How climate change intensified the Pacific Northwest heat wave


The deadly heat wave that ravaged the Pacific Northwest in late June was reportedly “Practically impossible” without man-made climate change, announced an international team of scientists on July 7.

In fact, temperatures were so extreme – Portland, Oregon, reached 47 ° Celsius (116 ° Fahrenheit) on June 29, while Seattle soared to 42 ° C (108 ° F) – that early analyzes suggested that they were impossible even with climate change, said Geert Jan van Oldenborgh, climatologist at the Royal Netherlands Meteorological Institute in De Bilt, at a press conference to announce the team’s findings. “It was an extraordinary event. I don’t know which English word covers it.

Climate change from greenhouse gas emissions has made the region’s heat wave at least 150 times more likely to occur, the team found. As global emissions and temperatures continue to rise, such episodes of extreme heat could occur in the region as often as every five to ten years by the turn of the century.

It’s not just that many temperature records have been broken, said van Oldenborgh. It was because the temperatures observed were so far from historical records, breaking those records by up to 5 degrees C in many places – and a full month before the usual maximum temperatures for the region. The observations were also several degrees higher than the upper temperature limits predicted by most climate simulations for heat waves, even after accounting for global warming.

Just a week after the onset of the heat wave, the new study is the latest real-time climate attribution effort by scientists affiliated with the World Weather Attribution Network. Van Oldenborgh and Oxford University climatologist Friederike Otto founded the group in 2014 to perform rapid analyzes of extreme events such as the Siberian heat wave of 2020 (NS: 07/15/20).

In the current study, 27 researchers focused on comparing observed temperatures from June 27 to 29 with annual maximum temperatures of the past 50 years for locations in the northwestern United States and southwestern Canada. The team then used 21 different climate simulations of temperatures to analyze the intensity of such a heat wave in the region with and without the influence of warming greenhouse gases.

The Earth has already warmed by about 1.2 ° C compared to pre-industrial times. This warming, the researchers determined, increased the intensity of the heat wave by about 2 degrees C. Once global warming reaches 2 degrees C, future heat waves could become even more intense (ND: 12/17/18). These heat waves could be even hotter by 1.3 ° C, the researchers found.

This represents a real danger. The heatwave at the end of June wreaked havoc (SN: 06/29/21), killing several hundred people – “almost certainly” an understatement, researchers say. On June 29, Lytton, a small town in British Columbia, set a Canadian temperature record of 49.6 ° C (121.3 ° F). The heat may have exacerbated wildfires that a day later swept through the Fraser Canyon region of British Columbia, burning 90 percent of the village, according to local authorities. Meanwhile, the western United States and southwestern Canada are already bracing for another spike in temperatures.

One possible reason for the surprising intensity of this heat wave is that, as climate change amplified temperatures, what happened was still a very rare and unlucky event for the region. It’s not easy to tell how rare it is, given that the observed temperatures were so far off the charts, the researchers said. Under current climatic conditions, simulations suggest that such a heat wave could occur once every 1,000 years, but these events will become much more frequent in the future as the climate changes.

aerial image of a forest fire in British Columbia
By the end of June 2021, more than 40 wildfires had burned in British Columbia, Canada, exacerbated by extreme drought and intense heat. A fire burned 90 percent of the town of Lytton, which had set a new temperature record for the country the day before. The fire also generated a huge plume of smoke producing a storm, called a pyrocumulonimbus cloud.Nasa

Another possibility is darker: Climate simulations may not accurately capture what actually happens during extreme heat waves. “Climate science has been a little complacent” about simulating heat waves, assuming heat wave temperatures would increase linearly with rising global temperatures, Otto said. But now the Earth’s climate system may have entered a new state in which other climatic factors, such as drier soils or changes in jet stream circulation, exacerbate the heat in more difficult ways. to be expected and less linear.

The new study did not seek to determine which of these possibilities is true, although the team plans to tackle that question in the coming months. However, many scientists have already noted the inability of current climate models to capture what is really going on.

“I agree that it is practically impossible that the [Pacific Northwest] the heat wave would have occurred with the intensity observed in the absence of climate change, ”commented by email Michael Mann, a climate scientist at Penn State who was not involved in the attribution study. “But the models used do not capture the phenomenon of the jet stream… which WE KNOW played an important role in this event.”

The disproportionate warming of the arctic region alters the high temperature gradients in the atmosphere, which can lead to a more wavy jet stream, Mann wrote in the New York Times June 29. This ripple can exacerbate and prolong extreme weather events, such as the thermal dome centered over the Pacific Northwest in late June.

This recent heat wave was not only a major disaster, but also posed major scientific questions, said van Oldenborgh. Such an event “would have been deemed impossible last year. We have all just reduced our certainty about the behavior of heat waves, ”he added. “[We] are much less certain of the impact of climate on heat waves than we were two weeks ago.


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