Peru and Alaska, among others, have experienced periods in which it was much warmer than average in recent decades, a new study shows.

Where and when did the most extreme heat waves since 1960 take place? That investigated Vikki Thompson from the University of Bristol and colleagues. They came up with a list of eight pieces – which includes a number of relatively unknown heat waves. Meanwhile, a number of well-known ones, such as that of Europe in 2003 and 2019† What’s up with that?

39.5 degrees in Canada

It is important to note that the premise of the study by Thompson and colleagues was not ‘where was the temperature highest?’, but ‘where was it much hotter than normal in the area in question?’ Then, for example, the heat wave that ravaged North America last year is a very extreme one. In the area concerned, the average highest temperature per day was 39.5 degrees Celsius, whereas in the ten years before that temperature, averaged over the hottest three months of the year, was only 23.4 degrees.

Thompson and her team found seven more such “heat extremes” in the data: in Canada (1969), the southeastern United States (1980), southern Brazil (1985), Southeast Asia (1998), the southwest of Peru (2016), southern Alaska (2019) and southeastern Brazil (2020).

Gone outliers

One can only wonder if these were really the heat waves with the biggest impact on people, says Peter Siegmund† He is associated with the KNMI as a climate expert and was not involved in the study. “I am inclined to think: if it is normally an average of 20 degrees somewhere and then it is 25 degrees for a while, that won’t really be a problem. But if you go from 35 to 40 degrees, it is. Look at India. It is warm there every year, but the current heat wave very serious indeed.”

Another point is that Thompson and colleagues cover fairly large areas. As a result, local outliers disappear from the picture. For example, during the North American heat wave of 2021, a maximum temperature of almost 50 degrees was measured. But the average temperature for the entire area used in the new study is more than 10 degrees lower.

The biggest killers

What is also striking is that there are quite a few white spots on the map with the study. These are the result of the fact that the researchers used not one, but two datasets of observations. If they disagreed about a certain area, they omitted that result.

Map heat waves

This map shows the most extreme heat wave per area: the darker the color, the more it deviated from the average. Illustration: University of Bristol

“On the one hand, it’s very good that two data sets have been used,” says Siegmund. “On the other hand, it is a pity that a number of areas where it is very hot have been left out.”

Nevertheless, he calls it a good, well-thought-out study, which rightly draws attention to heat waves. “Heat waves are the biggest killers among weather phenomena. Hurricanes cause more damage, but heat waves cause most deaths.”

positive side

There is also a positive side to the study. Yes, the average temperature on earth is rising and that also has consequences for heat waves. But the changes you see with heat waves, Thompson and colleagues write in their… article in Science Advances“are the same size as the changes in the average temperature”.

“That’s surprising,” says Siegmund. “In the Netherlands, we see that extreme temperatures are increasing faster than the annual average. But these researchers do not think that for the whole world; nor in their predictions for the future.”

Worse prepared

Finally, the authors issue warnings towards areas where, according to their definition, no heat waves have occurred, such as parts of India, Australia and Central Africa. “Those regions may be less prepared for high temperatures because they haven’t had to adapt to them sooner.” While on average they should have to deal with a heat wave once every thirty years. They just seem to have been lucky so far.

A logical point, Siegmund thinks. “Countries should not think that they will never have to deal with heat waves. If you haven’t been sick for five years, that doesn’t mean you’re guaranteed to stay healthy the year after.”