Rural agriculture largely reduces the urban heating effects in China: A tale of the three most developed urban agglomerations

Agriculture, as a widespread high-intensity land use activity in urban surroundings, may dramatically alter urban climate through advection and atmospheric feedbacks. However, it remains unclear how and to what extent agriculture warms or cools the urban temperature. This study investigated the potential impacts of rural croplands on summer and winter urban heat island (UHI) effects in three most developed urban agglomerations (JJJ, Jing-Jin-Ji; YRD, Yangtze River Delta; PRD, Pearl River Delta) of China by the Weather Research and Forecast (WRF) model. Results show that urban lands warm 2-m air temperature substantially relative to natural forests, with a mean intensity of 1.33-1.70 degrees C and the strongest in winter night of 2.99-3.07 degrees C, but one to two thirds of the warming are canceled by rural croplands, especially in the semi-humid JJJ. The cropland impacts depend strongly on the concurrent UHI effects, with the coupled effects (added impacts due to the UHI effect) are 2.0-8.7 times the independent effects (assuming no UHI effect), especially in the humid-hot PRD. Consequently, the rapid cropland loss for urbanization during 2000-2020 not only increases the UHI-affected area by 1.5-4.2 times, but also elevates the UHI intensity of existing urban lands, for example, by 0.43-0.65 degrees C in winter night. The cropland impacts on urban climate can be attributed to a range of factors like decreases in ground heat storage and net radiation, an increase in wind speed (turbulent heat mixing), and a change of planetary boundary layer height. Our results highlight the importance of rural agriculture in modulating urban climate and therefore in formulating the coordinated development plans of city clusters. However, the simulated cropland impacts differ greatly between near-surface air and land skin temperatures, and from the satellite-sensed radiant temperature, calling for field observations and more numeric experiments to elaborate the findings from this study.