“Record-shattering” extremes – which break weather records by large margins – will become more likely as a result of climate change, a new study finds.

The paper, published in Nature Climate Change, finds that the northern mid-latitudes are particularly vulnerable to record-shattering heat. This is exemplified by the recent heatwave over north-western US and Canada, in which many long-standing temperature records were broken by as much as 5C.

The study finds that record-shattering extreme events are likely to occur more frequently in the coming decades, but notes that they would be “nearly impossible” without climate change. It adds that the speed of warming is more important than the level of warming reached when determining the likelihood of these extremes.

The lead author tells Carbon Brief that “extremes in a changing climate [are] like an athlete on steroids – who suddenly breaks previous records in a step-change manner”.

“I think it is an extremely important paper that couldn’t be more timely”, a scientist who was not involved in the research tells Carbon Brief. She adds that, after the heatwave in the Pacific north-west, “many people have suggested our climate models are not able to simulate such events”. However, “this paper shows very nicely that they do. We just haven’t asked the question in quite this way before.”

Unprecedented

As global temperatures rise, extreme heat events are becoming more frequent, more intense and longer-lasting. This has caused an uptick in the number of record-breaking extreme temperatures logged around the world in recent years.

In the summer of 2018, for example, heatwaves swept across much of the northern hemisphere, breaking numerous all-time temperature records. In both Taiwan and Japan, national all-time temperature records were broken by 0.1C – and similar records were set throughout Europe and the US.

The following summer, both France and the UK set all-time records, breaking their previous records by 1.5C and 0.2C respectively. And in the summer of 2020, a heatwave in Siberia broke the all-time record for the region by 0.7C – reaching 38C in the Russian town of Verkhoyansk.

And in June this year, a severe heatwave struck the US and Canada, shattering long-standing temperature records across the region by as much as 5C.

All of these events were “record-breaking”. However, the 2021 heatwave is an example of a “record-shattering” event. Speaking to Carbon Brief, Dr Erich Fischer – a lecturer from ETH Zurich and lead author of the study – explains:

“We define record-breaking events as events exceeding the previous record by any margin. That may often be just 0.1C warmer than the previous event, which is not relevant in terms of impacts. Where the records become really relevant is if they are shattered like in the case of the Pacific north-west heatwave.”

Dr Dim Comou, a climate researcher from the University of Amsterdam, who was not involved in the study, tells Carbon Brief that the research on record-shattering extremes is “very timely”. He adds:

“The Canadian heatwave of just a few weeks ago was exactly such an event: There is no similar type of extreme in the historical data that comes even close to the temperatures that were measured in 2021.”

It is important to focus on record-shattering extremes because they have the biggest impacts on society, Fischer says:

“Impacts tend to be largest when an anomaly first occurs. If the same heatwave anomaly occurs again a couple of years later, society is better prepared and adapted – for example, with heat warning systems in place, public health plans, recommendations to the elderly, etc.”

Record-shattering extremes

In their analysis, the authors define three categories of extreme weather event, according to the difference between the extreme temperature reached and the annual maximum weekly temperature over 1961-90. These categories are calculated using a measure called “standard deviation”, which is denoted by the Greek lowercase letter, sigma (σ).

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