Evidence for a significant urbanization effect on climate in China

Abstract

China has experienced rapid urbanization and dramatic economic growth since its reform process started in late 1978. In this article, we present evidence for a significant urbanization effect on climate based on analysis of impacts of land-use changes on surface temperature in southeast China, where rapid urbanization has occurred. Our estimated warming of mean surface temperature of 0.05°C per decade attributable to urbanization is much larger than previous estimates for other periods and locations. The spatial pattern and magnitude of our estimate are consistent with those of urbanization characterized by changes in the percentage of urban population and in satellite-measured greenness. Land-use changes from urbanization, creating an urban heat island (UHI), have been suspected as partially being responsible for the observed warming over land during the last few decades because of (i) the observed decrease in the diurnal temperature range (DTR) resulting from a larger increase or a smaller decrease in minimum temperature relative to maximum temperature and (ii) a lower rate of warming observed over the past 20 years in the lower troposphere compared with the surface (1). The area-weighted average warming effect of UHI over land during the 20th century has been estimated to be Ͻ0.06°C per century (1-4) globally and approximately 0.06ϳ0.15°C per century (5, 6) in the U.S. based on differences in temperature trends between rural and urban stations. A much larger estimate of 0.27°C per century in the U.S. has been reported recently (7) by comparing trends in observed and reanalysis surface temperatures over the period from 1950 to 1999.China has experienced rapid urbanization and dramatic economic growth since its reform process started in late 1978. From 1978 to 2000, China's gross domestic product grew at an average annual rate of 9.5%, compared with 2.5% for developed countries and 5% for developing countries; the number of small towns soared from 2,176 to 20,312, nearly double that of the world average during this period; the number of cities increased from 190 to 663; and the proportion of urban population rose from 18% to 39% (see the Peopledaily article at http:͞͞english. peopledaily.com.cn͞200111͞27͞eng2001112785410.shtml and the State Family Planning Commission of China web site at www.sfpc.gov.cn͞EN͞enews20030320-1.htm). In this article, we present evidence for a significant urbanization effect on climate based on analysis of impacts of land-use changes on surface temperature in southeast China, where most of China's urbanization has occurred. Data and MethodsThe UHI effect has been estimated by comparing observed temperatures in urban stations with those in their surrounding rural stations, but such results largely depend on how rural versus urban stations are classified and whether the data are homogeneous (7-9). Population data often are used to identify a station as urban and rural, but such information generally is out-of-date, and thus satellite measurements of night lights have been substituted re...

The reliability of global and hemispheric surface temperature records

“…The assumption here is that reanalyses do not know about changes in land use. Initial work in this area was suggestive of a large effect [e.g., Zhou et al (2004) for southern China], but more detailed studies over different parts of China and for different periods showed results were very susceptible to the choices of region and period. Figure 1 shows hemispheric and global averages from the four groups, with results expressed as anomalies from the 1961-90 base period used by HadCRUT4.…”

Urbanization and heterogeneous surface warming in eastern China

“…For its unique low-light imaging capacity, the nighttime light imagery can not only be used as the characteristic index in the analysis of urbanization effect, but also meet the needs of detection in human activities [49,50]. In terms of Zhou et al [29] and Hu et al [35], the urbanization process in China primarily started around 1979. Therefore, observed data during the period from 1979 to 2008 in eastern China (15°-54°N, 110°-136°E) is used in the following analysis.…”

“…The OMR method is based on the fact that the reanalysis data did not assimilate observed surface information. This method does provide some signatures consistent with the distribution of underlying surface properties [22,[29][30][31][32][33][34][35][36]. However, whether or not the OMR fully represents the impact of LUCC is still controversial [37,38].…”

“…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.…”

Detecting change in urban areas at continental scales with MODIS data

“…The consequences of urbanization are largely contingent on the size, location, and configuration of development (Weng, 2001;Zhou et al, 2004), with many environmental impacts exacerbated when new growth is expansive and/or fragmented in form (Alberti, 2005). A meta-analysis of urban expansion indicates that localto regional-scale studies are geographically biased, leaving even many large cities unstudied (Seto et al, 2011).…”