An Updated Estimate of the Urban Heat Island Effect on Observed Local Warming Trends in Mainland China's 45 Urban Stations

Abstract

Observed surface air temperature (SAT) warming at urban stations often contains both the signal of global warming and that of local urban heat island (UHI) effects; these signals are difficult to separate. In this study, an urban impact indicator (Uii) developed by the authors was modified to represent the extent to which the observed temperature from a given station was influenced by UHI effects. The Uii of a city was calculated by simplifying the city's shape to a circle. In addition, a modified Uii (MUii) was calculated by considering the realistic horizontal distribution of the urban land. We selected 45 urban stations in mainland China, along with an adjacent station for each to give a station pair. Background climate changes across each pair were near-homogeneous. Thus, differences in the trends of annual averaged daily mean SAT (Trend_mean), maximum SAT (Trend_max), and minimum SAT (Trend_min) between the urban and adjacent stations (ΔTrend) could be mainly attributed to differences in MUii changes between the urban and adjacent stations (ΔMUii). Several linear regressions between ΔTrend and ΔMUii for the 45 station pairs were calculated to estimate UHI effects on Trend_mean (UT_mean), Trend_max (UT_max), and Trend_min (UT_min). The results showed that the mean MUii of the 45 urban stations increased from 0.06 to 0.35 during 1992–2013. Positive correlations between ΔMUii and ΔTrend for the 45 station pairs were significant at the 0.001 significance level (except for Trend_max). The average UT_mean and UT_min of the 45 urban stations during 1954–2013 were approximately 0.05 and 0.11°C decade⁻¹, respectively, accounting for 18 % and 31 % of the overall warming trends, respectively. The UT_min estimated in this study is about twice that of previous results based on regression equations between Uii and SAT trends.