By Brendan Bane
Compare volcanoes to wildfires, and it’s no competition. In many cases, volcanoes are vastly more powerful, and their eruptions’ effects on climate are much more pronounced. But in August 2017, the tables turned.
The 2017 North American fire season was one of the most destructive in recent history, breaking numerous regional records for acreage burned and costs incurred. Millions of acres were left smoldering, crumbling hundreds of homes and structures to ash. More than 280,000 personnel and 1,900 fire engines were deployed during the peak of the 2017 fire season. In the U.S., the Air National Guard assisted the U.S. Forest Service in wildfire suppression, dropping 530,000 gallons of fire retardant throughout the fire season. It was the service’s most costly firefighting year on record.
Now, a new study finds the fires that spread throughout North America last summer burned so powerfully their smoke pushed all the way into the stratosphere, circled the globe in roughly two weeks and remained in the stratosphere at measurable levels for several months.
The overall impact of the 2017 North American fires on the stratosphere surpassed all other documented wildfire events since the beginning of stratospheric observations in the 1980s and had an effect equivalent to a volcanic eruption, according to the study’s authors.
“This event was so big and its fires were so powerful that not only did they inject material into the stratosphere, they injected enough material that the stratosphere was polluted on a hemispheric scale,” said Sergey Khaykin, an atmospheric scientist at Versailles University (UVSQ) in France, and lead author of the new study in Geophysical Research Letters, a journal of the American Geophysical Union. “The effect really was comparable to a moderate volcanic eruption.”
When a volcano erupts, it spews ash and gaseous sulfur skyward. The ash later falls, but the sulfur reacts when airborne, eventually condensing into aerosol particles. Aerosols tend to reflect sunlight away from Earth, effectively cooling its surface and temporarily providing a cooler climate.
Fires also emit aerosols into the atmosphere. But unlike volcanoes, they’re rarely powerful enough to send aerosols into the stratosphere, the next highest layer of Earth’s atmosphere, which starts at about 10 to 17 kilometers (six to 10 miles) up. Because of this, fires tend to have far less pronounced effects on regional climate than volcanic eruptions, and thus scientists tend to pay them less attention, Khaykin said.
But the 2017 North American wildfires were uniquely powerful. In the new study, Khaykin describes the evolution of the wildfire plumes from the 2017 season as they circled the northern hemisphere. The traveling smoke clouds were observed above the Haute-Provence Observatory in France using laser radar called lidar powerful enough to pierce the entire atmosphere. Khaykin later tracked and analyzed the same plumes using CALIPSO space-borne lidar, which provided global Earth coverage.
As fires burned throughout California, Oregon, British Columbia, Montana and other areas last summer, thick smoke clouds lofted skyward. On August 16, the thickest plumes were seen above Athabasca Lake, Canada. In three days, the forefront of the plume reached France. A day later, it had advanced to Siberia. The plume reappeared above Canada shortly after — it had circled the globe in roughly two weeks.
The spreading smoke
The new study shows direct evidence of a fire’s smoke spreading on a hemispheric scale. Despite suppression, the fires burned hot enough that they began whipping up vertical drafts of wind, a process called pyroconvection. Those drafts pumped the smoke’s contents high into the stratosphere, just as if it were a moderate volcanic eruption. But the smoke particles were heavier than volcanic aerosols, irregularly shaped and potentially absorbed more light, according to Khaykin.
When smoke particles absorb light, they heat up and ascend even faster, facilitating their hemispheric-scale distribution. It was this quality that enabled the fires to have such a strong impact on the stratosphere, Khaykin said.
The smoke clouds even broke an aerosol index record, a measurement denoting how much light passes through the plumes and thus how thick and how high in altitude they are. The recent fires surpassed Australia’s 2006 Black Saturday and Canada’s 2001 Chisholm fires — the previous record holders — by roughly 30 percent, according to the new study.
“Many scientists believe the effect of biomass burning is small because the events are rare, even though bush or forest fires happen every summer,” Khaykin said. In light of these results, Khaykin hopes other researchers will allocate more attention toward the influence of wildfire pollution on the stratosphere. Although aerosol loads do affect regional changes in climate, Khaykin’s study did not explore that element of the plumes’ impact.
— Brendan Bane is a freelance science writer based in Santa Cruz, California.