An integrated assessment on the warming effects of urbanization and agriculture in highly developed urban agglomerations of China.

Urbanization and agriculture, the two major and concurrent land use activities, can dramatically alter land surface temperature (LST) through multiple biophysical processes. However, previous studies mainly focused on the warming effects of urbanization in large cities and/or urban core areas that may greatly underestimate the land use impacts on regional climate. Using natural forest as a reference, we assessed the LST changes of both urbanization and agriculture in the three most developed urban agglomerations of China (Jing-Jin-Ji, JJJ; Yangtze River Delta, YRD; Pearl River Delta, PRD) according to satellite observations. Results show that the urban-dominated lands warm the daytime LST substantially, especially in the south subtropical PRD (with an annual mean intensity of 5.5 °C), and the highest do not occur in the core cities. The crop-dominated lands also warm the daytime LST dramatically, especially in the temperate semi-humid JJJ (with an annual mean intensity of 3.9 °C). The daytime warming effects increase significantly from 2003 to 2018 mainly due to urban expansion in crop-dominated and mixed lands. The two land uses continue to warm the LST at night though in a lower magnitude in the PRD. However, the urban-dominated lands warm the LST slightly and the crop-dominated lands cool the LST substantially at night in the JJJ and YRD. Overall, the crop-dominated and/or mixed lands dominate the regional LST changes owing to their large areas. We further show that the daytime warming effects of the two land uses are likely caused by the changes of evapotranspiration, whereas the nighttime cooling effects might be mainly due to the changes in surface albedo and roughness. Our results highlight the importance of considering the urbanization in small-medium sized satellite cities and the more widespread agricultural activities in rural areas when assessing the regional climate change and formulating the mitigation strategies.

[Spatiotemporal Variations in Atmospheric Urban Heat Island Effects and Their Driving Factors in 84 Major Chinese Cities].

Accelerating urbanization seriously intensifies urban heat island effects in China, which in turn affects regional environment and human health. However, the spatiotemporal patterns of atmospheric urban heat island effects remain poorly understood in China as previous research is mostly based on satellite-sensed radiation temperatures. Using long-term daily meteorological observations from 1960 to 2017, this study explored the geographical distribution of atmospheric urban heat islands over diurnal, intra-annual, and inter-annual timescales in 84 major cities in China. The results show that on average, the intensity of the urban heat island reaches(0.9±1.1)℃, although large geographical variations were detected. The intensity was, overall, larger in northern China than in southern China; North China showed the largest annual intensity of(1.4±1.4)℃, and seasonal intensity variations were high in northeast and northwest China. Here, significantly higher intensity effects were measured at night[(1.2±1.1)℃] than during the day[(0.5±1.2)℃], and in summer than winter during the day, and in winter than summer during the night. Inter-annually, the annual mean urban heat island effect has increased by an average of 0.040℃ per decade, although this trend weakens after 2009. Furthermore, we found that the spatial patterns of atmospheric heat island intensity was greatly affected by climatic background conditions and the location of meteorological stations, while the long-term trends were strongly influenced by the impervious surface area. This study improves understanding of the atmospheric urban heat island effect in China and provides important insights for formulating urban land-use strategies to alleviate high temperatures and heatwaves.