Climate change is increasing the frequency and temperature of extreme heat waves


As California wakes up to the increasing risk of extreme climate events, researchers are shedding new light on last year’s anomalous and extreme heat wave in the Pacific Northwest. A study published this week said such heat waves could be 20 times more likely if current carbon emissions continue unabated. Another said they can also be almost 10 degrees hotter.

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 wildfires and killed an estimated 1 billion marine animals.

Such an event would have been “virtually impossible” in the 1950s, but the chance of atmospheric warming has already increased to about 0.5% chance per year, according to a Columbia University study published Thursday in the journal Nature Climate. Change. Should warming exceed 2 degrees Celsius — the upper limit set by the International Panel on Climate Change — that chance could rise to a 10% chance per year from 2050.

“The biggest control over how bad heat waves are going to get — more than how bad they already are — is the amount of CO2 we put into the atmosphere,” said Samuel Bartusek, a Ph.D. student at Columbia’s Lamont-Doherty Earth Observatory and the study’s lead author. “There’s really only one solution to the problem of putting more carbon dioxide into the atmosphere, and that’s to stop.”

Bartusek said the extraordinary heat wave was “shocking” both for the people who experienced it and the scientific community. Therefore, researchers hope to gain a better understanding of the physical mechanisms and its relationship to climate change.

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“This was an extremely weird event,” said Michael Wehner, a climate scientist at Lawrence Berkeley National Laboratory, who co-authored another article about 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, that paper found that the heat wave was so unprecedented that it essentially broke most standard tools used to measure human impact on heat waves.

“In the end, we calculated that the event was not only impossible without climate change, but also impossible with climate change. And of course it happened, 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 paper does include findings on temperature, noting that global warming during the heat wave caused a whopping 1.8 degree rise in maximum temperatures.

Future warming could lead to about a 9-degree rise in heat wave temperatures by the end of the 21st century, the paper said.

“The bottom line is that the degree of climate change we get is really determined by us, and by the people we choose to reduce carbon dioxide and other greenhouse gas emissions or not,” he said. “And the less we do, the worse it gets.”

The Columbia researchers also found that several factors came together to help create the sweltering heat dome, including abnormally dry soils and jet stream disturbances.

In California and other parts of the western United States, increasing heat, drought and desiccation contribute to long-term drying out of the soil, which means less water evaporates into the air, Bartusek explains.

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“And if less evaporation can get out of the ground, there’s a greater heating effect — heating the air just above the surface more effectively,” he said. He added that in some areas there was probably “this feedback process going on where the land surface helped amplify some of the highest temperatures.”

The jet stream — the fast-moving air currents in the upper level of the atmosphere that guide 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 “kinked” into a wave pattern and slowed, essentially locking the weather system in place and allowing the heat dome to build up over the region.

The researchers noted that the effect of climate change on the jet stream is still under debate, although some scientists believe 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.”

“It certainly remains a possibility that we will see more of these unusual flows with global warming,” he said.

Kai Kornhuber, an adjunct associate research scientist at the Lamont-Doherty Earth Observatory and another author of the Columbia study, said the findings highlight how anomalies between soil moisture levels, the jet stream and other factors can push temperatures even above their usual magnitudes.

“Sometimes these factors just match up and you get those conditions that make a perfect storm,” he said. “However, what is important to mention here is that each of those drivers show increasing trends associated with climate change… Coincidences like this may occur with a higher probability in the future just because these common drivers are all linked to climate trends .”

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As for the likelihood of such heat waves reaching that 10% annual probability in 2050, a lot depends on the emission path that society ultimately follows. But “given the accelerated trends in extreme weather events worldwide, there are reasons to believe that these estimates may even be a little conservative,” Kornhuber said.

He and the other researchers noted that while some of the numbers and predictions from the studies may differ, their core messages are very similar: that the extreme heat wave was essentially impossible at pre-industrial emission levels.

“What’s important in this aspect is that these methods all agree that climate change plays an important role in every heat wave we observe today,” Kornhuber said.

While the findings are dire, the researchers said they could help inform future models 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 unable to handle such strain. Wehner said improved adaptation efforts and contingency plans will help, but such events will eventually “get worse because there’s a lot of climate change baked into the system.”

“The more we can reduce our emissions – ultimately to nothing, to zero – and the sooner we can do that, the better it is to avoid even worse tragedies,” he said.


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