Australia’s
most recent wildfire season was so severe that smoke from the fires reached new
heights in the atmosphere — and showed some very weird behavior while it was up
there.
A
particularly intense series of bushfires in southeastern Australia from
December 29 to January 4 spurred the formation of huge pyrocumulonimbus, or pyroCb, clouds (SN: 10/22/10). Those
fire-fueled thunderstorms launched between 300,000 and 900,000 metric tons of
smoke into the stratosphere, which was more than any seen from a previous inferno.
One especially large, long-lasting smoke plume rose to a record altitude while spinning
and wrapping itself in rotating winds.
Those winds have never been observed around similar plumes, researchers report
online May 30 in Geophysical Research Letters.
This vast
puff of smoke, which still hasn’t fully dissipated, spanned roughly 1,000
kilometers — about the width of Montana. That made it one of the largest, if
not the largest, wildfire smoke plume that satellites have ever seen in the
stratosphere, says atmospheric scientist Jessica Smith of Harvard University,
who was not involved in the study. “Any perturbation to the stratosphere has
implications for … stratospheric ozone,†which shields Earth from the sun’s
harmful ultraviolet radiation
(SN: 4/7/20).
It
remains to be seen whether a blob of pyroCb smoke like this could leave a
chemical scar on the stratosphere. But observing the plume’s behavior may give
insight into what could happen if much more smoke — say, from a nuclear war —
were pumped into the atmosphere.
Mike
Fromm, a meteorologist at the U.S. Naval Research Laboratory in Washington,
D.C., and colleagues kept tabs on the unusual pyroCb smoke plume with
satellites and weather balloons. One of the most striking things about the
plume is how high it rose, says coauthor George “Pat†Kablick III, an
atmospheric scientist also at the U.S. Naval Research Laboratory. In less than
two months, it was buoyed up from the lower stratosphere, about 15 kilometers
off the ground, to over 31 kilometers high.
Dark
particles in the smoke absorbed sunlight and heated up the plume to make it
rise, Kablick explains. Atmospheric scientists first observed such self-lofting
behavior in pyroCb
smoke from Pacific Northwest wildfires in 2017, but that smaller mass of smoke
ascended only from an initial altitude of about 13 to about 23 kilometers above
the ground (SN: 8/8/19).
The
smoke from the Australian plume largely resisted mixing with surrounding air for
months after its formation, perhaps shielded by 15-meter-per-second winds seen
whirling around the plume as it rotated, the researchers say. The team is still
trying to figure out what whipped up this newly discovered wind phenomenon.
As the
plume rose through the stratosphere, it lifted up unprecedented amounts of
water and carbon monoxide. The concentrations of those gases in the plume were
several hundred percent higher than normal stratospheric air and displaced the
ozone-rich air that typically makes up gas at these altitudes.
Lots of
sun-warmed smoke rising through the atmosphere has the potential to damage the
ozone layer not only by displacing the stratosphere’s normal, ozone-rich gas,
but also by triggering chemical reactions that destroy ozone. Future satellite
or weather balloon observations could reveal whether this plume has had any noticeable
impact on stratospheric chemistry, says Pengfei Yu, a climate scientist at
Jinan University in Guangzhou,
China who studied the 2017 plume but was not involved in the new work. Â
Even if
this one wildfire-driven plume doesn’t leave a lasting mark on the stratosphere,
the smoke does offer clues about the fate of much larger quantities of smoke
that would result from a nuclear war, says Alan Robock, a climate scientist at
Rutgers University in New Brunswick, N.J., who was part of the team that
analyzed the 2017 plume.
The
smoke released by Pacific Northwest wildfires in 2017 helped validate nuclear
warfare simulations, which predict that smoke from burning cities would heat up
in the stratosphere and ascend to extremely high altitudes — where it could last
for years and damage the ozone layer.
“We
called that [2017 event] ‘the mother of all pyrocumulonimbus,’†because it injected
so much smoke into the stratosphere, Robock says. The fact that the larger
Australian smoke plume reached even greater heights now gives the researchers
“much more confidence†that their computer simulations are accurate, he says.
