Severe haze during the winter season has been troubling the citizens of Beijing over the past few decades, and caused wide-spread attention. To tackle the severe air pollution, Chinese government has implemented active clean air policies since 2013, giving rise to remarkable emission reduction rate in most parts of China.

However, severe haze episodes persisted in the Beijing-Tianjin-Hebei (BTH) region even in recent years. In 2015, 13 cities in BTH failed to meet the Chinese Air Quality Standard for 47.6% of the year. During the winter of 2016, the concentrations of PM2.5 and PM10 were still 2.1 and 1.7 times higher than the national standards, respectively.

Nevertheless, Beijing witnessed unusually blue skies in the winter of 2017. Researchers at the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences and the Institute of Urban Meteorology (IUM) in Beijing identified the critical role of unusual large-scale climatic conditions in the unusually clean winter of 2017.

Recently, these researchers published further results in Environmental Research Letters. They pointed out that the atmospheric circulation anomalies during the winter of 2017 were extremely favorable for the dispersion of pollutants in Beijing. Only three winters during the past 38 years were found to be similar to that of 2017, which could then be regarded as an extreme climate event.

The occurrences of such favorable winter circulation anomalies were found to have decreased by about 50% from the 1st to the 2nd half of the 20th century. A further 60% reduction from 1951-2000 to 2050-2099 was projected by using multi-model simulations under the Representative Concentration Pathway 8.5 scenario of Intergovernmental Panel on Climate Change (IPCC).

According to this study, winters will go on being dismal for the 20 million people living in Beijing with further global warming, if without serious cuts in pollution emissions. The local government has to consider from a perspective of large-scale climate change when coping with the issue of air pollution in Beijing.