In our study, the final available datasets are all from China, and there are few study on relocated meteorological stations in developed countries where urbanization was finished before early 19 century, resulted in less concern on global scale. However, obvious urbanization warming tendency could be proved over the worldwide. According to IPCC (1), the daily average temperature increasing rate of China is 0.17ºC per decade in the near half century, higher than that of global (0.12ºC per decade). In other studies, the urbanization has been proved to contribute 27~ 55% to the temperature increasing tendency(5,8-9). From 1980s to 2010, the temperature increased not higher than 1.04ºC in China (10), and based on our results, the urbanization contributes not less than 63.5% to the temperature increasing tendency, with a rate of 0.22ºC per decade (0.66ºC/30a). The overestimation on temperature increasing tendency caused by urbanization has also been found in western America (11), North America (12), South America(13), Europe (8), Korea (14), and South Africa (15). So the overestimation on global warming tendency caused by fast urbanization could be a global scale phenomenon. And the interference caused by urbanization is hard to exclude even by the popular homogenization methods (16), and could be only accurately estimated by comparing the synchronous observed data sets between relocated old and new stations.
There are coincidences between the periods of temperature increasing and regional fast urbanization process. The fast economic development and urbanization processes bring heavy interference to meteorological stations. Our results indicate that the warming tendency in the near 30 years in China was affected by the unprecedented urbanization process, which causes the inconsistent of the temperature change tendency between China and worldwide. China experienced a rapid economic growth and consequently a dramatic increase in the population and areas of its cities since 1980s. With the fast urbanization in China, many meteorological stations were moved away from old to new places due to gradually surrounded by the urban. Most of the meteorological stations were built up after 1950, and at the building time the stations were located in rural areas where unbiased observation data could be obtained (Fig. 2-a). After the reform and opening policies in 1980s, the meteorological stations “entered cities” and were relocated continually (Fig. 2-b,c; 17). Before 1980s, the temperature increasing rate in China was lower than global average level, while was faster than that of worldwide after 1980s (18). The East Asian region including China shows a faster warming tendency in the near half a century (18), whereas European region experienced that from the end of 19th to the beginning of 20th century (8, 19). In Europe, the earlier urbanization process erases the heat island effect on temperature increasing tendency over the 20th, because the urbanization have not changed over this time. For instance, based on Dienst et al. (20), the temperature increasing tendency would display a rise in TAVG trend by 0.03ºC per decade in Northern Europe, if the 20th century is regarded exclusively. In developed areas, the heat accumulation and release process repeated several times with stations relocation; while in developing countries, the heat accumulation is on going with the urban process.
The IPCC reports have not emphasize the effect of fast urban urbanization on temperature recording, the global warming tendency could be over estimated. The temperature observational bias caused by the urban heat island effect contributes to much of the global warming overestimation. Contrast to the IPCC’s conclusion that urbanization contributes not more than 10%, our results revealed that the urbanization contributed more than 60% to the temperature increasing tendency. Whether the global warming tendency is as severe as the IPCC (1-2) report is doubtful. At regional scale, the conclusion that urbanization contributes much to temperature increasing is derived from research analyses that based on strict selected city-rural paired data sets (21). Whereas the urbanization biases are few corrected at global scale and should be considered carefully to estimate the global warming tendency accurately.