CLIMATE CHALLENGES“Statistically Impossible” Heat Extremes Are Here: Study Identifies the Regions Mmost at Risk
In the summer of 2021, Canada’s all-time temperature record was smashed by almost 5℃. Its new record of 49.6℃ is hotter than anything ever recorded in Spain, Turkey or indeed anywhere in Europe. One of the most important questions when studying these extreme heatwaves is “how long do we have to wait until we experience another similarly intense event?”. This is a challenging question but, fortunately, there is a branch of statistics, called extreme value theory, that provides ways in which we can answer that exact question using past events.
In the summer of 2021, Canada’s all-time temperature record was smashed by almost 5℃. Its new record of 49.6℃is hotter than anything ever recorded in Spain, Turkey or indeed anywhere in Europe.
The record was set in Lytton, a small village a few hours’ drive from Vancouver, in a part of the world that doesn’t really look like it should experience such temperatures.
Lytton was the peak of a heatwave that hit the Pacific Northwest of the US and Canada that summer and left many scientists shocked. From a purely statistical point of view, it should have been impossible.
I’m part of a team of climate scientists who wanted to find out if the Pacific Northwest heatwave was unique, or whether any other regions had experienced such statistically implausible events. And we wanted to assess which regions were most at risk in future. Our results are now published in the journal Nature Communications.
Tracking these outlier heatwaves is important not just because the heatwaves themselves are dangerous, but because countries tend to prepare to around the level of the most extreme event within collective memory. An unprecedented heatwave can therefore provoke policy responses to reduce the impact of future heat.
For instance, a severe heatwave in Europe in 2003 is estimated to have caused 50,000-70,000 excess deaths. Although there have been more intense heatwaves since, none have resulted in such a high death toll, due to management plans implemented in the wake of 2003.
One of the most important questions when studying these extreme heatwaves is “how long do we have to wait until we experience another similarly intense event?”. This is a challenging question but, fortunately, there is a branch of statistics, called extreme value theory, that provides ways in which we can answer that exact question using past events.
But the Pacific Northwest heatwave is one of several recent events that have challenged this method and should not have been possible according to extreme value theory. This “breakdown” of statistics is caused by conventional extreme value theory not taking into account the specific combination of physical mechanisms, which may not exist in the events contained in the historical record.