Enhanced Surface Urban Heat Islands Due to Divergent Urban-Rural Greening Trends
Using satellite and numerical model simulations, this study quantifies the impacts of vegetation cover changes on land surface temperature over 228 urban and rural areas globally. Recent changes in vegetation cover account for about 25% of the observed upward trends in temperature differences between urban and rural areas. This finding is mainly attributed to an increasing contrast of vegetation evaporative capacity.
We isolate the influence of Earth greening on urban heat island trends from climate variability and anthropogenic activities. We also identify the spatial distribution of the key biophysical factors affecting the vegetation-induced urban heat island changes. Further analysis shows that the urban heat island impacts in hot-humid megacities surrounded by croplands and in hot-dry cities surrounded by non-forest vegetation respond more strongly to vegetation change than cities in other regions. This research paves the way for studying the vegetation impacts on changing urban climate, providing information on designing effective mitigation strategies to reduce urban heat islands.
Satellite observations show that the urban heat island effect has been increasing over the last two decades. This warming is often accompanied by an increased urban-rural contrast of vegetation greenness. However, the contribution of uneven vegetation trends in urban and rural areas to the urban heat island trend is unclear, due to confounding effects of climate change and changes in infrastructure and anthropogenic heat sources. In this study, we use a data-model fusion approach to quantify such contributions during the peak growing season. We find that differences in contrast between urban and rural vegetation leaf area are increasing (P<0.05) in 189 of the selected 228 global megacities. The increasing trend of urban-rural vegetation leaf area difference from 2000 to 2019 accounts for about one-quarter of the satellite-derived urban heat island trend, and the impact is particularly evident in places with rapid urbanization and rural cropland intensification. The marginal sensitivity of the urban heat island effect to vegetation leaf area is the strongest in hot-humid megacities surrounded by croplands and in hot-dry megacities surrounded by mixed woody and herbaceous vegetation. This study highlights the role of long-term vegetation trends in modulating the trends of urban-rural temperature differences.