The record-shattering 2021 Pacific Northwest heatwave – challenges and opportunities for attribution and event storylines

Parts of western North America experienced a heatwave in late June 2021that many would have conceived impossible based on observations so far. In Lytton, Canada, temperatures peaked at 49.6°C, and the area-average daily maximum temperature record across the Pacific Northwest was broken by nearly 5°C. Given the exceptional intensity of the eventsome media outlets and scientists raised the questions whether heat extremes intensify faster than previously projected based on climate models, or whether current generations of climate models miss crucial processes and are thus unable to even reproduce such an event.^. Here I address these questions and highlight some of the challenges for widely methods in model evaluation and attribution.

First, I review some of the recent literature detailing the key physical mechanisms driving the Pacific Northwest heatwave. I address some of the key scientific challenges regarding the quantification of return periods, event attribution, model evaluation and near-term projections. Widely used methods estimating stationary return periods based on the observational record up to the year before imply that such an event had an infinite return period, i.e., that it would never happen. Even when taking into account the non-stationarity of a warming climate, the exceedance probability would be zeroor nearly zero depending on the estimation of the confidence intervals, the duration of the event and whether the event itself is included in the fit^.

I discuss some potential ways forward in addressing the above challenges and in quantifying the potential intensity of record-shattering events in the near future.