How hot can a heat wave really get? Before June 2021, scientists thought they knew.
That’s when one of the most extreme heat spikes ever observed hit western North America, leaving at least 1,400 people dead. Lytton, British Columbia, smashed the 84-year-old Canadian heat record on June 26, reaching 46.6 C (116F).
And it smashed that the next day by 1.3C.
And smashed that the next day by another 1.7C.
And the next day, Lytton burned to the ground.
When a team of climate scientists assembled days later to analyze the heat wave, they found that the local historical weather data offered a paradox: Their standard approach for estimating a heat wave’s rarity concluded that the new records were too extreme to occur in the region where they actually did. They were in a sense “impossible” even though they actually occurred, as three American scientists put it earlier this year.
Read more: Extreme Heat Is Killing European Workers Despite Government Efforts
They adjusted their method to accommodate the new reality (and use that approach still), but noted that “follow-up research will be necessary to investigate the potential reasons for this exceptional event.”
In the four years since then, dozens of studies have taken up that challenge, with a tightening focus on a simple question that eludes easy answers: How hot can it get? The answer has grave implications for humanity, from those living in places where high temperatures are currently rare to those in places that are increasingly on the edge of habitability as climate change makes heat more intense and frequent. Everyone, everywhere, needs to know the risks about where they live.
There are as many answers to this question as there are thermometers around the globe. To make finding an answer slightly more manageable, scientists look not at absolute temperatures, as anybody would when leaving the house in the morning. Instead, they parse each weather station’s departures from the average.
The 2021 event “shocked everyone, including specialists working on the subject. People were completely stunned,” said Robin Noyelle, a post-doctoral researcher in climate science at ETH Zurich.
