Urbanization Effects on Observed Surface Air Temperature Trends in North China

Abstract

A dataset of 282 meteorological stations including all of the ordinary and national basic/reference surface stations of north China is used to analyze the urbanization effect on surface air temperature trends. These stations are classified into rural, small city, medium city, large city, and metropolis based on the updated information of total population and specific station locations. The significance of urban warming effects on regional average temperature trends is estimated using monthly mean temperature series of the station group datasets, which undergo inhomogeneity adjustment. The authors found that the largest effect of urbanization on annual mean surface air temperature trends occurs for the large-city station group, with the urban warming being 0.16°C (10 yr)−1, and the effect is the smallest for the small-city station group with urban warming being only 0.07°C (10 yr)−1. A similar assessment is made for the dataset of national basic/reference stations, which has been widely used in regional climate change analyses in China. The results indicate that the regional average annual mean temperature series, as calculated using the data from the national basic/reference stations, is significantly impacted by urban warming, and the trend of urban warming is estimated to be 0.11°C (10 yr)−1. The contribution of urban warming to total annual mean surface air temperature change as estimated with the national basic/reference station dataset reaches 37.9%. It is therefore obvious that, in the current regional average surface air temperature series in north China, or probably in the country as a whole, there still remain large effects from urban warming. The urban warming bias for the regional average temperature anomaly series is corrected. After that, the increasing rate of the regional annual mean temperature is brought down from 0.29°C (10 yr)−1 to 0.18°C (10 yr)−1, and the total change in temperature approaches 0.72°C for the period analyzed.

The reliability of global and hemispheric surface temperature records

“…Locally, however, the effects may be larger, and much recent work has emphasized China (e.g., Ren et al, 2008) because here the effect may be larger than in other parts of the world . Urban growth has been dramatic in the recent 30 years across eastern China, but an important consideration is that very few of the series are located in rural locations [see discussion in recent papers by Li et al (2014) and Zhao et al (2014)].…”

“…Averages produced for China or parts of China, e.g., by Li et al (2014) and Zhao et al (2014), use networks of different station densities, including both rural and urban stations. Others (e.g., Ren et al, 2008) omit the more urban stations. For both types, averaging doesn't consider land use except at the stations.…”

Urbanization and heterogeneous surface warming in eastern China

“…The key of this method depends on whether or not those sites or stations can be objectively classified. In general, such a classification utilizes either population data [6][7][8][9][10][11][12][13][14][15][16][17] or satellite data (such as nighttime light imagery and land cover dataset) [18][19][20][21][22][23] as well as the geographic location of the stations. In addition, Empirical Orthogonal Function (EOF) [24,25], Principal Component Analysis (PCA) [26,27] and station metadata [28] can also be applied to define reference or rural stations.…”

“…Some previous studies indicated that urbanization has little impact on regional warming [7,13,26,27]. However, recent investigations have suggested that the urbanization process can not only increase the local daily temperature, but also play an essential role in regional climate change [6,9,12,[14][15][16][17][20][21][22][23][24][28][29][30]35,36]. Using the OMR method, Zhou et al [29] and Zhang et al [30] found that the urbanization exerts a significant influence on temperature trends in eastern China.…”

“…Zhang et al [39] performed a series of numerical simulations to show that the urban heat island effect in the YRD region features obvious diurnal variation, thus affects the structure and development of urban boundary layer. With records at 282 stations in northern China, Ren et al [15] determined that the urban warming rate is 0.11°C per decade, which accounts for 37.9% of the overall regional warming in northern China. However, they found that the urban heat island effect is more remarkable in winter than summer, unlike that in the YRD region [20].…”