This summer has once again broken heat records, with July being by far the hottest month on record. In 2021, north-western Canada was particularly hard hit: Lytton recorded temperatures of 49.6 degrees Celsius. How do such extreme temperatures occur?
Sebastian Sippel: The new temperature record in Lytton was indeed remarkable, as the temperature record for all of Canada prior to the heatwave was 45 degrees Celsius in the 1930s. This means that a long-standing record was surpassed by almost 5 degrees Celsius in one go!
And many other regions have set new extreme temperature records this summer and in recent years. Were it not for climate change, these record-shattering extremes would be highly unlikely. And even with climate change, such extreme new heat records are still rather unlikely – but they can happen. In a study in 2021, we showed that such record-shattering extremes occur when a combination of physical causes that favour and amplify heatwaves come together. These include the right weather conditions and very dry soils, which intensify the heat, and of course the onward march of climate change.
In your most recent study, you have shown that record temperatures of more than 50 degrees Celsius in the summer are possible in Europe as well. What new predictive models underpin this finding?
This study was about assessing risk: would it have been physically possible to predict the possibility of such extreme temperatures before the 2021 Canadian heatwave? And we were in fact able to show that such temperatures are possible in physical climate model simulations. To do this, we stopped climate model runs a few weeks before heatwaves occurred and then simulated these heatwaves in many – new – runs in order to better understand extreme events. In this way, it is actually possible to show that a risk assessment – one for the worst-case scenario – could indeed have predicted the figure of almost 50 degrees Celsius in Canada in advance.
So it is important that authorities, infrastructure and businesses are prepared for such events.
How predictable are such extreme heatwaves in our latitudes? Can you, for example, make any predictions for next summer and for a specific region?
It is important to stress that the new technique is not a (weather) forecast. So we cannot predict what the temperature will be next summer, nor which regions will be particularly hot.
What do the predictions of these worst-case scenarios mean for cities, municipalities and broader climate policy?
We are trying to assess risk. So what we do is show the physical scenarios that are possible. This is important to be prepared for unprecedented events, as heat is a major health risk, especially for the elderly and infirm. Take the example of France: heatwaves have been on the rise there in recent years, and in 2003 around 15,000 people died from heat-related causes. In response, French authorities have drawn up heat action plans, and the city of Paris has recently planned and run through scenarios on how to protect people in the event of extreme summer temperatures – possibly as high as 50 degrees Celsius.
Original publication in Nature Communications:
“Storylines for unprecedented heatwaves based on ensemble boosting”, DOI: 10.1038/s41467-023-40112-4
Study from 2021 on record-shattering extremes:
“Increasing probability of record-shattering climate extremes”, DOI: 10.1038/s41558-021-01092-9
Junior Professor Sebastian Sippel is involved in Breathing Nature, the proposed Cluster of Excellence under the second competition phase of the Excellence Strategy of the German federal and state governments. His main research interest is to improve the understanding of climate variability, extreme events and their changes at global and regional scales. His research also focuses on how global climate change affects the water and carbon cycles and how they interact. Most of his research is closely related to climate change attribution. He uses empirical and quantitative statistical and machine learning techniques to address these research questions, integrating different climate and earth science data streams, such as climate observations and climate model simulations.