From the CO2Science archive: Introducing the topic of their paper, Ren et al. (2015) state that “the possible systematic biases due to the urbanization effect [on surface air temperature (SAT) records] have not been adequately addressed, and arguments on the nature and magnitude of the urbanization effects for global and continental scale SAT change studies have not been completely settled,” citing the works of Hughes and Balling (1996), Houghton et al. (2001), Kalnay and Cai (2003) and Vose et al. (2004). For the focus of their work, therefore, the team of nine Chinese researchers set out to analyze the influence of urbanization on a regional network of surface temperature stations across mainland China over the period 1961-2004. Their choice of China for analysis was simple, as they say that during the past half-century, “mainland China has experienced an unprecedented [rate of] urbanization.” Whereas only 11 percent of the 540 million Chinese population resided in urban areas in 1949, these numbers jumped to 52.6 percent and 1.35 billion persons by 2012. So what did their study reveal?

Paper reviewed: Ren, G., Li, J., Ren, Y., Chu, Z., Zhang, A., Zhou, Y, Zhang, L., Zhang, Y. and Bian, T. 2015. An integrated procedure to determine a reference station network for evaluating and adjusting urban bias in surface air temperature data. Journal of Applied Meteorology and Climatology 54: 1248-1266.

Ren et al. found a significant urbanization influence for most stations or regions in China. The country-wide average annual and seasonal mean effects of urbanization, as well as their relative contributions to the overall warming trends observed over the 1961-2004 period, are presented in the table below, all of which values are significant at the 0.05 confidence level. In commenting on these numbers, the authors caution that they “should be regarded as the lowest [or most conservative] estimates” because the background stations from which the trends were derived were themselves “subject to local human interference.” In addition, they say “the principles and criteria formulated for selecting the reference stations [from which to calculate the urbanization effect] had been somewhat compromised for a variety of reasons” and that “remnant urbanization effects must have been kept in the SAT series of the reference stations.”

Table 1. Country-averaged annual and seasonal mean urbanization effects for Chinese SAT stations over the period 1961-2004. Asterisk indicates statistical significance at the 0.05 confidence level.

The significance of the authors’ findings is found in the introductory material to their analysis, wherein they rightly state the need to factor out and adjust for the effects of urbanization in order to produce reliable data for climate change studies. Without such corrections, “it is impossible to establish a global or regional SAT series that realistically represents the changes in large-scale temperature fields,” and as a result, “a convincing conclusion is less likely to be drawn from the studies of regional or global climate change detection and attribution.” Indeed, and despite the authors’ best efforts to fully account for the urbanization influence in this study, it appears they too have fallen short of this important goal. Nevertheless, they have done a good service by illustrating the existence and relative magnitude of an urban warming bias in Chinese SATs.

References
Hughes,W.S. and Balling, R.C. 1996. Urban influences on South African temperature trends. International Journal of Climatology 16: 935-940.

Houghton, J.T., Ding, Y., Griggs, D.J., Noguer, M., van der Linden, P.J., Dai, X., Maskell, K. and Johnson, C.A. 2001. Climate Change 2001: The Scientific Basis. Cambridge University Press, 881 pp.

Kalnay, E. and Cai, M. 2003. Impact of urbanization and land-use change on climate. Nature 423: 528-531.

Vose, R.S., Karl, T.R., Easterling, D.R., Williams, C.N. and Menne, M.J. 2004. Impact of land use change on climate. Nature 427: 213-214.