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Human Influence on the 2021 British Columbia Floods

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Nathan Gillett and Markus Schnorbus - coauthors: Alex J. Cannon, Elizaveta Malinina, Faron Anslow, Qiaohong Sun, Megan Kirchmeier-Young, Francis Zwiers, Christian Seiler, Xuebin Zhang, Greg Flato, Hui Wan, Guilong Li, Armel Castellan
February 23, 2022 - 11:00am to 12:00pm


A strong atmospheric river made landfall in southwestern British Columbia, Canada on 14th November 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. This talk describes a rapid attribution study of this event, carried out as a collaboration between Environment and Climate Change Canada and the Pacific Climate Impacts Consortium. We show that 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 approximately a one in 50-100 year event, and the probability of events at least this rare 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 has been increased by human-induced climate change by a best estimate of 100-300%. 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.

Read the preprint of the related research paper.

Watch a recording of this talk.

Access Nathan Gillett's slides and Markus Schnorbus's slides.

Speaker Bios:

Nathan Gillett holds a PhD in atmospheric physics from the University of Oxford. After his doctorate, Nathan worked as a researcher at the University of Victoria on the attribution of climate change, before being appointed to a faculty position at the Climatic Research Unit of the University of East Anglia in the UK. In 2008, Nathan returned to Canada to work as a research scientist at Environment and Climate Change Canada’s Canadian Centre for Climate Modelling and Analysis. From 2014 to 2018 he served as manager of the centre, overseeing the development and application of Canada’s earth system model. He served as Lead Author of the IPCC Fourth and Fifth Assessment Reports, and the 2014 WMO/UNEP Ozone Assessment, and he co-led the chapter on human influence on the climate system of the IPCC Sixth Assessment Report as a Convening Lead Author.

Markus Schnorbus is the Lead of PCIC's Hydrologic Impacts Theme. Markus joined PCIC in April 2009 and became Lead Hydrologist in July 2010. Prior to joining PCIC Markus was a Hydrologic Modelling Scientist and Forecaster with the BC Ministry of Environment, River Forecast Centre, where he was engaged in the analysis of observed climate and hydrometric data and the application of various hydrologic models for flood, drought and seasonal streamflow forecasting. He also recently led the implementation of the Variable Infiltration Capacity (VIC) macro-scale hydrology model to investigate the effects of mountain pine beetle and salvage harvest operations within the Fraser River watershed. During this time, Markus developed a clear and pragmatic appreciation of the hydrologic effects of climate change and variability.

Markus possesses a sound knowledge of mountain and forest hydrology, particularly in the context of British Columbia, and has a strong understanding of the challenges inherent in applying hydrology models in this complex environment. Markus leads the effort at PCIC to quantify the direct and indirect effects of climate change and variability upon the various components of hydrologic cycle at the local and regional scale.