Dynamic changes and drives of surface urban heat islands in China

Under the backdrop of rapid urbanization and global warming, urban environments have undergone significant changes, with the one of the most pronounced being the urban heat island effect. This phenomenon can disrupt the functions of urban ecosystems, affect residents’ comfort and health, and even pose life-threatening risks. existing research has predominantly focused on developed cities during urbanization, often overlooking smaller towns. As urbanization accelerates, the urban heat island effect is becoming increasingly pronounced in medium and small cities. Therefore, investigating the urban heat island effect in small and medium-sized cities can provide reliable data for long-term monitoring of various levels of urbanization across China, which is crucial for developing green, low-carbon, and livable ecological cities. This study leveraged long-term MODIS land surface temperature data, delving into the temporal variations and spatial patterns of surface urban heat island intensity (SUHII) across 1444 counties in China from both annual and seasonal time scales, as well as from national and climatic zone spatial dimensions. Meanwhile, the primary reasons influencing SUHII changes were investigated. The results indicated that a significant upward trend in SUHII nationwide, with the exception for winter, and the most rapid increase was observed in spring. In different climatic zones, the mid-temperate, warm temperate, and northern subtropical zones experienced notable growth in spring and autumn, while the mid-subtropical to marginal tropical zones demonstrated a pronounced rise in all seasons. Furthermore, weak hot islands dominated in China, except in winter, although their proportion declined over time. Strong and moderate hot islands were on the rise, primarily in the south of the Qinling-Huaihe line at 34°N. Winter was mainly characterized by no clear cold/hot islands, concentrated in the warm temperature and northern subtropics, with weak hot islands expanding southward into temperature regions. Besides, over the 20 years, approximately 47 % of cities maintained stable cold/hot island levels, while 53 % underwent transitions, predominantly hot island level conversions. In addition, natural factors such as precipitation and relative humidity played a critical role in SUHII changes, alongside an increasing impact of aerosol optical depth and white-sky albedo in the last decade