The phenomenon of “statistically impossible” heat waves and which regions are at risk of experiencing them

In the summer of 2021, Canada’s all-time temperature record was surpassed by nearly 5℃. The new figure of 49.6 ℃ is hotter than ever measured in Spain, Turkey or elsewhere in Europe.

The record was set in Lytton, a small village a few hours’ drive from Vancouver, in a part of the world where it really doesn’t look like temperatures like this should be reached.

Lytton was the culmination of a heat wave that swept across the northwestern US and Canada that summer, leaving many scientists baffled. Purely statistically, it should have been impossible.

I am part of a team of climate scientists who wanted to investigate whether the Northwest heat wave was unique, or whether other regions had experienced a similar statistically implausible event.

And we wanted to assess which regions would be most at risk in the future. Our results have been published on the specialized site Nature Communications.

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It is important to monitor these marginal heat waves not only because they are dangerous in their own right, but also because countries tend to prepare based on the level of the most extreme event in their collective memory.

So, an unprecedented heat wave may motivate policy action to mitigate the impact of future high temperatures.

For example, a severe heat wave in Europe in 2003 caused an estimated 50,000 to 70,000 deaths. While there have been more intense heat waves since then, none have resulted in such a high death rate as a result of plans developed after 2003.

One of the most important questions when studying these extreme heat events is “How long before we experience an equally intense event?”.

This is a tricky question, but luckily there is a branch of statistics called theory of extreme valueswhich normally provides ways for us to answer them using past events.

However, the heat wave in northwestern North America is one of many recent events that has called this method into question and should not have been possible under the extremes theory.

This “collapse” of statistics is caused when conventional extreme value theory fails to account for the specific combination of physical mechanisms that may not exist in the events of historical records.

Unlikely heat is everywhere

By studying the historical data between 1959 and 2021, we found that the 31% of the Earth’s surface has already experienced that statistically unlikely heat (although the heat wave in the US and Canada is exceptional even between these events).

These regions are scattered around the world without following any clear spatial pattern.

We also come to similar conclusions when we analyze “large pool” of data produced by climate models, which involve multiple computer simulations of global climate.

Those exercises are extremely useful to us because the effective duration of this simulated “historical record” is much longer and thus provides many more examples of rare events.

But while this analysis of the most exceptional events is interesting and cautions against using purely statistical methods to determine the boundaries of physical extremes, the main conclusions of our work come from the other end of the spectrum; regions that have not previously experienced particularly extreme events.

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Some places have been lucky so far

We identified a number of regions, evenly distributed around the world, that have not experienced particularly extreme heat (relative to their “expected” climate) over the past six decades.

As a result, these regions are more likely to experience a record-breaking event in the near future.

And without having experienced these atypical events, and with less incentive to prepare for them, they could be hit particularly hard by an unprecedented heat wave.

Socioeconomic factors, including the population size, population growth and level of development will exacerbate these effects.

That’s why we take population and economic development projections into account when assessing the regions most at risk globally.

Regions we believe are at risk include Afghanistan, several countries in Central America and the Russian Far East.among other things.

These regions can be surprising because they are not what people typically think of when they think about the impacts of extreme heat from climate change, such as India or the Persian Gulf. But these countries have recently experienced severe heat waves, so they are doing all they can to prepare.

Central Europe and several provinces in China, including the Beijing area, also appear vulnerable given the radicality of the data and the size of the population, but as they are more developed regions, they probably already have plans to mitigate the dire impact .

Overall, our work raises two main issues:

The first is that statistically unlikely that heat waves could occur anywhere on Earth, and that we should be very careful about using only the historical record to estimate the “maximum” possible heat wave. Authorities around the world should prepare for exceptional heat waves that could be considered implausible based on current data.

The second is that there are some regions whose all-time record is not exceptional and therefore more likely to be broken. These regions have been lucky so far, but that’s precisely why may be less prepared for the consequences of an unprecedented heat wave in the near future.

It is especially important that these regions prepare for more intense heat waves than they have already experienced.

*Nicholas Leach is a postdoctoral researcher in climate science at the University of Oxford, UK. His original article was published in The Conversation, of which you can view the English version read here.