Emissions were not entirely to blame for one of China's worst haze pollution events on record, according to a new study; rather, the heavily polluted air observed over the East China Plains in 2013 was a consequence of decreasing Arctic sea ice and increasing Eurasian snow that together produced stagnant atmospheric conditions in the region. The findings are an example of how large-scale perturbations caused by global climate change can have significant regional impacts. In January 2013, the East China Plains suffered from unprecedented large-scale haze lasting almost an entire month, a so-called "airpocalypse." Researchers have studied short-term weather conditions, such as a weak East Asian winter monsoon, contributing to the event, but the underlying climate effects are still not well understood. Despite years of reduced emissions, severe winter haze has persisted in the region, indicating other factors may also be at play. Yufei Zou and colleagues studied ventilation conditions over the last 35 years in the region, which is made up of interconnected horseshoe-shaped basins where ventilation of air pollutants relies heavily on large-scale weather systems. By looking at wind speed and air temperature data, Zou et al. created their own "Pollution Potential Index" to quantify the effect of ventilation on air pollution. The results connect the 2013 haze event to poor ventilation conditions that have not been seen in the past three decades. The authors also analyzed climate-related patterns that contribute to poor ventilation and extreme haze. The findings suggest that Arctic sea ice loss the autumn before January 2013, as well as extensive boreal snow in the preceding months, drove the haze event. If Arctic sea ice continues to melt, similar poor ventilation conditions in winter may occur more frequently in eastern China. The authors say this finding provides a "strong incentive" for more stringent emission reductions in China and note that haze could affect Beijing's hosting of the 2022 Winter Olympics.