Cool-season precipitation is a critical component of western North American water supplies. In recent decades, snow water equivalent (snowpack) has declined dramatically, culminating in record lows along the northern Cascade Range, USA, during the winters of 2014 and 2015. Given the brevity of observational records, the extent to which these recent trends exceed historical ranges of variability remains poorly understood. Here, we use a network of 30 tree-ring chronologies to reconstruct April 1 snowpack in the northern Cascade Mountains (Washington, USA) over the past ca. 400 years that accounts for 62% of the instrumental period variability. The reconstruction is characterized by considerable interannual- to interdecadal-scale variability. Yet, the most conspicuous feature is a decline that began in the mid-1970s concurrent with a shift to above-average air temperature which is accelerated through to the most recent years of the record. The 2014–2016 snow drought, especially snow conditions during the 2015 winter, across the northern Cascades is unprecedented within the context of the last 400 years. The reconstruction is characterized by considerable interannual- to interdecadal-scale variability, but a conspicuous decline emerged in the mid-1970s and accelerated through to the most recent years of the record to levels below the pre-industrial envelope of variability. This decline culminated in the 2014–2016 northern Cascade Mountains snow drought, which is unprecedented within the context of the last 400 years. Not only is the twentieth–twenty-first century period characterized by the rapid decline in snowpack conditions, it is also highly volatile. Extreme flips in snowpack conditions (year-to-year changes between < 5th and > 95th percentiles) during the twentieth and twenty-first centuries were anomalous within the context of the record. These patterns described for the North Cascades are concurrent with anomalously low snowpack in the Sierra Nevada Mountains of California, and thus part of a broader trend toward reduced snowpack in western North America. This decline will fundamentally impact regional drought and hydrography in North Cascadia and is likely the combined result of rising temperatures, weakening

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北喀斯喀特（美国）2014-2016 年雪旱的多世纪、树轮衍生观点

冷季降水是北美西部供水的重要组成部分。近几十年来，雪水当量 (snowpack) 急剧下降，在 2014 年和 2015 年冬季美国喀斯喀特山脉北部达到创纪录的低点。变异的范围仍然知之甚少。在这里，我们使用 30 个树轮年表的网络来重建过去约 4 月 1 日在喀斯喀特山脉北部（美国华盛顿）的积雪。400 年，占工具周期变率的 62%。重建的特点是相当大的年际到年代际尺度的变化。然而，最显着的特征是始于 1970 年代中期的下降，同时气温向高于平均水平的转变并加速到创纪录的最近几年。2014-2016 年的雪旱，尤其是 2015 年冬季的雪灾，横跨北喀斯喀特山脉，这是过去 400 年来前所未有的。重建的特点是相当大的年际到年代际尺度的变化，但在 1970 年代中期出现了明显的下降，并加速到最近几年的记录，低于工业化前的变化范围。这种下降最终导致了 2014-2016 年北喀斯喀特山脉的雪旱，这是过去 400 年来前所未有的。20-21 世纪不仅以积雪条件迅速下降为特征，而且还非常不稳定。20 世纪和 21 世纪期间积雪条件的极端翻转（< 5th 和 > 95th 百分位数之间的逐年变化）在记录的背景下是异常的。为北喀斯喀特描述的这些模式与加利福尼亚内华达山脉的异常低积雪同时发生，因此是北美西部积雪减少的更广泛趋势的一部分。这种下降将从根本上影响北卡斯卡迪亚的区域干旱和水文，很可能是气温上升、减弱的综合结果 20 和 21 世纪的第 5 和 > 95 个百分位数）在记录的上下文中是异常的。为北喀斯喀特描述的这些模式与加利福尼亚内华达山脉的异常低积雪同时发生，因此是北美西部积雪减少的更广泛趋势的一部分。这种下降将从根本上影响北卡斯卡迪亚的区域干旱和水文，很可能是气温上升、减弱的综合结果 20 和 21 世纪的第 5 和 > 95 个百分位数）在记录的上下文中是异常的。为北喀斯喀特描述的这些模式与加利福尼亚内华达山脉的异常低积雪同时发生，因此是北美西部积雪减少的更广泛趋势的一部分。这种下降将从根本上影响北卡斯卡迪亚的区域干旱和水文，很可能是气温上升、减弱的综合结果