Decreasing fire season precipitation increased recent western US forest wildfire activity
Decreasing fire season precipitation increased recent western US forest wildfire activity
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DOI：
10.1073/pnas.1802316115
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发表时间：
2018-09-04
影响因子：
11.1
通讯作者：
Affleck, David
中科院分区：
文献类型：
--
作者：
Holden, Zachary A.;Swanson, Alan;Affleck, David
美国西部野火的增加通常归因于气温升高，要么是由于冬季积雪的影响，要么是由于夏季蒸发的影响。然而，近地表空气温度和蒸发需求受到水分有效性的强烈影响，这些相互作用及其在调节火灾活动中的作用尚未得到充分探讨。在这里，我们表明，1979年至2016年，美国西部31-45%的森林地区夏季降水的下降与燃烧面积的变化密切相关，此前未被注意到。在火灾季节，湿雨日数（WRD；降水日数>= 2.54 mm）在一定程度上调节了温度和随后的蒸汽压亏缺（VPD），而VPD此前被认为是年度野火面积燃烧的主要驱动因素。利用通径分析方法分解积雪减少、气温上升和降水减少对观测到的火灾活动增加的相对影响。考虑相互作用后，WRD异常对野火燃烧面积的净效应是VPD的2.5倍以上，并且WRD和VPD的影响都大大大于冬季积雪的影响。这些结果表明，火灾季节降水对烧伤面积的控制作用最强，要么直接通过湿润作用，要么间接通过反馈到VPD。如果这些趋势持续下去，夏季降水的减少和相关的夏季干旱的增加将导致美国西部更多的被烧毁地区，并产生深远的生态和社会经济影响。
Western United States wildfire increases have been generally attributed to warming temperatures, either through effects on winter snowpack or summer evaporation. However, near-surface air temperature and evaporative demand are strongly influenced by moisture availability and these interactions and their role in regulating fire activity have never been fully explored. Here we show that previously unnoted declines in summer precipitation from 1979 to 2016 across 31-45% of the forested areas in the western United States are strongly associated with burned area variations. The number of wetting rain days (WRD; days with precipitation >= 2.54 mm) during the fire season partially regulated the temperature and subsequent vapor pressure deficit (VPD) previously implicated as a primary driver of annual wildfire area burned. We use path analysis to decompose the relative influence of declining snowpack, rising temperatures, and declining precipitation on observed fire activity increases. After accounting for interactions, the net effect of WRD anomalies on wildfire area burned was more than 2.5 times greater than the net effect of VPD, and both the WRD and VPD effects were substantially greater than the influence of winter snowpack. These results suggest that precipitation during the fire season exerts the strongest control on burned area either directly through its wetting effects or indirectly through feedbacks to VPD. If these trends persist, decreases in summer precipitation and the associated summertime aridity increases would lead to more burned area across the western United States with far-reaching ecological and socioeconomic impacts.