The small village of Lytton in British Columbia, Canada was once a stop for hikers and tourists to admire the beautiful mountains and rivers nearby.
But in 2021, a devastating wildfire — fueled by an unprecedented heat wave that swept across much of the Pacific Northwest — destroyed nearly every home and building, killing two of the 250 residents there.
Amid the disaster, temperatures in Lytton reached 49.6C – the highest temperature ever recorded in Canada, breaking the country’s previous record of 4.6C.
Climate scientists studying the heatwave were stunned by the record temperatures.
“To our knowledge, this [heatwave] is basically impossible,” the late pioneering extreme weather scientist Dr. Geert Jan van Oldenborgh said at a press conference at the time.
Since then, Earth has experienced many more “record-breaking” moments.
In 2022, temperatures in the UK hit 40.3C amid a scorching summer – 1.6C higher than the previous record. A year later, the national record in China was broken by 1.9C when temperatures hit 52.2C.
At the same time, leading climate scientists are racing to understand why these events are happening, how they are related to rapidly rising global temperatures, and what this could mean for the future of Earth.
Below, Carbon Transient talks to experts and reviews the latest scientific evidence to explore why climate change is causing record-breaking extreme heat around the world.
What is a ‘record-breaking’ extreme temperature?
In an era of rapid human-caused climate change, setting new regional and national temperature records during heat waves occurs so frequently that it can sometimes be considered the norm.
Every now and then, however, a record is broken in massive proportions – often with unprecedented impacts, as well as warnings about what it might signify about the pace of climate change.
Researchers are calling this “record-breaking” or “record-breaking” heat, explains Professor Erich Fischer, a climate extremes scientist at ETH Zurich and lead author of the most recent landmark assessment by the Intergovernmental Panel on Climate Change (IPCC). He told Carbon Transient:
“I define it as a type of record-breaking event where the record breaks the previous record by a large margin.”
The difference, he said, depends on the event in question.
For example, a new national temperature record that is a few tenths of a degree higher than the previous record would not be considered a record-breaker.
However, the average global temperature in 2023 was a record high – albeit only 0.14-0.17C higher than the previous record in 2016.
This is because taking an average over a large area and time period is more likely to mitigate the effects of natural climate variability – making smaller upward trends more meaningful, Fischer explains:
“You can expect global temperatures to rise pretty steadily, with some fluctuations. It’s not like the temperature in our backyard is going up and down.”
To quantify the size of record-breaking events over different time periods and in different geographic areas, scientists often use standard deviation, a measure of how spread out the data is from the mean.
In what is known as a “normal distribution” of data, 68% of the data points will fall within one standard deviation of the mean, 95% will fall within two standard deviations, and 99.7% will fall within three standard deviations.
So for a particular heat wave, scientists calculate how many standard deviations the event deviates from the average climate for that location. A record-breaking extreme event will be many standard deviations above average.
Recent years have seen several record-breaking heat waves.
New record-breaking national temperatures have been recorded during heatwaves across western Europe, including in France and Britain, as well as in China and Canada.
The 2021 Pacific Northwest heat wave, when Canada’s temperature record was broken in Lytton, was one of the largest on record — although there have been some events that were more extreme by a standard deviation, research shows.
That study found that the heat wave in the Pacific Northwest broke the temperature record by just over four standard deviations, while the most severe heat wave ever recorded, in Southeast Asia in 1998, broke the record by just over five standard deviations.
Other record-breaking heat events in recent years include a year-long marine heatwave in the north Atlantic that began in March 2023, when sea surface temperatures were up to 5C warmer than normal.
The event triggered deadly heat stress that hit nearly all tropical coral reefs in the Atlantic and contributed to a very active 2023 Atlantic hurricane season.
It is worth noting that understanding of such extremes is generally better for countries in the global north than for countries in the global south.
Reasons for this include longer and more complete temperature records in the global north, plus more weather stations. Previous Carbon Transient analysis found that Africa had the lowest density of weather stations of any continent, followed by South America and Asia.
This means scientists do not fully understand the geographical distribution of the record-breaking heatwave, said Dr Robert Vautard, a senior climate scientist at the National Centre for Scientific Research at the Institut Pierre-Simon Laplace in Paris and co-chair of the consortium. The IPCC’s Climate Science Working Group, he told Carbon Transient:
“We don’t know enough about the record-breaking events that are more anticipated elsewhere…That’s an area where I expect there to be progress in the coming years.”
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Why are heat records broken by huge margins?
After the 2021 heat wave in the Pacific Northwest, scientists were left scratching their heads about what kind of temperatures they were studying.
At the time, researchers at the World Native Climate Attribution service, a consortium of scientists studying the effects of climate change on extreme events, said it could indicate that the Earth had “crossed a nonlinear threshold” — where even small increases in global temperatures are causing a much larger increase in extreme temperatures than scientists had predicted.
“It was an extraordinary event,” van Oldenborgh, co-founder of WWA, said at a press conference, adding that it was “surprising and shocking” to discover that “our theoretical picture of how heat waves will behave” in a warming climate “was broken” in such a spectacular way.
Since then, several studies have looked at possible causes of extreme heat waves in the Pacific Northwest and the occurrence of record-breaking heat waves more broadly.
The emerging consensus is that – while shocking and dangerous – these extremes are within the range of what the world can expect as global temperatures continue to rise rapidly.
Vautard, who recently led a study in the journal Environmental Analysis Letters to see if extreme temperatures are increasing in ways that exceed what scientists predicted, told Carbon Transient:
“The research shows that this is exactly what we expect with climate change. There is nothing else that we don’t understand. It’s terrible, but we understand it.”
Fischer, who has also published research papers on the phenomenon, explains that why records are sometimes broken by large margins – rather than gradually increasing – can be understood by looking at the rapid rise in global temperatures. He says:
“The rate of warming plays a big role. For example, if you have a 50-year event – if it happens once and then recurs [on average] 50 years later, the climate in between is warming very rapidly, so the difference in the magnitude of those two events is going to be much larger.”
In other words, very often, a variety of climate factors – both natural and human-caused – will combine to cause an extreme heat event.
Scientists use return periods to describe such events, with a larger return period indicating a more extreme event is less likely to occur at any given time.
This is what happened during the Pacific Northwest heat wave, when a “blocking” weather pattern—a zone of high pressure in the atmosphere—stalled over the region, creating a dome of unusually high temperatures. At the same time, the heat was exacerbated by the effects of dry land.
Each time an event like this happens, it comes against the backdrop of human-caused climate change, which is making heat waves more intense and frequent.
As global temperatures continue to rise, their impact on extreme heat events becomes increasingly greater.
So when a very rare heat event is combined with an ever-increasing temperature rise, it can cause a record-shaking event, says Dr Clair Barnes, a climate statistician at the Imperial Faculty of London and co-author of a recent Environmental Research Letters study. She tells Carbon Transient:
“The extreme heat we are seeing is not a surprise.”
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How could extreme temperatures increase records in the future?
In 2021, Fischer led a study published in the journal Nature Native local weather change that explored how record-breaking heat waves might increase in the future.
The study considers such events to be weeks-long heat waves that break previous records by two, three or four standard deviations.
Events in the two standard deviation categories include the 2003 European heat wave, which killed 30,000 people, and the 2010 Russian heat wave, which killed at least 5,000 people in Moscow.
Researchers have looked at the likelihood of such events occurring in various future scenarios.
This includes a future scenario in which greenhouse gas emissions are extremely high (known as “RCP8.5”) and a scenario in which global temperature increases are limited to below 2C by 2100 (known as “RCP2.6”). (Limiting global warming to “well below 2C” is part of the long-term goal set by countries under the Paris Agreement.)
The study found that, under a very high greenhouse gas emissions scenario, record-breaking week-long heatwaves of three standard deviations or more would be two to seven times more likely to occur between 2021 and 2050, and three to 21 times more likely to occur between 2051 and 2080, compared with the past three decades.
Conversely, if the world could limit global warming to 2C, which would involve stabilizing the climate by reaching net-zero greenhouse gas emissions, then the occurrence of record-breaking heatwaves would drop rapidly, Fischer said:
“If you try to stabilize the climate, the record events will decrease. You will still have the heat waves that are worrisome, but that record-breaking aspect will decrease.”
This is because the rate of global temperature rise – the main cause of record-breaking extreme events – will no longer increase.
Even slowing the rate of global warming by cutting global emissions would reduce the likelihood of such events, he added:
“This is one of the few early benefits [of mitigating climate change]. I think it’s an important one, because we often say we’ll only see the benefits when we actually stabilize the climate, but this is something we’ll see even before we achieve that.”
This is a clear example of how “mitigation can be a huge help to adaptation,” Vautard added:
“Very often, we adapt to what we have just seen – or what we have seen all our lives. But record-breaking events are unimaginable.
“If we stop warming [the planet], the probability of record-breaking events will quickly go to zero. “That would be a huge help for adaptation.”