On Feb 2, 2023, Science published a paper titled "Smoke-weather interaction affects extreme wildfires in diverse coastal regions" by Professor Aijun Ding and Professor Xin Huang's team. It was found that wildfires, which are globally important natural disasters, are not only affected by meteorology, instead, the radiative effects of smoke aerosols can modify meteorological parameters, thus resulting in a positive feedback mechanism at the weather scale that can significantly enhance extreme wildfire events in diverse coastal regions around the world.
Wildfires directly endanger human life and property, and the pollutants emitted by burning can negatively affect air quality, human health, and ecosystems in downwind areas. What is more, the aerosols and greenhouse gases emitted by wildfires can disturb the radiation balance and play a vital role in the Earth's climate system. The evolution of wildfire in vegetation ecosystems can be affected by many factors, and natural factors such as meteorology is an important driving force in addition to human activities. Meteorology affects vegetation productivity (fuel availability), determines the length of fire seasons (fuel flammability), and influences the spread of the fires (fire behavior). Because of this, prior studies have mostly correlated the increased frequency of extreme wildfire events with climate change. However, the frequency and intensity of wildfires occur at multiple spatial and temporal scales. In the context of a global warming trend that is unlikely to change in the near future, it is more important to understand the complex physicochemical mechanisms and key control processes that influence the occurrence, development, and extinction of wildfires within the current predictable time window (e.g., on a 1-2 week weather scale), so as to provide scientific support for proactive human response and precise intervention for disaster prevention and mitigation.
Fig. 1. Synoptic-scale variability of wildfires and meteorology during extreme fires in typical fire regions in the globe.
The authors applied empirical mode decomposition and Fourier transform to daily burned area in major wildfire regions during 2002–2021. Figure 1 shows that synoptic-scale (<20 days) variation of wildfires is evident in most fire-prone areas across the globe. Among them, the US West Coast and Southeastern Asia are characterized by the most pronounced fluctuations down to 1 to 2 weeks, corresponding to the time period of extreme fires that have repeatedly ravaged both regions. Given the dense populations in these regions, the impacts of these fires on air quality and the associated human health exposure are enormous. The team further conducted meteorology-chemistry–coupled simulations using the WRF-Chem model on the most devastating fire, which engulfed the Indo-China Peninsula in March 2004 and a series of wildfires in Oregon and California (defined as a "gigafire" for an area >1 million acres) in September 2020. In a Mediterranean climate with dry and hot summers, such as that in the US West Coast, increased aerosol loading from wildfire suppresses the development of the PBL and enhances orographic winds, thereby increasing the large fire potential at the western slope of the Cascade Mountains through higher wind speed and lower humidity. Conversely, in the fire-intensive region of Southeastern Asia, which has a monsoon climate, thick fire smoke tends to cool the land surface but warm the atmosphere over the sea. The opposite air temperature responses over the land and sea modify the monsoon circulation and block the onshore transport of moisture, thereby suppressing rainfall. More flammable vegetation due to the hydrologic drought intensifies the fire activities and prolongs the burning period. Although the feedbacks for the two different climatic zones on the east and west coasts appear different, in essence, they can be well explained by a unified mechanism. Both are driven by the radiative effects of fire smoke over different land covers and terrains, which cause thermal contrast and thus enhanced fire emission by modifying circulations and water vapor transport.
Fig. 2. Conceptual model of the fire-weather feedback in the Mediterranean and monsoon climate regimes.
Professor Aijun Ding's team has been working on aerosol-PBL-radiation interactions for nearly a decade. Based on the first-hand observation data from the Station for Observing Regional Processes of the Earth System (SORPES), the team applied numerical simulation to reveal the aerosol-PBL physicochemical processes caused by radiatively active aerosols emitted from mixed biomass burning, and found that the related processes can affect secondary aerosol generation, change the thermal structure of PBL and local temperature and precipitation (Atmos. Chem. Phys. 2013, 2016). Further studies found that the interaction between black carbon and boundary layer (named the dome effect) significantly aggravates heavy haze pollution in China's megacities (Geophys. Res. Lett. 2016, 2018; Atmos. Chem. Phys. 2018), and enhances the cross-regional transport of haze pollution in North China Plain (NCP) and the Yangtze River delta (YRD) (Nature Geoscience, 2020). The team further investigated the influence of related processes on weather and climate at regional and global scales, and found that aerosols can affect daily weather prediction in several typical regions around the world (Sci. Bull. 2021), and a synergetic effect of aerosol-cloud-boundary layer interaction with the monsoon is the main reason for the strong semi-direct effect and enhanced low cloud formation in southeastern Asia (Nature Communications, 2021), which could lead to stronger transcontinental ozone transport in El Niño years (Nat. Sci. Rev., 2021). More than 10 papers have been selected as ESI Highly Cited Papers, and many have been selected as "China's 100 most influential international academic papers". Researchers of the team received the ICTP award, the Distinguished Young Scholars from National Natural Science Foundation of China (NSFC) and etc.
The paper published in Science is an important breakthrough based on the above-mentioned series of work. The innovation lies in the discovery that aerosols emitted from combustion can significantly change key meteorological parameters such as wind speed, humidity and precipitation that affect fire behavior through a complex feedback process. This finding provides important scientific support for the prediction and early intervention of extreme wildfire events in densely populated coastal areas worldwide, and is also expected to provide a reference for the prevention and control of mountain fires in complex terrain areas such as western China and the Greater Khingan Range; and have important application value for air pollution prevention and control, weather prediction and climate change response in the globe.
Professor Xin Huang and research scientist Ke Ding are the co-first authors of this paper. Professor Aijun Ding and Professor Xin Huang are corresponding authors. Academician Congbin Fu, Academician Zhemin Tan, and scholars from Max Planck Institute for Chemistry, University of California and Tsinghua University are co-authors of the paper. This work was supported by the National Natural Science Foundation of China (grant 41725020 to A.D., grant 41922038 to X.H., and grant 13001146 to K.D.), by Fundamental Research Funds for the Central Universities (grant 14380187 to X.H.), and by the Tencent Foundation through the XPLORER PRIZE to A.D.
Huang X, Ding K, Liu J, Wang Z, Tang R, Xue L, Wang H, Zhang Q, Tan Z-M, Fu C, Davis S J, Andreae MO, Ding A, Smoke-weather interaction affects extreme wildfires in diverse coastal regions, Science, 379, 6631, 457-461, https://doi.org/10.1126/science.add9843, 2023.
Ding K, Huang X, Ding A, Wang M, Su H, Kerminen V-M, Petaja T, Tan Z-M, Wang Z, Zhou D, Sun J, Liao H, Wang H, Carslaw K, Wood R, Zuidema P, Rosenfeld D, Kulmala M, Fu C, Poschl U, Cheng Y, Andreae MO, Aerosol-boundary-layer-monsoon interactions amplify semi-direct effect of biomass smoke on low cloud formation in Southeast Asia, Nature Communications, 12, 6416,https://doi.org/10.1038/s41467-021-26728-4, 2021.
Huang X, Ding A, Aerosol as a critical factor causing forecast biases of air temperature in global numerical weather prediction models, Science Bulletin, 66，18, 1917-1924, https://doi.org/10.1016/j.scib.2021.05.009, 2021.
Xue L, Ding A, Cooper O, Huang X, Wang W, Zhou D, Wu Z, McClure-Begley A, Petropavlovskikh I, Andreae MO, Fu C, ENSO and Southeast Asian biomass burning modulate subtropical trans-Pacific ozone transport, National Science Review, 8(6), nwaa132, https://doi.org/10.1093/nsr/nwaa132, 2021.
Huang X, Ding A, Wang Z, Ding K, Gao J, Chai F, Fu C, Amplified transboundary transport of haze by aerosol–boundary layer interaction in China, Nature Geoscience, 13, 428-434, https://doi.org/10.1038/s41561-020-0583-4, 2020.
Wang Z, Huang X, Ding A, Dome effect of black carbon and its key influencing factors: a one-dimensional modelling study, Atmos. Chem. Phys., 18, 2821-2834, https://doi.org/10.5194/acp-18-2821-2018, 2018.
Huang X, Wang Z, Ding A, Impact of aerosol-PBL interaction on haze pollution: Multiyear observational evidences in North China, Geophys. Res. Lett., 45, 16, 8596-8603,https://doi.org/10.1029/2018GL079239, 2018.
Ding A, Huang X, Nie W, Sun J, Kerminen V-M, Petaja T, Su H, Cheng Y, Yang X, Wang M, Chi X, Wang J, Virkkula A, Guo W, Yuan J, Wang S, Zhang R, Wu Y, Song Y, Zhu T, Zilitinkevich S, Kulmala M, Fu C, Enhanced haze pollution by black carbon in megacities in China, Geophys. Res. Lett., 43, 6, 2873-2879, https://doi.org/10.1002/2016GL067745, 2016.
Huang X, Ding A, Liu L, Liu Q, Ding K, Niu X, Nie W, Xu Z, Chi X, Wang M, Sun J, Guo W, Fu C, Effects of aerosol–radiation interaction on precipitation during biomass-burning season in East China, Atmos. Chem. Phys.,16,10063-10082, https://doi.org/10.5194/acp-16-10063-2016, 2016.
Ding A, Fu C, Yang X, Sun J, Petaja T, Kerminen V-M, Wang T, Xie Y, Herrmann E, Zheng L, Nie W, Liu Q, Wei X, Kulmala M, Intense atmospheric pollution modifies weather: a case of mixed biomass burning wither fossil fuel combustion pollution in the eastern China, Atmos. Chem. Phys.,13,10545-10554, https://doi.org/10.5194/acp-13-10545-2013, 2013.