A strong atmospheric river made landfall in southwestern British Columbia, Canada on November 14th, 2021, bringing two days of intense precipitation to the region. The resulting floods and landslides led to the loss of at least five lives, cut Vancouver off entirely from the rest of Canada by road and rail, and made this the costliest natural disaster in the province's history. Here we show that when characterised in terms of storm-averaged water vapour transport, the variable typically used to characterise the intensity of atmospheric rivers, westerly atmospheric river events of this magnitude are approximately one in ten year events in the current climate of this region, and that such events have been made at least 60% more likely by the effects of human-induced climate change. Characterised in terms of the associated two-day precipitation, the event is substantially more extreme, approximately a one in fifty to one in a hundred year event, and the probability of events at least this large has been increased by a best estimate of 45% by human-induced climate change. The effects of this precipitation on streamflow were exacerbated by already wet conditions preceding the event, and by rising temperatures during the event that led to significant snowmelt, which led to streamflow maxima exceeding estimated one in a hundred year events in several basins in the region. Based on a large ensemble of simulations with a hydrological model which integrates the effects of multiple climatic drivers, we find that the probability of such extreme streamflow events in October to December has been increased by human-induced climate change by a best estimate of 120–330%. Together these results demonstrate the substantial human influence on this compound extreme event, and help motivate efforts to increase resiliency in the face of more frequent events of this kind in the future.

2021 年 11 月 14 日，一条强大的大气河流在加拿大不列颠哥伦比亚省西南部登陆，给该地区带来了两天的强降水。由此引发的洪水和山体滑坡导致至少 5 人丧生，温哥华与加拿大其他地区的公路和铁路完全隔绝，成为该省历史上损失最惨重的自然灾害。在这里，我们表明，当以风暴平均水汽输送为特征时，通常用于表征大气河流强度的变量，这种规模的西风大气河流事件在该地区当前气候下大约是十年事件中的一个，并且由于人类引起的气候变化的影响，此类事件的可能性至少增加了 60%。以相关的两天降水为特征，该事件实质上更为极端，大约是五十分之一到一百分之一的事件，并且由于人类引起的气候变化，事件发生的概率至少增加了 45%。这种降水对径流的影响因事件前已经潮湿的条件而加剧，事件期间气温升高导致大量融雪，导致该地区几个盆地的径流最大值超过估计的一百年事件中的一个。基于使用综合了多种气候驱动因素的水文模型进行的大量模拟，我们发现 10 月至 12 月发生此类极端径流事件的概率因人为气候变化而增加了 120– 330%。