Pacific Northwest heat wave a 1,000 year event — hopefully

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The sun shines near the Space Needle, Monday, June 28, 2021, in Seattle. Seattle and other cities broke all-time heat records over the weekend, with temperatures soaring well above 100 degrees Fahrenheit (37.8 Celsius). (AP Photo/Ted S. Warren)

Recent analysis from an international team of weather and climate experts known as “World Weather Attribution” (WWA) have come to the preliminary conclusion that the recent deadly heat wave in the Pacific Northwest was a 1-in-1,000-year event in today’s climate.

These results are preliminary, and it’s because the results have yet to be formally reviewed by other experts. But the methods used by WWA have been used in numerous other published studies like this.

According to climate.gov, if they are correct, it would have been at least 150 times rarer before global warming. Theoretically, a 1-in-150,000-year event—so rare, they concluded, that it’s fair to say it would have been “virtually impossible” in pre-industrial times. Taken at face value, it would also mean that events like that aren’t about to become common any time soon.

Portland International Airport set a new all-time high temperature record (pink dots) three days in a row. Not only was this record much warmer than the average high from 1991-2020 (top of the dark gray area), it was well outside the margin of other daily records (light gray). The most extreme heat occurred on June 28, when the daytime maximum temperature reached 116 degrees Fahrenheit,

It’s to be noted that the 1-in-1,000-year event comparisons are based on climate data and models that a given increase in global average temperature will lead to a statistically proportional increase in extreme heat—not a sudden paradigm shift in which the kind of heat that’s physically possible suddenly goes non-linear.

While there are many models at the disposal of WWA, only models that could reasonably mimic the real-world behavior of heat extremes in the Pacific Northwest—the seasonal cycle, the geographic patterns, the long-term trends, and the statistical “shape” of the full spread of events that can occur—were included in the final results.

However, according to climate.gov, the Pacific Northwest heat wave was so exceptionally rare that experts at WWA are also having to consider whether extreme events like this are part of a new climate norm. It’s possible what we’ve considered “extreme weather events” are now more like 1-in-50 or 1-in-100 year, and thus part of the new norm. We just don’t know yet.

Animated gif showing how a change in mean temperature changes the frequency of heat extremes
This animation shows how a relatively small increase mean temperature can lead to a large shift in the frequency of hot extremes. Image by Climate Central. 

Silver lining is, if the follow-up studies fail to find a connection, or a plausible hypothesis for what really could have caused the heat wave, then it’s also possible the event was an anomaly – true 1-in-150,000-year event.

No matter the conclusion, the outlook for the future would still be worrisome, though. Earth’s surface temperature today is about 1.2 degrees Celsius (2.2 degrees Fahrenheit) warmer than it was at the start of the Industrial Revolution.

This amount of warming appears to have turned an event like the June heatwave in the Pacific Northwest from something that was “virtually impossible” into something that is rare, but possible. With another 0.8 degrees Celsius of warming, model experiments projected that a heatwave like the June event would return not once per millennium, on average, but as often as once or twice per decade.

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