Spatiotemporal Characteristics And Drivers Of Urban Heat Island Effect In Beijing-Tianjin-Hebei Urban Agglomeration Based On Footprint Analysis

Rapid urbanization causes substantial disturbance to the local heat balance and induces serious urban heat island（UHI）effect,which adversely affects urban ecological safety and sustainable development,as well as the high temperature it brings exacerbates thermal risks and has serious negative impacts on human health and habitat.Therefore,it is necessary to have a comprehensive understanding of the spatiotemporal characteristics of the UHI effect,drivers and the potential risks,so as to facilitate scientific urban thermal environment management as well as the development of thermal risk prevention and control measures.This study identifies and extracts the UHI footprint extent（FPE）and intensity（FPI）based on a single exponential decay model to characterize the UHI effect,and uses Mann-Kendall trend analysis,Sen slope estimation,and spatial visualization to analyze and compare the spatiotemporal characteristics of the multi-temporal UHI effect in the Beijing-Tianjin-Hebei（BTH）urban agglomeration;the potential drivers of environmental,social and economic aspects were selected to explore the drivers of multi-temporal UHI effect through correlation analysis and regression analysis based on panel data;The heat risk population and heat risk level of the region were further identified through spatially overlaid population raster data.The results indicate that:（1）There are significant spatial and temporal differences in the FPE and FPI of the BTH urban agglomeration.Daytime FPE is higher than nighttime（25.30 km>22.79 km）and shows higher values in spring（compared to other seasons,reaching 33.13 km in spring daytime and23.32 km in spring nighttime）and in large cities（compared to other size cities,reaching 36.80km in annual daytime and 70.06 km in annual nighttime）.In terms of FPI,nighttime FPI is generally higher than daytime（2.43°C>0.92°C）,with the highest seasonal diurnal FPI values occurring in summer daytime（2.55°C）and winter nighttime（2.77°C）,and spatially in medium-sized cities（reaching 1.38°C on annual daytime）and large cities（reaching 2.57°C on annual nighttime）,respectively.Temporally,the daytime FPE in most cities show expansion tends,with a regional average change rate of 0.65 km/year,and high change rates occur more often in spring（daytime change rate of 1.497 km/year）;however,the FPE decreases at night in most cities,with a regional average change rate of-0.068 km/year,where only the summer nighttime FPE maintains expansion（0.157 km/year）.Diurnal FPI show overall decreasing trends（diurnal annual change rate of-0.018°C/year）,except for the spring daytime（0.018°C/year）.In addition,medium-sized cities exhibit higher values of FPE change rate（1.115 km/year for daytime and-0.027 km/year for nighttime）and FPI change rate values（0°C/year for daytime and-0.004°C/year for nighttime）.（2）Social and economic factors are important drivers of changes in the UHI effect in the BTH urban agglomeration,but the specific key drivers of FPE and FPI vary across seasons and diurnal periods.Electricity consumption and the proportion of tertiary industry are the main contributors to the annual daytime FPE and FPI,respectively,and both are proportion of tertiary industry at nighttime.In addition,the main drivers of FPE and FPI differ across seasons and diurnal time periods.For daytime,the most important drivers of FPE in each season are road area（spring）,construction land area（summer）,electricity consumption（autumn）and sunshine hours（winter）;for nighttime,the most important drivers in each season are the proportion of three industries.For FPI,the most important daytime drivers for each season are the proportion of three industries（spring）,vehicle number（summer）,population density（autumn）,and CO₂emissions（winter）;at night,the most important driver of FPI in summer is electricity consumption,and for the rest of the seasons,the most important driver is the proportion of three industry.（3）The BTH urban agglomeration has varying degrees of annual and seasonal diurnal thermal risk,with mainly low risk and a continuously increasing population at risk,especially in large cities.The annual daytime thermal risk population in the region’s cities reached an average of 2135 thousand people and 2137.7 thousand people at night.During the daytime,the peak thermal risk populations in most cities in the region occur in summer,reaching an average of2259.6 thousand people,and at night in spring,reaching 2372.8 thousand people.The thermal risk populations are generally larger in large cities than in small and medium-sized cities（7101thousand people during the day and 8834.7 thousand people during the night）,with Beijing having the most thermal risk population,reaching an annual of 9938 and 11081.8 thousand people during the day and night,respectively.During the study period,there are general and significant growth trends in the thermal risk population,with an average annual daytime increase of 77.6 thousand and 53.9 thousand at night for cities in the region.The thermal risk population has a faster growth in summer（compared to other seasons,80.5 thousand people/year in daytime and 70 thousand people/year at night）and in large cities（compared to other size cities,the annual growth is 263.7 thousand people in daytime and 298.5 thousand people at night）.Combined with the risk level,the annual diurnal thermal risk population of the region exceeds68.52%and 66.00%of the total population within the FPE,respectively.Broader risk populations tend to exist in summer（compared to other seasons,reaching 84.50%during daytime and 83.03%during nighttime）and in large cities（compared to other size cities,reaching78.53%during daytime and 91.05%during nighttime on average per year）.And the risk levels faced are generally dominated by low risk,which will negatively affect the thermal comfort level of the inhabitants.The study indicates that as urbanization continues,the UHI problem and the thermal risks associated with it will persist and exhibit spatiotemporal heterogeneity in the BTH urban agglomeration,and urban planners and managers must pay attention to it and take targeted and effective preventive,mitigating and management measures.