A dusky shroud hung high over Alaska and western Canada in early August, a plume of smoke, soot and other tiny particles tainting the lower stratosphere and thick enough for satellites to detect. But the particles suspended in the Alaskan and Canadian pall, called aerosols, didn’t emanate from one of the wildfires that often strike the region’s boreal forests during the long days of summer. Instead, space-based images traced the smoke to massive blazes that erupted in late July in central Russia, more than 9,000 kilometers to the west.
While it has been known for decades that large wildfires can create or enhance thunderstorm clouds, leading to what are called pyrocumulonimbus clouds, only recently have scientists discovered that the clouds can boost smoke into the stratosphere. Once in this layer of the atmosphere — immediately above the troposphere, where most of Earth’s weather happens — the smoke can be caught by jet stream winds and carried long distances, says Mike Fromm, a meteorologist at the Naval Research Laboratory in Washington, D.C.
Before the late 1990s, anomalous plumes of stratospheric aerosols were usually blamed on remote and therefore undetected volcanic eruptions, Fromm noted in August at the American Geophysical Union’s Meeting of the Americas in Iguaçú Falls, Brazil. But new analyses of satellite data, presented at the meeting and chronicled in the September Bulletin of the American Meteorological Society, reveal that pyrocumulonimbus clouds, or pyroCbs, regularly send smoke to the stratosphere. During the 2002 North American fire season alone, pyroCbs lofted aerosols to this layer more than a dozen times.
“In 2000, few scientists believed that these clouds could inject aerosols into the stratosphere,” says Pao Wang, an atmospheric scientist at the University of Wisconsin–Madison. “Now it’s almost taken for granted that they do.”
Along with aerosols, the high-flying plumes carry a heavy load of the chemically active gases that are produced in substantial quantities during a fire, especially in the smoldering phase. While their chemical and climatic effects aren’t fully known, the plumes’ dark particles tend to absorb sunlight, warming themselves and the atmosphere around them while cooling Earth’s surface slightly. A fuller understanding will help scientists fine-tune climate models, adjusting contributions of various aerosol sources.
In many ways, says Fromm, pyroCbs are just like other cumulonimbus clouds: They can provide prodigious amounts of precipitation and spawn a lot of lightning. Where pyroCbs differ from standard storm clouds, however, is in their source of convection. While it’s the heat produced by condensing water vapor that drives the updrafts in the towering thunderheads of cumulonimbus clouds, those in pyroCbs are largely driven by the intense heat of the wildfire at ground level.
That gives pyroCbs an extra push: The momentum from particularly strong updrafts enables the fire-fueled clouds to routinely make it to the lower reaches of the stratosphere. Even the tops of typical cumulonimbus clouds, in contrast, rarely rise out of the troposphere.
As a column of smoke rises from a wildfire, it pulls in surrounding humid air. The moisture in that air condenses to form the pyroCb cloud as the plume reaches high altitudes.
“Nobody really knows what happens inside these clouds,” Fromm says. But satellite images clearly show that smoke carried upward inside the clouds emerges from the top as if from a chimney, he notes.
Many high-altitude aerosol plumes detected by satellites in the late 1980s and early 1990s were mistakenly attributed to volcanoes. Take, for instance, a plume that wafted over the central United States in the summer of 1989. Several times during late July and early August, ground-based lasers near Manhattan, Kan., spotted the layer of aerosols in the lower stratosphere. Instruments based near Salt Lake City also recorded the plume. The first analyses of airflow patterns in the lower stratosphere suggested a Central American source. And indeed, Fromm notes, Guatemala’s Santiaguito volcano had erupted on July 19 of that year.
But other data didn’t support a volcanic origin, Fromm and his colleagues reported at the August meeting. For one thing, witness accounts and satellite images suggested that Santiaguito’s plume never rose more than six kilometers into the sky. Also, sensors at the Utah site saw signs that the particles weren’t spherical, as typical sulfate droplets from volcanoes would be. New analyses of weather patterns, including the location and speed of the jet stream at the time, track the aerosol plume to large wildfires sparked by lightning in Manitoba and Saskatchewan on July 17 of that year, Fromm said.
Similarly a thick plume of aerosols detected in the stratosphere over the Atlantic Ocean northwest of Spain in late June 1991 was mistakenly linked to eruptions of the Philippines’ Mount Pinatubo earlier that month. Fromm’s team has traced that plume to wildfires in Quebec.
Satellite images from the 2002 fire season in North America suggest just how often pyroCbs may pollute the stratosphere. That year, 17 different plumes of stratospheric aerosols could be linked to massive wildfires, including Colorado’s Hayman Fire and Arizona’s Rodeo-Chediski Fire — the largest fires recorded in the histories of those states.
In addition to aerosols such as smoke and soot, fire-fueled clouds inject unusually large quantities of chemically reactive gases such as carbon monoxide, methyl cyanide and hydrogen cyanide into upper layers of the atmosphere, data suggest. Scientists are somewhat concerned about the ultimate effects of these gases on atmospheric chemistry and the planet’s climate.
“These clouds are an important route to the stratosphere for smoke, soot and other pollutants,” Wang says.
The thickest plume of aerosols lofted from a pyroCb in recent years originated with fires that ravaged southeastern Australia and killed hundreds of people on February 7, 2009 — a day known as Black Saturday. These fires, fueled by high temperatures, low humidity and strong winds, slung an unprecedented amount of pollutants into the stratosphere, says Nathaniel Livesey, an atmospheric scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.
Satellite data hint that carbon monoxide concentrations in some parts of the plume in the lower stratosphere reached 800 parts per billion, 10 to 15 times that in normal air, he reported at the August meeting. Preliminary analyses suggest that concentrations of methyl cyanide and hydrogen cyanide were between 2.5 to 7 times normal, says atmospheric physicist Hugh Pumphrey of the University of Edinburgh, who worked with Livesey on the study.
Notes Livesey, “It’s still an unanswered question as to what combination of circumstances produced such a strong injection of aerosols from this particular fire.” And the ultimate fate of those aerosols isn’t clear.
While the plume of aerosols from the Australian fires was unusually thick, those from most other pyroCbs are more modest. “In the broad scheme of things,” Pumphrey contends, “pyroCbs may not affect the stratosphere all that much.”
Yet Fromm thinks the overall effect of pyroCbs should be detectable. Though individual plumes from pyroCbs aren’t nearly as thick as the typical plume of volcanic aerosols, the clouds occur more often than major eruptions do. All together, he suggests, the climatic influence of the fire-triggered plumes may equal a few percent of the global cooling produced by volcanic aerosols.
Determining the precise magnitude of the effect, if any, would be useful, Fromm notes. Once scientists know the cooling power of aerosols from pyroCbs, that result can be incorporated into models of regional and global climate.