during the daytime. However, at night, the response of Ts and Ta to increased FTC appears to 34 be much weaker and ambiguous. On average, for European cities, daytime CETs reaches 0.075 °C %-1, which is significantly higher than the corresponding CETa (0.006 °C %-1). In contrast, the average nighttime CETs and CETa for European cities are similar, both approximating zero. Overall, urban trees have the capability to cool daytime temperatures, but their CE has been largely overestimated (by an order of magnitude based on our estimates) by remotely sensed Ts observations compared to in-situ Ta measurements. Our findings provide critical insights into the practical benefits of alleviating the urban heat islands (UHI) effect through urban planting.