As California wakes up to the worsening risk of extreme climate events, scientists are shedding new light on last year’s anomalous and extreme heat wave in the Pacific Northwest. One study published this week said such heat waves could be 20 times more likely to occur if current carbon emissions continue unabated. Another said they could also be almost 10 degrees warmer.
The nine-day event in late June and early July 2021 scorched parts of northern California, Oregon, Washington and British Columbia, where Canada recorded its highest temperature on record, 121.3 degrees. The heat wave claimed hundreds of lives, sparked several devastating fires and killed an estimated 1 billion sea creatures.
Such an event would have been “virtually impossible” in the 1950s, but atmospheric warming has already increased its probability to about 0.5% per year, according to one Columbia University study published Thursday in the journal Nature Climate Change. If warming were to exceed 2 degrees Celsius – the upper limit set by the International Panel on Climate Change – this probability could rise to 10% per year as early as 2050.
“The single biggest control over how bad heat waves will be—over how bad they already are—is the amount of CO2 we put into the atmosphere,” said Samuel Bartusek, Ph.D. student at Columbia’s Lamont-Doherty Earth Observatory and lead author of the study. “There’s really only one solution to the problem of putting more carbon dioxide into the atmosphere, and that’s to stop doing it.”
Bartusek said the extraordinary heat wave was “shocking” both to the people who experienced it and to the scientific community, so scientists hope to gain a better understanding of its physical mechanisms and its relationship to climate change.
“It was an extremely strange event,” said Michael Wehner, a climate scientist at Lawrence Berkeley National Laboratory who co-authored another paper on the heat wave published last week in the journal Geophysical Research Letters. “It was also tragic, of course, because of the mortality that resulted.
Among other conclusions, the paper found that the heat wave was so unprecedented that it essentially broke most of the standard tools used to measure the human impact on heat waves.
“At the end of the day, we calculated that this event was not only impossible without climate change, but it was impossible with climate change. And of course it did, so that means the model is wrong,” Wehner said.
Wehner said such statistical outliers make it difficult to predict with certainty the future frequency of such events. However, his work includes findings on temperature, noting that global warming has caused peak temperatures during heatwaves to increase by as much as 1.8 degrees.
Future warming could lead to an increase in summer temperatures of around 9 degrees by the end of the 21st century, the paper says.
“The bottom line is that the amount of climate change we get is really determined by us and by the people we choose to either mitigate carbon dioxide and other greenhouse gas emissions or not,” he said. “And the less we do, the worse things will get.”
The Columbia scientists also found that several factors converged to help create the bubbly heat dome, including anomalously dry soil and disrupted flow.
In California and other parts of the western United States, increasing heat, drought and aridification are contributing to long-term drying of soils, meaning less water must evaporate into the air, Bartusek explained.
“And if there’s less ability for evaporation to come up from the ground, there’s a greater thermal effect — where the air that’s just above the surface will be heated more efficiently,” he said. He added that in some areas it is likely that “there is this feedback process going on where the land surface has helped amplify some of the highest temperatures”.
The jet stream — fast-moving air currents in the upper atmosphere that drive weather systems from west to east — also played a role in the heat wave, according to the study. Before and during the event, the jet stream “twisted” into a wave pattern and slowed, essentially locking the weather system in place and allowing a dome of heat to accumulate over the area.
The researchers noted that the effect of climate change on the jet stream is still debated, although some scientists believe that such wave patterns are becoming more frequent and extreme due to human activity. Wehner said the question is “one of the most interesting problems in climate science right now.”
“Certainly the possibility remains that we will see more of these unusual flows with global warming,” he said.
Kai Kornhuber, an associate scientist at the Lamont-Doherty Earth Observatory and another co-author of the Columbia study, said the findings highlight how variations in soil moisture levels, flow and other factors can drive temperatures well above their normal values.
“Sometimes those factors just align and you get the conditions that create the perfect storm,” he said. “However, what is important to note here is that each of these drivers is showing increasing trends that are associated with climate change. . . . These types of accidents may be more likely to occur in the future, simply because these common factors are all related to climate trends.”
As for the likelihood of such heat waves reaching a 10% annual probability by 2050, much depends on which path society ends up taking. But “given the accelerated trends in extreme weather events around the world, there are reasons to believe that these estimates may even be a bit conservative,” Kornhuber said.
He and other researchers noted that while the numbers and predictions of some studies may differ, their key messages are very similar — that an extreme heat wave was essentially impossible at pre-industrial emissions levels.
“What’s important about this is that all these methods agree that climate change plays a major role in every heat wave we’re currently seeing,” Kornhuber said.
Although the findings are ominous, the researchers said they could help inform future modeling of such events and help people better prepare. Many parts of the Pacific Northwest were not equipped for such extreme heat, including homes without air conditioning and infrastructure systems that could not handle the strain. Wehner said improved adaptation efforts and contingency plans will help, but ultimately such events “will get worse because there’s a lot of climate change baked into the system.”
“The more we can reduce our emissions — eventually we’ll get to zero, zero — and the sooner we can do it, the better in terms of preventing even worse tragedies,” he said.