The high temperatures in late June that killed hundreds of people in Oregon, Washington and Canada were so unusual that they couldn’t have happened without a boost from human-caused global warming, researchers said Wednesday, when they released a rapid climate attribution study of the heat wave in the Pacific Northwest.

The temperatures were so far off the charts that the scientists suggested that global warming may be triggering a “non-linear” climate response, possibly involving drought magnifying the warming, to brew up extreme heat storms that exceed climate projections. 

Climate change, caused by greenhouse gas emissions, made the Pacific Northwest heat wave at least 150 times more likely, and increased its peak temperatures by about 3.6 degrees Fahrenheit, the study by World Weather Attribution concluded. 

“I think it’s by far the largest jump in the record that I have ever seen,” said Fredi Otto, a University of Oxford climate researcher and co-author of the study. “We have seen temperature jumps in other heat waves, like in Europe, but never this big.”

The extreme temperature spike shook up some fundamental assumptions about heat waves, said co-author Geert Jan van Oldenborgh, of the Royal Dutch Meteorological Institute. 

“It was way above the upper bounds,” he said. “It was surprising and I’m shaken that our theoretical understanding of how heat waves behave was so roughly broken. We’ve dialed down our certainty.”

If global warming has pushed the climate past a heat wave tipping point, he added, “we are worried about these things happening everywhere.”

The Pacific Northwest heat wave should be a big warning, said co-author Dim Coumou, with the Institute for Environmental Studies at VU Amsterdam and the Royal Netherlands Meteorological Institute. It shows that climate scientists don’t understand the mechanisms driving such exceptionally high temperatures, suggesting “we may have crossed a threshold in the climate system where a small amount of additional global warming causes a faster rise in extreme temperatures.”

In an unrelated study published July 6, European Union researchers studying climate tipping points found additional evidence that human-caused warming could be “abrupt and irreversible,” partly because the current warming is so fast that the climate system can’t adjust. Even the “safe operating space of 1.5 or 2.0 degrees above present generally assumed by the IPCC might not be all that safe,” said co-author Michael Ghil, with the University of Copenhagen.

About 800 people died across the Pacific Northwest during the heat wave, a number that will probably still go up as officials examine medical records and statistics in the coming weeks and months. The peak temperature was 121.3 degrees Fahrenheit on June 29 in Lytton, British Columbia. After setting heat records for Canada on three consecutive days, the town was mostly destroyed by a wildfire driven by hot winds in the dried out forests nearby. In addition to contributing to several major wildfires in the region that are still burning, the heat cooked growing fruit and scalded foliage on trees and other vegetation. 

The European Union’s Copernicus Climate Change Service said Wednesday that the average June temperature was the highest on record for North America and the fourth-highest on record globally. In early July, extreme heat boiled over in northern Scandinavia, with parts of Finland reporting record-breaking temperatures. Persistent heat across northeastern Russia is fueling fires there that are emitting record levels of carbon dioxide for this time of year. And in the West, yet another spasm of dangerous heat is building, potentially peaking this weekend in central and eastern California.

The Deadliest Climate Extreme

Release of the attribution study of the Pacific Northwest heat wave coincided with other new research with dire heat warnings. 

A study led by Monash University scientists published Wednesday in The Lancet Planetary Health gives a comprehensive evaluation of heat deaths around the world from 2000 to 2019, a period when the global average temperature rose by nearly a full degree Fahrenheit. It attributes about 637,550 deaths during each of those years to high heat, including about 224,000 deaths per year in Asia, 78,000 in Europe and 19,000 in the United States. 

The high death toll in the Pacific Northwest was “sadly, no longer a surprise but part of a very worrying global trend,” said Maarten van Aalst, with the Red Cross Red Crescent Climate Centre and University of Twente, noting that heat waves were the world’s deadliest climate disasters in 2019 and 2020. 

In the U.S., heat is the leading weather-related killer, said Kristie L. Ebi, of the Center for Health and the Global Environment at the University of Washington. But with good planning, nearly all those deaths are preventable, she said. 

Communities need effective heat action plans that prepare for what are now completely unexpected heat extremes. Early warning and response systems, and community outreach programs, with neighbors checking on each other during heat emergencies, are among the best tools for saving lives, van Aalst said. There is also research showing that staff and scheduling changes at hospitals and ambulance services, based on extreme heat forecasts, can prevent deaths. 

Loading the Dice for Weather Extremes

The new attribution study bolsters previous warnings about the need to prepare for more extreme heat waves in a rapidly warming climate, said Otto, one of scientists working on the attribution study. The findings should be considered in the context of what societies are resilient to, and what they can adapt to, she said.

“This is not something you would plan for, or expect to happen,” she said. “The models of today are not a good indicator of what to expect at 1.5 degrees (Celsius) of warming. Most societies are sensitive to small changes, and this is not a small change, it’s a big change. We should definitely not expect heat waves to behave in the same way they have in the past.” 

Global warming has jacked up the odds for rare events, like 100-year floods, to happen every few years, said Carl-Friedrich Schleussner.

“We haven’t seen what a once in a 50 year event looks like now, in a climate altered by humans,” he said. “People are relating to those extreme events as really exceptional, and they are not. We are on the way to leaving the climate window of the Holocene, of the last 8,000 years where we’ve been enjoying a stable climate.”

Already, the world has warmed about 1.2 degrees from the pre-industrial average, he said, enough to fuel exceptional and dangerous heat extremes.

“It’s not really comprehended or understood what a climate change of 1.2 degrees is,” he said.

He warned that change is non-linear with global warming, meaning that a small rise of the average global temperature can spur a proportionately bigger increase in dangerous heat. Studies show that extremes like the 2003 European heat wave that killed about 70,000 people would have been nearly impossible without human caused warming and, with just another 1 degree Fahrenheit of warming, are likely to happen every other year by the 2040s.

“Our climate experience doesn’t prepare us to understand the scale of what’s going on,” he said. “People talk about loading the dice and throwing sixes. Global warming is loading the dice so we’re throwing sevens now, something impossible previously.”