“Study pinpoints regions at highest risk for extreme heat: A statistical improbability now a reality”

In the summer of 2021, it broke the Canadian temperature record by nearly 5°C. 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 seem like it should see such temperatures.

Layton was the height of a heatwave that hit the Pacific Northwest for the United States 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 climatologists who wanted to know if the Pacific Northwest heat wave was unique, or if any other regions had experienced such statistically implausible events. And we wanted to assess which areas are most at risk in the future. Our results are now published in the journal Nature Communications.

Tracking outdoor heat waves is important not only because heat waves themselves are dangerous, but because countries tend to prepare for nearly the level of the most extreme event in collective memory. Therefore, an unprecedented heat wave could trigger policy responses to reduce the impact of heat in the future.

For example, it is estimated that there was a severe heat wave in Europe in 2003 50,000-70,000 excess deaths. Although there have been more extreme heatwaves since then, none have resulted in such a large number of deaths, due to management plans implemented in the aftermath of 2003.

One of the most important questions when studying these extreme heat waves is, “How long should we wait to see another extreme intensity event?” This is a difficult question, but fortunately, there is an offshoot of statistics, called extreme value theory, that provides ways we can answer this delicate question using past events.

But the Pacific Northwest heat wave is one of many recent events that have challenged this method and should not have been possible according to extreme value theory. This “collapse” of statistics results from traditional outlier theory not taking into account the specific combination of physical mechanisms, which may not be present in the events in the historical record.

Unbelievable heat everywhere

Looking at historical data from 1959 to 2021, we find that 31% of the Earth’s surface has already experienced such statistically implausible heat (although the Pacific Northwest heat wave is exceptional even among these events). These regions are spread all over the world without a clear spatial pattern.

We also came to similar conclusions when we analyzed the “big ensemble” data produced by climate models, which involve computers that simulate the global climate many times. These simulations are very useful to us, as the effective length of this simulated “historical record” is much greater, and therefore it produces many more examples of rare events.

However, while this analysis of the most extraordinary events is interesting, and cautions against using purely statistical approaches to assess the limits of extreme physical phenomena, the most important conclusions of our work come from the other end of the spectrum—regions that have not experienced particularly extreme events before.

Some places have been lucky – until now

We identified a number of regions, again spread around the world, that have not experienced particularly extreme heat over the past six decades (relative to their ‘expected’ climate). As a result, these regions are likely to experience a record-breaking event in the near future. And with no experience of such a massive extreme, and less incentive to prepare for it, they could be especially hurt by a record-setting heat wave.

Socio-economic factors, including population size, population growth and level of development will exacerbate these effects. As a result, we factor population and economic development projections into our assessment of the regions most at risk globally.

Our vulnerable regions include Afghanistan, many countries in Central America, the Far East of Russia, and more. These regions may come as a surprise, because they are not the ones people usually think of when thinking of extreme thermal impacts of climate change like India or the Persian Gulf. But those countries have done so recently I was exposed to severe heat waves And so they’re really doing what they can to prepare.

Central Europe and several provinces in China, including the area around Beijing, also appear to be at risk when considering record severity and population size, but as the more developed regions it is likely that they already have plans in place to mitigate severe impacts.

In general, our work raises two important points:

The first is that statistically implausible heat waves can occur anywhere on Earth, and we have to be very careful about using the historical record in isolation to estimate the “extreme” heat wave possible. Policymakers around the world should prepare for exceptional heatwaves that may be considered implausible based on current records.

The second is that there are a number of regions whose historical record is not exceptional and therefore more likely to be broken. These areas have been lucky so far, but as a result, they are likely to be less prepared for 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.

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