Thanks for reading My World’s on Fire, a newsletter about disasters from journalist Colleen Hagerty. Let’s make this a regular thing – subscribe here for disaster deep-dives, Q&As, and context in your inbox on Thursday evenings. You can also become a paying member to access exclusive content.

Looking at images of the Calf Canyon fire—which has merged with the Hermits Peak fire to become the largest wildfire complex* in New Mexico’s history— it can be hard to imagine that it started as a smolder. But that's exactly what officials say happened.

*Note: I share that statistic with my usual disclaimer

“The Calf Canyon Fire was caused by a pile burn holdover from January that remained dormant under the surface through three winter snow events before reemerging in April,” reads the InciWeb report.

US Forest Service Chief Randy Moore reiterated that finding in a Congressional hearing this morning.

“One of those fires that started had snow on it two to three different times,” Moore said when speaking about the devastating conflagration in New Mexico. “Normally, you would think that that should be enough to put it out, only to have the conditions change within our climate and it reignite outside of the containment lines.”

What he’s saying might sound pretty wild—the idea of a fire not only surviving multiple bouts of snow but then starting up again elsewhere—and Moore acknowledged that this blaze in particular has raised questions even within the Forest Service of what exactly happened. But the general phenomenon of fires smoldering for extended periods of time—days, weeks, or even months—and then reigniting isn’t actually unusual. These are known as holdover, sleeper, or even “zombie” fires, and they were actually the subject of one of my very first newsletters in the summer of 2020.

Now that they're back in the headlines, I thought I'd revisit that reporting and round it out with some new context and conversations.

At the time, I wanted to provide context about a series of holdover fires observed in the Arctic, and I enlisted Dr. Merritt Turetsky, Director of the Institute of Arctic and Alpine Research at the University of Colorado-Boulder, to help.

As she explained to me then, “Scientists surmise that fires in 2019 went underground and smoldered in peat all winter long and are now growing again and becoming detected. We don’t know how much of the fires currently being monitored in the Arctic are zombie fires, but they are intriguing because they represent momentum in the system by which one severe fire season can impact the subsequent season.”

You can read more from that 2020 newsletter here (and apologies in advance for the wonky formatting that came with migrating old editions to my new provider).

Experts have since established a clearer connection between these fires and climate change. According to one study published last year, some of the “main drivers” of holdover fires are influenced by the warming climate, including extreme heat and drought conditions that can fuel longer and more severe wildfire seasons. National Geographic detailed more about this study in an article that explains why the Arctic in particular tends to see holdover fires and touches on the increasing prevalence of these events.

NASA also has some pretty incredible satellite images showing what these smoldering fires look from above.

In recent days, I’ve spoken with some firefighters to get their perspectives about holdover fires since they're on the frontlines of experiencing them. One firefighter told me they’ve personally seen about “a dozen” of these fires that have kicked back up after a week or two. Another noted that a holdover fire can happen in spots officials attempted to light a prescribed burn and failed (as can happen due to moisture levels and other conditions). Even if the fire doesn't appear to catch, the area can still smolder enough to ignite in warmer, drier, and/or windier conditions in the future.

When it comes to recognizing and addressing holdover fires, a representative from the advocacy group Grassroots Wildland Firefighters, which represents federal wildland firefighters, told me there’s a need for “experienced firefighters” to patrol areas that have been the sites of previous burns or are in prime condition for such fires to reignite. But that requires a level of staffing and attention they told me is not the reality in many areas, as federal agencies struggle to fill their ranks.

If you have any questions or want to share your expertise about holdover fires, I’d love to hear it—you can always reply to this email to reach me directly, or find me on Twitter.

Summer reading

I'm very excited to have my own affiliate page with Bookshop.org, a one-stop-shop to purchase reads from your local booksellers. I'll be filling it with titles that have helped inform my reporting or expanded my thinking about disasters, including some I have highlighted in previous editions!

I just started adding books, so be sure to bookmark the page and keep coming back to stock up for your summer reading.

As always...

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Now, here's a little something for reading to the end.

Colleen