Contribution of urban expansion to surface warming in high-altitude cities of the Tibetan Plateau

Abstract

Large-scale climate change on the Tibetan Plateau (TP), the world’s Third Pole, has attracted extensive attention, while studies of urban climate on the TP have been sporadic. Here, we quantify urban expansion on the TP and its contribution to surface warming of typical plateau cities based on existing literature, observations, and numerical simulations. Large-scale warming of the surface atmosphere over the TP during 1975–2021 is estimated to be 0.31 [0.19 to 0.47] °C decade⁻¹ based on a set of rural meteorological sites, while the atmospheric warming of six typical highland cities on the TP during the same period is estimated to be 0.44 [0.31 to 0.53] °C decade⁻¹. This means that urbanization contributed about 30% of the warming in these cities. Lhasa is one of the cities with the strongest warming over the TP. The contribution of urban expansion to Lhasa’s air temperature increase is approximately 40%, which can be estimated using both observations and numerical simulations. The urban expansion footprint of Lhasa and its contribution to the spatial pattern and diurnal variation of atmospheric and land surface warming are quantified. The contribution of urbanization to the near-surface air temperature and land surface temperature of Lhasa from 2000 to 2020 is estimated to be 0.44 [0.39 to 0.49] °C and 1.44 [1.32 to 1.56] °C, respectively. These results provide potential global implications for urban studies in plateau areas.

Appendices

Appendix A1 Selection of buffer radius

In this study, the degree of urbanization in a buffer zone around a given weather station is used as the basis for selecting it as an urban site (Cao et al. 2016). To obtain a reasonable choice for the buffer radius, we analyzed the Spearman correlation coefficients of urban expansion rate and the annual T_2m trends for all 31 stations in a buffer area within a 1–10-km radius, with reference to Li et al. (2021). The urban expansion rate was defined as the trend of the urban ratio for all years of ESA’s available land cover products from 1992 to 2020. As shown in Appendix Fig. 10, the correlation between the urban expansion rate and the annual trend of T_2m increases significantly at the beginning and then stabilizes at 5 km. Thus, a buffer radius of 5 km was determined to be most appropriate.

 

Fig. 10

Spearman’s correlation coefficients between urban expansion and annual trend of T_2m at different radius (1–10 km)

Appendix A2 Elevation-dependent warming (EDW)

EDW, a phenomenon in which the warming rate varies systematically with elevation, is of importance for realistically estimating the warming rate on the TP. In our study, as shown in Appendix Fig. 11(a), the average elevation of urban sites is 3341.33 m and the average elevation of rural sites is 3528.58 m. The difference in elevation between urban and rural sites is 187 m. A review paper collected all existing studies on the elevation-dependent warming of the TP and documented that the observed EDW based on annual mean temperature was not apparent between 3300 and 3500 m above sea level (You et al. 2020). We further found that the variation in the warming rate with elevation was almost the same for urban and rural sites, as shown in Appendix Fig. 11(b). Therefore, the effect of elevation-dependent warming on urban-induced warming was excluded in this study.

 

Fig. 11

Elevation comparison between urban and rural sites (a). Comparison of elevation-dependent warming at urban and rural sites based on annual mean temperature observations (b)