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大火扰动植被加剧沙尘排放
题 目：Enhanced dust emission following large wildfires due to vegetation disturbance (发表于Nature Geoscience)
参与方式：线上参会（具体参会方式请扫描文末二维码报名选择）
会议时间：报告总时长约35分钟, 留言板&语音提问讨论25~40分钟
北京时间 11月11日（周五）上午10:00
加州时间 11月10日（周四）晚上6:00
悉尼时间 11月11日（周五）中午13:00
报告人简介
Dr. Yan Yu, Assistant Professor in the Department of Atmospheric and Oceanic Sciences at Peking University. Yan got her bachelor's degree in Environmental Engineering from Tsinghua University in 2010. She completed two master's degrees in Environmental Sciences (2013) and Statistics (2016) and received her PhD degree in Atmospheric and Oceanic Sciences (2017) from the University of Wisconsin-Madison. She was awarded the prestigious NASA postdoc program fellowship at the California Institute of Technology Jet Proposition Laboratory. After postdoc appointments at UCLA and Princeton University, Yan came back to China and became a faculty member at Peking University in 2021. Yan’s research involves land-atmosphere interactions with a particular focus on the climate feedbacks through dust and wildfire processes.
俞妍博士，北京大学物理学院大气与海洋科学系助理教授、研究员、“博雅”青年学者。清华大学学士（环境工程）， 威斯康星大学麦迪逊分校硕士（环境学、统计学）、博士（大气与海洋科学）。曾获美国航天航空局博士后奖学金，在加州理工大学喷气推进实验室、加州大学洛杉矶分校和普林斯顿大学开展博士后研究。主要探索陆地与大气的相互作用，特别是其中沙尘和野火过程在全球气候环境变化下的响应和反馈。
报告摘要
Large wildfires reduce vegetation cover and soil moisture, leaving the temporally degraded landscapes an emergent source of dust emission. However, the global extent of post-fire dust events and their influencing factors remain unexplored. Using satellite measurements of active fires, aerosol abundance, vegetation cover and soil moisture from 2003 to 2020, here we show that 54% of the examined ~150,000 global large wildfires are followed by enhanced dust emission, producing substantial dust loadings for days to weeks over normally dust-free regions. The occurrence and duration of post-fire dust emission is controlled primarily by the extent of precedent wildfires and resultant vegetation anomalies and modulated secondarily by pre-fire drought conditions. The intensifying wildfires and drying soils during the studying period have made post-fire dust events one day longer, especially over extratropical forests and grasslands. With the predicted intensification of regional wildfires and concurrent droughts in the upcoming decades, our results indicate a future enhancement of sequential fire and dust extremes and their societal and ecological impacts.
研究团队分析燃烧火点、气溶胶、植被、土壤湿度等多源遥感资料，发现大面积、持续性的生物质燃烧破坏植被、降低土壤湿度，导致燃烧后沙尘排放。上述现象在全球干旱地区以外的大范围地区造成了沙尘浓度显著升高，揭示全球气候变化下干旱—野火—沙尘这一联合气象生态灾害加剧的可能性。
研究背景
Wildfires represent a major ecosystem disturbance and aerosol emission source, affecting the global carbon budget, the climate and human life. Among the natural consequences of the destroyed vegetation, especially the short species such as grasses and shrubs, is the expansion of bare ground that is particularly susceptible to wind erosion—the detachment of soil particles from the ground and dust storms in an extreme condition. In situ observations and modelling studies have confirmed dust emission from post-fire landscapes mostly in North America; yet post-fire dust emission has not yet been globally examined using observational data.
野火是发生在自然植被类型上的生物质燃烧现象，是陆地生态系统的主要扰动源和天然气溶胶的主要自然排放源之一，影响全球碳循环、气候和经济社会。理论上说，野火破坏植被导致裸土面积增加，进而降低地表粗糙度和空气动力学阻力导致近地面风速上升，同时野火燃烧有机质降低土壤粘结力和土壤湿度。以上结果造成野火后有利于沙尘排放的陆面和大气条件。前人利用地面观测资料在北美捕捉到野火后沙尘排放的现象；全球其他地方是否也有这样的现象？野火后沙尘排放的强度和频率有多高？
研究发现
A. 2019–2020年澳大利亚丛林大火后的沙尘排放
Following long-lasting drought conditions, a series of large wildfires burned a historic 186,000 km2 across eastern Australia during the 2019–2020 bushfire season. Satellite observations indicate a record-breaking low vegetation cover and high dust concentration across the burned regions during December 2019 to February 2020. Such a massive amount of dust particles is emitted mainly from the burned regions that witness the most severe vegetation damage. For example, the savannahs (around 27.8° S, 152.3° E) to the west of Brisbane experience a substantial reduction in vegetation cover (EVI drops from about 0.25 to 0.17, compared with a long-term average of 0.35) after the extensive fires during 7–13 November 2019. Following the persistent vegetation disturbances and abnormally dry soils, extreme DOD episodes are observed in December 2019 (Fig. 1).
2019年末到2020年初，澳大利亚东南部发生创纪录的极端大火，导致这一地区植被指数降低至往年同期的一半左右，过火区域沙尘浓度是往年同期的两倍以上（图1）。
Fig. 1: Extreme dust activity associated with vegetation disturbances caused by the 2019–2020 Australian bushfires. a, True-colour image of aerosol plumes originating from the active fires and burned areas in southeastern Australia on 22 December 2019. b, Time series of active fire count (orange bars, referring to the left y axis), EVI (green line, referring to rightmost y axis) and daily maximum DOD (black dots, referring to the inter right y axis) within ±0.05° of 27.8° S, 152.3° E. c, Anomaly in EVI during December 2019 to February 2020 compared with the long-term average during December to February of 2000–2020. d, Anomaly in DOD during December 2019 to February 2020 compared with the long-term average during December to February of 2000–2020.
图 1.2019-2020年澳大利亚丛林大火引发的植被扰动和沙尘排放。a.过火区域气溶胶羽真彩图。b. 27.8° S, 152.3° E周围±0.05°区域内火点数、植被指数(EVI)和日最大沙尘气溶胶光学厚度(DOD)的时间序列。c. 2019年12月到2020年2月间EVI距平。d. 2019年12月到2020年2月间DOD距平。
B.燃烧后沙尘排放事件的全球分布
On the basis of the analysed satellite measurements of fire, vegetation and aerosols during the 18 years of 2003–2020, we identify 151,727 large wildfire events with more than 20 active fires detected in a 0.1° × 0.1° pixel during consecutive seven days. Among the analysed large fire events, 91% and 54% are followed by significant EVI reduction and consequent dust events, respectively, during the subsequent 60 days. These 87,400 post-fire dust emission events are distributed across 36,386 0.1° pixels in the fire-prone regions of tropical savannahs in Africa, South America and northern and eastern Australia, shrublands in western Australia, grasslands and croplands in central Asia and various landscapes in western North America. Among different landscapes, global savannahs contribute 66% of the currently identified large fire events, 59% of consequent significant EVI reduction and 51% of post-fire dust events. The post-fire dust events typically last 1–25 days, and the maximum DOD ranges between 0.2 and 1.8; the intensity and duration of post-fire dust events vary by land-cover type (Fig. 2).
The occurrence and duration of post-fire dust emission depends largely on the temporal and spatial extent of precedent wildfires (Fig.3). As the extent of wildfires increases, the posterior vegetation cover lowers, weakening its physical protecting and aerodynamical sheltering effects on soils. Consequently, the probability of less vegetation, more dust situations elevates after the occurrence of large wildfires and further enhances with the increment of precedent fire counts.
本研究考察了逾15万次大面积、持续性的全球生物质燃烧事件，其中54%的燃烧事件导致燃烧结束后两个月内发生显著的沙尘浓度升高（图2）。野火后沙尘排放主要发生在非洲稀树草原地区、美国西部、亚马逊以南的南美地区、澳大利亚、中亚、喜马拉雅山脉南侧和中南半岛以及东北亚地区。野火后沙尘排放导致的沙尘浓度最大值，接近干旱半干旱地区（如撒哈拉沙漠和塔克拉玛干）沙尘暴发生时的沙尘浓度；野火后沙尘排放可持续几天到几周。野火后沙尘排放的强度主要取决于野火燃烧面积和持续时间、以及野火后植被破坏程度。随着野火的燃烧面积和持续时间上升，植被破坏程度加剧，野火后沙尘排放的发生及强度都有所上升（图3）。
Fig 2. Global distribution of post-fire dust events. a, Occurrence of post-fire dust events during 2003–2020 indicated by the size of dots, with colour representing the dominant land-cover type. b, Total occurrence of large wildfire events (orange bars), significant EVI reduction (green bars) and dust emission (grey bars) by land-cover type. c, Maximum DOD (colour of dots), representing the columnar dust loading associated with the most intensive post-fire dust emission, and mean duration (days, size of dots) of post-fire dust events. d, Box plot (thin lines, 10th–90th percentiles; thick lines, 25th–75th percentiles; intersection, median of both metrics) of maximum DOD and duration of post-fire dust events by land-cover type.
图2. 全球野火后沙尘排放导致的最大沙尘气溶胶光学厚度和单次燃烧事件后沙尘排放最长持续天数的空间分布及按植被类型统计。
Fig 3: Severity of post-fire dust events regulated by the extent of precedent wildfires and vegetation disturbance. a, Probability distribution of post-fire, 30-day average DOD (top) and EVI (bottom) as a function of number of precedent fires. b, Scatterplot of post-fire minimum EVI (referring to the left y axis) and number of precedent fires. The boxes in b indicate the 10th, 25th, 50th, 75th and 90th percentiles of post-fire dust event duration (days, referring to the right y axis) with precedent fires ranging between 21 and 30, 31 and 40, 41 and 50, 51 and 60, 61 and 70, 71 and 80, 81 and 90, 91 and 100 and above 100 during the burning period.
图3. 沙尘排放强度受野火燃烧面积和持续时间以及野火后植被破坏程度调控。a. 沙尘浓度（DOD）和植被植被（EVI）随野火时空范围变化的概率分布。b. 野火后最低植被指数(EVI)及其对应的沙尘浓度(DOD)散点图和箱图。
C. 2003到2020年间野火后沙尘排放事件演变
The duration of these post-fire dust events has been increasing significantly over all landscapes, with positive trends of 0.82, 0.54, 0.28, 1.05 and 0.80 days per decade over forests, shrublands, savannahs, grasslands and croplands, respectively, during 2003–2020 (Fig. 4). Indeed, the most long-lasting dust-emission events are widely seen in either 2019 or 2020 over 4,699 pixels out of the 36,386 examined pixels, such as those during the 2019–2020 Australian bushfire season and 2020 western United States extreme fire season. This recent elongation of post-fire dust emission is attributed here to the increased extent of wildfires, as indicated by the positive trends in active fire counts. Meanwhile, the analysed forest, grassland and cropland pixels exhibit moderately significant reduction in pre-fire soil moisture during 2003 to 2020.
伴随着地区性更频发、更持久的干旱，以及上升的燃烧面积和强度，2003到2020年间主要地表覆盖类型都经历了野火后沙尘排放持续时间变长的趋势。其中2019到2020年澳大利亚丛林大火和2020年北美西部山火，都贡献了上述地区2003年以来持续时间最长的野火后沙尘排放事件。
Fig 4: Observed evolution of the occurrence and duration of post-fire dust events during 2003–2020. a,b, Time series of the occurrence (a) and mean duration (days) (b) of post-fire dust events for each dominant land-cover type during 2003–2020. c, Year of the longest-duration post-fire dust event.
图4. 2003–2020年间野火后沙尘排放事件的(a)发生频率, (b)持续时间，和(c)持续时间最长的年份。
结论
Our findings have direct implications for the ecological and societal impacts of intensifying droughts and wildfires over certain landscapes. The post-fire dust storms identified are as intensive as those observed in arid to semi-arid lands and cause similar infrastructure damage and air quality decline. The emitted soil particles from these disturbed lands might enter the global dust cycle, altering the radiation budget, cloud and precipitation patterns, as well as oceanic and terrestrial biogeochemistry. Furthermore, the relationship between drought, fires and post-fire dust emission will potentially become more interactive and complicated in the upcoming decades, in response to anthropogenic activity and global warming.
在全球气候环境变化下，很多地区面临更频发、更持久的干旱，同时全球变暖、部分地区变干导致这些地区野火的燃烧面积和强度也有上升趋势。本研究揭示了干旱—野火—沙尘这一联合气象—生态灾害现象，在全球变暖的趋势下这一联合灾害在部分地区将会加剧。同时，干旱—野火—沙尘的耦合过程及其对气候变率和气候变化的反馈作用也值得更深入的研究。
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