A study leads to new conclusions about the range of the 2021 heat dome and points a finger at “blocking ridges”
Worse than we thought: Like this guy at the Salmon Street Springs in Portland, we knew the 2021 heat dome was big. We just didn’t realize how big. Photo: Alex Milan Tracy/Sipa USA via AP Images
By Nathan Gilles. May 23, 2024. In June and July of 2021, a large, immovable “dome” of hot, high-pressure air enveloped the Pacific Northwest, blocking cooler, wetter systems from moving in.
The now infamous “heat dome” lingered long enough to subject the region to record heat.
On June 28, 2021, Seattle reached an unbelievable 108 degrees. That same day, Portland reached 116 degrees.
The following day, in British Columbia, the small town of Lytton reached 121 degrees, Canada’s warmest temperature ever recorded. The next day much of Lytton was destroyed in a wildfire.
What became known as the 2021 heat dome is estimated to have killed hundreds of people in Oregon, Washington and British Columbia.
But according to a recent study published in the Nature journal Communications Earth & Environment, the effects of the 2021 heat dome extended well beyond the Pacific Northwest.
The study estimates the heat dome was responsible for 21–34 % of the total area burned during an outbreak of wildfires that started during the heat dome in June 2021 and spread across the United States and Canada, reaching as far as Ontario.
The study also estimates the 2021 heat dome was longer and larger than it might have been had it not been for human-caused climate change.
“Blocking ridges” gaining intensity
The 2021 heat dome is an example of a naturally occurring atmospheric phenomenon called a “blocking ridge.”
Blocking ridges are intense high-pressure systems. As their name implies, they block other weather systems from moving into a region. This includes cooler, moisture-laden storms that can bring relief.
Blocking ridges also tend to linger, creating their own feedback patterns.
Evolution of the blocking ridge associated with the 2021 heat dome from June 21 to July 7, 2021. The heat dome’s intensity is measured in geopotential meters. GPMs measure how high into the troposphere (the bottom layer of the atmosphere) a blocking ridge reaches. The taller a blocking ridge gets, the more intense the temperatures it produces can be. The tallest period of the blocking ridge occurred over the Pacific Northwest and corresponds with the hottest period of the heat dome. Blocking ridges are referred to in the figure and study as persistent positive anomaly, or PPAs. Graphic : Springer Nature
Although naturally occurring, blocking ridges are becoming more intense due to climate change, says study lead author Piyush Jain, research scientist at Natural Resources Canada at the Northern Forestry Centre in Edmonton, Alberta.
“These blocking ridges … they’re part of the natural variability of the climate system,” says Jain. “But the story of the [2021] heat dome and some of the emerging trends that we are seeing is an amplification of these events [due to climate change].”
The longer a blocking ridge hangs around, the hotter it can get.
Because a blocking ridge’s heat also dries out the soil and the air, they can become even hotter and more immovable, making them linger longer, causing more warming and more drying, and so on.
This, says Jain, is what happened during the 2021 heat dome.
“When you have these events occurring for a long time, you get this feedback process that amplifies the event itself. And you get these very, very intense temperatures,” says Jain.
In his study, Jain and his colleagues calculate the 2021 heat dome was 59% longer and 34% larger than it might have been without anthropogenic climate change.
Andrés Holz, associate professor at Portland State University and director of the university’s Global Environmental Change Lab, who was not involved in the study, says it’s useful to think of high-pressure blocking ridges like the 2021 heat dome like a “rock in a river.”
In Holz’s analogy, the rock is the blocking ridge, and the water is the wind.
“The wind goes north or south around the blocking ridge like water goes around the rock,” says Holz. “And so, there is a stagnation, a lack of wind and, hence, warmer conditions right behind and to the leeward side of that rock. During the 2021 heat dome, that’s what happened in the Pacific Northwest.”
Holz, who was in Portland when the city reached 116 degrees, remembers spending the hottest day of the heat dome in the basement with his family.
“It was insane,” says Holz. “I have never experienced temperatures like that before.”
Holz describes Jain’s research as “outstanding in the discipline,” adding that his current study is “solid.” Holz characterizes Jain and his coauthors as some of the best climate researchers working.
Heat dome bigger than thought
Jain and colleagues’ current study is part of a growing body of “climate attribution” studies.
Attribution studies are scientific studies that link human-caused climate change to specific events, including droughts, heat waves and wildfire activity.
“From an attribution perspective, when we look at models that consider no climate change versus models that consider climate change, there’s very strong evidence to suggest that extreme heat events are much more likely under a warming climate due to human-caused climate change,” says study coauthor John Abatzoglou, professor of climatology at University of California, Merced, and a leading researcher on the role that climate change has played in the rise in wildfires in the American West.
Both Abatzoglou and Jain are quick to point out that although the number of heat waves has increased in recent decades, blocking ridge-induced heat waves have not. However, heat waves caused by blocking ridges are getting bigger.
This is the conclusion of a previous study published in 2022 that both Jain and Abatzoglou contributed to. The study looked at blocking ridges that occurred in North America from 1979 to 2020.
Jain and colleagues’ current study on the 2021 heat dome builds off this previous work.
Comparison of blocking ridges in North America from 1979 to 2021. Blocking ridges are referred to in the figure and study as persistent positive anomaly, or PPAs. Blocking ridge intensity is measured in geopotential meters. GPMs measure how high into the troposphere (the bottom layer of the atmosphere) a blocking ridge reaches. Black dots represent the tallest and most intense blocking ridge observed in a given year. Gray dots represent all other blocking ridges observed. The overall trend shows an increase in the tallest/warmest blocking ridges, ending with the 2021 heat dome, the tallest and warmest blocking ridge on record. Graph : Springer Nature
The difference between these past blocking ridges and the 2021 heat dome, according to Jain, is how much larger and longer it was.
In their current study, Jain and colleagues quantified this by comparing the 2021 heat dome to 449 previously recorded blocking ridges.
The researchers determined that the heat dome was 2.9 times stronger than the average blocking ridge observed from 1979 to 2021.
In fact, by all measures, the 2021 heat dome was the largest blocking ridge ever recorded in North America. The next largest ridge, which occurred in 2003, was 36% weaker by comparison.
The 2021 heat dome also lasted an astounding 27 days, according to the study.
Previous estimates for the heat dome’s duration have focused on the hottest eight days of the heat wave, June 25 to July 2, 2021. By tracking the heat dome as a blocking ridge, Jain and colleagues demonstrated that the heat dome actually started on June 18 and lasted until July 14.
During its near month-long lifespan, the researchers estimate that the heat dome covered 4.5 million square miles of North America, making it roughly eight times the size of Oregon, Washington and British Columbia combined.
Just how big the 2021 heat dome was can be seen by viewing the phenomenon on a map of North America.
Although the epicenter of the heat dome was positioned over Oregon, Washington and British Columbia, the effects of the blocking ridge extended as far south as southern California, as far north as southern Alaska and as far east as western Quebec.
The 2021 heat dome also reached higher in the troposphere (the bottom layer of the atmosphere) than any previously recorded ridge.
“In lay terms, that means [the 2021 heat dome] was taller, and that means the atmosphere under it was much warmer. The atmosphere where people live, where the fuels [trees and other vegetation] were drying was much warmer,” says Abatzoglou.
“Fire weather”
The warm, dry conditions caused by the 2021 heat dome severely dried out vegetation, creating the right conditions for wildfires.
These conditions are referred to in the paper and similar studies as “fire weather.”
Extremely dry conditions caused by intense fire weather only need the right source—be it a downed power line or a lightning strike—to ignite, says Abatzoglou.
“I wish there was something more to it, but, yeah, when it’s hot, it’s also dry. And when you have hot, dry conditions, once a fire gets going, there is even less resistance for that fire to burn through an area,” says Abatzoglou.
Piyush Jain. Photo: Piyush Jain
Jain says the fire activity observed during the 2021 heat dome tracks with what is known about how blocking ridges create fire weather.
In their previous 2022 study, Jain and Abatzoglou determined that wildfires in North America were seven times as likely to start during blocking ridges than during periods without blocking ridges.
“It’s well known that when you have a blocking ridge, you have sustained warm temperatures and little rainfall for several days or longer, and these events are very important for fire activity in North America,” says Jain.
However, the 2021 heat dome was again different from these past ridges due to its intensity and reach across North America.
The extent of the heat dome’s drying effects led to fires occurring not only in Oregon, Washington and British Columbia, but also to fires in California, Saskatchewan, Manitoba and Ontario, according to the study.
What’s more, as the study points out, many of these fires were burning at the same time across the landscape.
Using previously compiled fire data, the study notes that simultaneously burning fires in the United States and Canada “severely constrained” the sharing of firefighting personnel and resources both inside and between the two countries.
“Fire management agencies weren’t able to share resources because they couldn’t spare them,” says Jain.
Jain says a similar problem occurred during last year’s fire season in Canada, which was caused not by a single large drying event, like the 2021 heat dome, but a series of smaller drying events that fell one on top of another.
He says we’re likely to see larger fire seasons made up of simultaneously burning fires and, consequently, fewer opportunities to share firefighting resources as climate change continues.
According to Abatzoglou, wildfires and the drying effect of climate change in general is “a catalyst for pushing ecosystems to the brink.”
“In agricultural systems we can do things like apply irrigation. We can’t do that to our natural systems,” says Abatzoglou.
