Smoke from a Distant Fire
Burning forests can send aerosols into the stratosphere and around the world
By Sid Perkins
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.