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Pacific Northwest heat wave was ‘virtually impossible’ without climate change, scientists find

The unprecedented episode was made at least 150 times as likely by rising temperatures, according to a rapid analysis

The Sparks Lake wildfire in British Columbia, seen from the air June 29. (BC Wildfire Service/AFP/Getty Images)

By all estimates, last week’s heat wave in the Pacific Northwest and British Columbia was essentially unprecedented. Seattle hit 108 degrees, Portland spiked to 116 and Canada broke its national temperature record three days in a row, hitting 121 degrees on June 29.

Hundreds of excess deaths were blamed on the brutal heat, which established records by margins of 10 degrees or more in spots. This was not “just another heat wave,” Christopher Burt, an expert on world weather extremes, wrote in a Facebook message, but rather “the most anomalous extreme heat event ever observed on Earth since records began two centuries ago.”

An analysis conducted by the World Weather Attribution group, which specializes in using computer modeling to examine the links between ongoing weather events and climate change, finds that the extreme heat wave would have been “virtually impossible” without human influence.

“I want the public to know that climate change has already affected extreme weather in a big way, especially heat waves,” Michael Wehner, a senior scientist at the Lawrence Berkeley National Laboratory and a contributor to the analysis, wrote in an email. “Global warming is not our grandchildren’s problem; it is ours, here and now.”

Another intense heat wave to roast Western U.S., southwest Canada

The analysis concluded that a robust relationship exists between top-tier heat extremes and human-caused climate change, stating that greenhouse gas emissions made the heat wave at least 150 times more likely to occur.

Twenty-seven scientists from more than half a dozen countries, including Canada, the United States, Germany, the Netherlands, Switzerland, France and the United Kingdom, worked on the attribution study.

The sobering findings reveal the real-time toll that human-caused climate change is having by making extreme weather more dangerous. The stakes will become even greater as the magnitude, duration and frequency of heat events increase.

“Every heat wave occurring today is made more likely and more intense by climate change,” read the analysis, noting that the planet has warmed about 2.2 degrees since the late 1800s. It also made clear that the outbreak of extreme heat in the Pacific Northwest and Canada was so far outside the realm of anything observed previously that it was “difficult to quantify exactly how rare the event is in the current climate.”

About 175 record highs were set in Northern California, Oregon, Washington and Idaho between June 25 and 30.

In Seattle, for instance, temperatures observed broke the previous record by more than five degrees. The team estimated that as a once-in-300-year event; temperatures in Seattle have risen 6.8 degrees since the late 1800s.

So exceptional was the event that the analysis team grappled with how to characterize it, ultimately arriving at two scenarios.

The first possibility suggests that climate change made the heat wave about 3.6 degrees hotter than it would have been otherwise, making it a roughly one-in-1,000-year event. But with continued climate warming, the team warned, an event of this magnitude will occur every five to 10 years by 2050.

Here’s what’s causing record temperatures in the Pacific Northwest

A more alarming possibility is that the latest extreme event in the Pacific Northwest and southwest Canada reflects a climatic inflection point of sorts, marking a “nonlinearity” or tipping point in Earth’s climate system. It would mean we are reaching a point where extreme heat events increase in scale, intensity and frequency far more rapidly than climate models would predict.

The scientists warned that leaders should enact adaptation or coping plans at locations as far north as 50 degrees north latitude, where “extreme temperatures, far outside the temperature range currently expected,” are increasingly likely.

Scientists shied away from connecting any singular event to climate change as recently as five to 10 years ago. Now it’s possible within as little as a week’s time, thanks to advances in computer model capabilities.

For this analysis, the team was working with about 70 years worth of data that it pumped into model “ensembles,” which create numerous simulations of the future. Simulations were run with and without the effects of human-induced climate change to quantify the impact of rising temperatures on the heat wave’s intensity.

“I’m completely unsurprised by [the findings],” wrote Kate Marvel, a climate scientist at Columbia University, who was not involved in the analysis. “Rising temperatures make heat waves more severe and more likely — that’s just basic physics. And all of that warming is due to human activities. Climate change didn’t ‘cause’ the heat wave — it made it much worse.”

The Pacific Northwest heat wave is shocking but shouldn’t be a surprise

The team emphasized that more research is needed in the coming years to fully firm up the link between heat waves and climate change. There are some models that suggest that the jet stream is slowing and becoming more wavy, allowing for large-scale deviations that transport lobes of heat poleward. The validity of that theory is unclear. Regardless of the exact dynamics, the scientists fear this latest example of unprecedented heat offers a glimpse into our future.

“Currently we do not understand the mechanisms well that led to such exceptionally high temperatures,” wrote Dim Coumou at the Royal Netherlands Meteorological Institute. “This event should be a big warning.”

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The exceptional heat wave helped North America register its hottest June on record, according to the Copernicus Climate Change Service, a Europe-based climate monitoring agency.

June temperatures were also boosted by a sweltering blast of heat mid-month that set all-time records in the Southwest and Mountain West. Phoenix hit at least 115 degrees for a record six days in a row, while Tucson topped 110 for an unmatched eight days in a row. Death Valley, Calif., notorious for holding the record for the planet’s highest temperature, had its hottest June on record.

Meanwhile, another serious heat wave is anticipated across the West later this week.

A heat wave is pushing temperatures to record highs in the northwestern United States and western Canada. (Video: Chris Vazquez/The Washington Post)

Jason Samenow contributed to this report.