Xin Huang, Jingyi Liu, Ke Ding, Zilin Wang, Rong Tang, Lian Xue, Haikun Wang, Qiang Zhang, Steven J Davis, Meinrat O. Andreae, Aijun Ding

Wildfires threaten human lives, destroy infrastructure, disrupt economic activity, and damage ecosystem services. A record-breaking gigafire event ravaged the western United States (USA) in mid-September 2020, burning 1.2 million acres (4,900 km2) in Oregon and California, and resulting in severe smoke pollution with daily fine particulate matter (PM2.5) concentrations over 300 µg/m3 for multiple days in many cities. Although previous studies have shown that regional warming escalates wildfire in the western USA, such an unprecedented fire cannot be explained by climate variability alone. Here we show that the synoptic-scale feedback between the wildfires and weather played an unexpectedly important role in accelerating the spread of this fire and also trapped pollutants in the shallow boundary layer over valley cities. Specifically, we find that aerosol-radiation interaction of the smoke plumes over the Cascade Mountains enhanced the downslope winds and weakened the moisture transport, thereby forming a positive feedback loop that amplified the fires and contributed to ~54% of estimated air-pollution related deaths. Our study underscores the complexity of the Earth system and the importance of understanding fundamental mechanisms to effectively mitigate disaster risks in a changing climate.