Recent wildfires in the western United States have led to substantial economic losses and social stresses. There is a great concern that the new climatic state may further increase the intensity, duration, and frequency of wildfires. To examine temporal and spatial features of historical wildfire trends and future changes, a common fire index, the Keetch–Byram Drought Index (KBDI), is calculated over the contiguous United States (CONUS) and Alaska. We introduce an efficient initialization method and calculate 36 years (1982–2017) of historical KBDI at 4 km using a high-quality observational dataset. KBDIs are also calculated at 12 km using regional climate models and extended into the mid- and late-21st century. Based on the observational data, annual mean (and 95th percentile) KBDI over forested regions in the southwestern and northwestern CONUS has risen since 1982 at a rate of 5.2 (4.0) and 2.9 (3.3) per year, respectively, indicating a persistent drying trend fostering fire activities; the number of days exceeding the top 5% historical KBDI has increased by 16 and 25 d in the 36 years. Multiple regional climate model simulations project increasing wildfire potential and longer fire seasons over broader areas based on the estimated KBDI for the mid- and late-21st century. By the end of the century, most of the CONUS would experience about 90–189 more days per year exceeding the historical local maximum KBDI; areas with high KBDI (>600), whose occurrence correlates with large burned area, are projected to broaden by nearly 60 times in the southern CONUS. While both temperature and precipitation contribute to future KBDI changes, warming is the main driver of more frequent, intense and wide-spread extreme wildfires indicated by high KBDIs in future projections.

美国西部最近发生的野火导致了巨大的经济损失和社会压力。令人担忧的是，新的气候状态可能会进一步增加野火的强度，持续时间和频率。为了检查历史野火趋势和未来变化的时空特征，计算了连续的美国（CONUS）和阿拉斯加的共同着火指数，即Keetch-Byram干旱指数（KBDI）。我们引入了一种有效的初始化方法，并使用高质量的观测数据集计算了4 km时36年（1982–2017）的历史KBDI。还使用区域气候模型计算了12 km时的KBDI，并将其扩展到21世纪中叶和21世纪中叶。根据观测数据，自1982年以来，西南和西北CONUS森林区域的年均KBDI（和95％百分率）分别以每年5.2（4.0）和2.9（3.3）的速度增长，表明持续不断的干燥趋势助长了火灾。在过去的36年中，超过历史KBDI最高5％的天数增加了16天和25天。多个区域气候模型模拟基于对21世纪中叶和下世纪的估计KBDI，预测了更广泛地区的野火潜力和更长的火灾季节。到本世纪末，大多数CONUS每年将经历超过当地最大KBDI历史记录的90-189天。KBUS高（> 600）的地区，其发生与大面积烧毁有关，预计在CONUS南部将扩大近60倍。