What's behind the warming signals in eastern China megacity areas?

Urbanization doubtlessly has been the most significant demographic trend in the world for at least a century and promises to become even more significant in the future. The population of Asia has increased rapidly over the last three decades. In 1995, there were five megacities in Asia. Nowadays, half of the world's megacities are in Asia, population centers with more than 10 million people living in them. Urban development often causes great modification of land surface properties, landscape pattern, and even local climate through changing land use and cover. Urban expansion resulted by economic development and population growth has changed land surface properties in urban and surrounding area, including green vegetation cover, surface albedo, surface roughness, and emissivity in many parts of the world, especially in East Asia. The expansion of so-called urban heat island (UHI) is considered as a direct climate consequence of urbanization, as solar radiation is sequestered by urban architecture. Urban heat island is a unique micro-climate characteristic in urban areas, and it varied widely in different area due to varied weather or anthropogenic influence factors. Here, we investigated spatial pattern and temporal variety of urban heat island in the Bohai coastal region with multi-scale thermal infrared satellite data, including MTSAT, NOAA/AVHRR, Terra/MODIS, and Landsat/TM, along with hand-held thermal imager. We try to find the most optimal scale among those satellite platforms according to the thermal landscape properties and scale independent principles of thermal remote sensing. The influence of surface parameters on urban heat island effect and the seasonal changes of UHI were also investigated. Higher land surface temperatures were detected over city limits, especially in major cities compared to nearby rural areas in the Bohai Region. In some cases, the land surface temperature can get 2.5°C higher in city center than nearby green areas. There was a significant correlation between spatial patterns of land surface temperature and fraction of buildup. Meanwhile, both satellite and meteorological data indicated greater warming trends along urban-rural transition areas where major increases of buildup were detected in past 10 years. Because of rapid urbanization, urban warming has become an important part of regional warming signals and in many cases dominates warming trends detected by met stations densely located in urban areas.