Regulation of humid heat by urban green space across a climate wetness gradient

Abstract

Green spaces are a common strategy for adaptation to urban warming. Whereas they have been demonstrated to reduce air temperature, much less is known about their effect on air humidity. Because human heat stress is contributed by both temperature and humidity, it is important to quantify the relationship between the effects of both. Here, using mobile measurements in 15 cities, we show that the daytime temperature effect is negatively correlated with the humidity effect, resulting in an insignificant change in the wet-bulb temperature Tw or humid heat (daytime Tw difference between green space and built-up area ΔTw = −0.01 °C). A net reduction in humid heat was observed at night in intermediate (summer precipitation 180 to 570 mm) and wet climates (summer precipitation > 570 mm; mean ΔTw = −0.35 °C). A model simulation revealed that the nighttime Tw reduction resulted mostly from reduction in soil heat storage in vegetated land. These nuanced microclimate responses to urban vegetation caution against a one-size-fits-all solution to climate adaptation. Green spaces are known to help cool cities, but they contribute humidity while reducing heat—and both matter. Using smart sensors mounted on bicycles, this study finds that daytime temperature reductions in urban green spaces are largely offset by humidity increases but that urban vegetation causes a net reduction in humid heat at night.