Mass human migration and Beijing's urban heat island during the Chinese New Year holiday

Population movements around the Chinese New Year (CNY), which are much larger in recent years than before, are the largest annual human migration in the world. However, it is still largely unknown how or to what extent such mass human migration affects urban climate. Here, we investigate the role of mass human migration in influencing Beijing's urban heat island (UHI) during the CNY holiday for the period of 2004-2013. We find that the UHI effects expressed as daily mean (ΔTmean), maximum (ΔTmax), and minimum (ΔTmin) temperature differences between urban and rural areas show a weakening trend during the CNY week relative to the background period (4 weeks including 2-3 weeks before and 2-3 weeks after the CNY week). In particular, large reductions occurred during the CNY week for the period of 2009-2013, when nearly half of population left the city before the CNY holiday. ΔTmean, ΔTmax, and ΔTmin averaged over the period of 2009-2013 during the CNY week were 0.64, 0.45, and 0.83°C lower than during the background period, representing relative reductions of 35%, 66%, and 27%, respectively. Our findings highlight the important role of modern mass human migration for urban climate based on a case study in Beijing.