Greenspace, bluespace, and their interactive influence on urban thermal environments

Abstract

Urban land use land cover (LULC) change raises ambient temperature and modifies atmospheric moisture, which increases heat-related health risks in cities. Greenspace and bluespace commonly coexist in urban landscapes and are nature-based heat mitigation strategies. Yet, their interactive effects on urban thermal environments are rarely assessed and it remains unclear how extreme heat events (EHEs) affect their ability to regulate human thermal comfort. Using multi-year observations from a dense urban observational network in Madison, WI, we found that green and blue spaces jointly modify the intraurban spatiotemporal variability of temperature and humidity, and the resultant effects on thermal comfort show diurnal and seasonal asymmetry. Greenspace is more effective at cooling throughout the year, particularly at night. Accelerated cooling efficiency is found in areas with dominant greenspace coverage and little co-influence from bluespace. The thermal comfort benefit due to greenspaces can be offset by bluespaces because of intensified nighttime warming and humidifying effects during the warm months, although a weak daytime cooling of bluespace is observed. EHEs enhance bluespace cooling, but the overall joint thermal regulation remains the same due to the enhanced moisture effect. Our findings suggest that diverse outcomes of green and blue spaces cross multiple temporal scales should be holistically assessed in urban planning. The analysis framework based on generalized additive models is robust and transferable to other cities and applications to disentangle the nonlinear co-influences of different drivers of urban environmental phenomena.

Diurnal evolution of urban tree temperature at a city scale

“…We found that the daytime spatial variation of the canopy temperature is much higher than nighttime variation. The finding underlines the great sensitivity of the daytime canopy temperature to the surrounding environment, which coincides with diurnal patterns of LST 31 , 40 . This research has found a greater impact of urban greenspaces compared to urban bluespaces and building height on modulating daytime canopy temperature.…”

“…Previous findings revealed the enhanced cooling effects of greater greenspace coverage on urban ambient temperature 31 , 33 , 34 . A similar relationship is observed between GAF and the canopy temperature diurnally, showing a greater greenspace coverage associated with a cooler canopy temperature (Fig.…”

“…To understand the complex impacts of urban environment on the spatiotemporal variability of canopy temperature, we incorporated the major known environmental factors in the GAMs model analysis, namely, greenspaces (green area fraction, GAF), bluespaces (distance to waterbodies, DTW), and surrounding building morphological characteristics (building height, BH). GAMs are a flexible approach allowing the quantification of non-linear controls of confounding factors on spatially and diurnally 31 , 44 . Due to the common coexistence of various greenery, e.g., trees, shrubs, and grassland, in the urban landscape and their similar diurnal temperature patterns (Fig.…”

“…Strategic plantation of urban trees should instead aim for increasing the cluster size of tree to enhance their resilience to warmer environment. Greater greenspace clusters improve the ambient air temperature 31 , 33 and simultaneously reduce heat stress on the urban vegetation.…”

“…Even though these studies do not directly investigate the canopy temperature, such contrasting diurnal effects on ambient air temperature can further alter the canopy temperature at the watersides 22 . In addition, recent local-scale studies discovered a nonlinear effect of tree coverage on ambient air temperature 31 , 33 , 34 and many studies documented the strong negative relationship between urban LST and vegetation coverage 35 , 36 . Even though, it is still unclear how urban canopy temperature responds to tree/vegetation coverage, surrounding buildings, bluespaces diurnally, due to lack of spatially representative observations.…”