The last few summers have brought some big firestorms: record-setting fires ravaged Washington last year, as they have throughout the West over the last decade. Those fires have sometimes turned deadly, as happened in 2013, when rapidly changing weather conditions led to the deaths of 19 firefighters outside Yarnell, Arizona. Now, experimenters have taken a small step toward identifying the warning signals of so-called "blow up" fires—a step that could one day help slow the most erratic fires, and perhaps save lives.
For firefighters, a blow up is a sudden explosion in the intensity or spread of a flame, often accompanied by equally violent, convective winds driven in part by the fire itself. They're not inevitable, though. Many forest fires are relatively slow-burning and slow-moving and remain so for extended periods of time. The switch to a violent blow-up fire happens only some of the time, and it takes some kind of kick to get that switch going.
To Harvard University researchers Jerome Fox's and George Whitesides' ears, that sounds a lot like what physicists call a phase transition. Likely the most familiar example is the switch from water to ice as temperatures drop, but in recent years scientists have used the concept to study ecosystem collapse, language shifts, financial meltdowns, and more—all shifts from one stable situation to another. But as the focus has shifted to less-predictable cases with serious societal repercussions, there's been a corresponding move toward identifying the signs we might be near collapse.
Many forest fires are relatively slow-burning and slow-moving and remain so for extended periods of time. The switch to a violent blow-up fire happens only some of the time, and it takes some kind of kick to get that switch going.
One potential sign goes by the name "critical slowing down." Imagine nudging an ecosystem a bit by culling the population of a crucial species. The closer the system is to a tipping point, beyond which lies collapse, the longer that system takes to recover—that's critical slowing down. Perhaps, Fox and Whitesides reasoned, fires near the transition to blow ups might do something similar.
The pair chose a very simple system to test out their idea: strips of nitrocellulose, better known as flash paper, placed on a wire mesh grid and set ablaze at one end. Using high-speed infrared cameras to track the intensity and size of the flame, the researchers first mapped out where the transition to blow ups was in terms of variables such as temperature and the flash paper's incline. Then, they nudged the fire a bit with a small fold in the paper designed to fan the flames, so to speak.
As they expected, the fires flared temporarily post-nudge, and the closer their fires were to blow up transitions, the longer they took to return to pre-nudge levels. "This constitutes the very definition of critical slowing down," Fox and Whitesides write.
Although forests are a long way from nitrocellulose strips, the researchers argue they have enough in common to warrant further investigation of critical slowing down in real forest fires. "Future fire intervention strategies capable of accommodating such warning signals may be effective at slowing the spread of 'erratic' fires and minimizing risk to fire response teams," they write.