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2017 Montreal flood: event attribution

Significant flood damage occurred near Montreal in early May 2017, as flow from the upstream Ottawa river basin (ORB) reached its highest levels in over 50 years. In this blog post, the influence of anthropogenic greenhouse gas emissions on the probability of occurrence of the precursors to the ORB flood event is explored by using the 'attributable risk' approach to event attribution. This is accomplished by comparing large ensembles of simulations performed with the fifth generation Canadian Regional Climate Model (CRCM5). Different ensembles represent present-day (IR) and pre-industrial (PIRa, PIRb, PIRc) climate, and they differ in their GHG concentrations, sea surface temperatures and sea-ice concentrations.

Results showed that events such as the heavy April 2017 precipitation accumulation over the ORB are between two and three times as likely to occur in the present-day climate as in the pre-industrial climate. This increase in the risk of heavy precipitation is linked to increased atmospheric moisture due to warmer temperatures in the present-day climate, a direct consequence of anthropogenic emissions, rather than changes in rain-generating mechanisms or circulation patterns. Warmer temperatures in the present-day climate also reduce early-spring snowpack in the ORB, offsetting the increase in rainfall and resulting in no discernible change to the likelihood of extreme surface runoff.

Figure. Return times of April total precipitation (left), snow water equivalent at the beginning of April (center) and April surface runoff (right) in present-day (red) and pre-industrial ensembles (blue), over the Ottawa River basin. Black dashed lines correspond to reference CRCM5 simulations for April 2017. Risk ratios calculated from each of the pre-industrial ensembles are shown as black solid lines, along with their 95% confidence interval (CI). Red shading is used when the lower bound of the CI exceeds one, blue shading is used when the upper bound of the CI is less than one, and grey shading is used otherwise.

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