It’s not just how hot the fires burn – it’s also where they burn that matters. During the recent extreme fire season in Australia, which began in 2019 and burned into 2020, millions of tons of smoke particles were released into the atmosphere. Most of those particles followed a typical pattern and settled to the ground after a day or week; however, the particles created in fires burning in one area of the country managed to blanket the entire Southern Hemisphere for months. When studying particle-laden haze, two researchers at the Weizmann Institute of Science noticed puzzling spikes in a certain measurement, and tracked the elevated levels to the fires in that area. Next, as reported in Science, the Israeli scientists uncovered the “perfect storm” of circumstances that swept the particles emitted from those fires into the upper atmosphere and spread them over the entire Southern Hemisphere.
Particles reaching the stratosphere – the upper layer of the atmosphere – are typically from volcanic eruptions. The ash emitted in such extreme eruptions dims the sun and cools the planet – and produces spectacular sunsets. Prof. Ilan Koren of the Institute’s Department of Earth and Planetary Sciences, who conducted the study with his former student, Dr. Eitan Hirsch, now head of the Environmental Sciences Division at the Israel Institute for Biological Research in Ness Ziona, had noticed an extreme increase in aerosol optical depth (AOD), a satellite-based measure of particle loading in the atmosphere. In January 2020, AOD measurements, plotted in standard deviations, showed a deviation three times the normal – some of the highest readings ever obtained, higher even than those from the catastrophic volcanic eruption of Mt. Pinatubo in 1991. The timing, however, did not coincide with any volcanic activity. The scientists wondered if fires might be to blame, even though it is rare for such smoke to escape the troposphere in significant amounts. The troposphere – the atmosphere’s lower layer – extends from the ground to a height of several kilometers; if smoke particles even manage to rise that high, they hit the tropopause: an inversion layer that acts as a sort of ceiling between the troposphere and the stratosphere.
Using data from AOD, several satellites, and LIDAR readings that revealed how the particles were distributed vertically in “slices” of atmosphere, Prof. Koren and Dr. Hirsch worked backwards to prove that bushfires – specifically, those burning in Southeastern Australia – were the source of the spikes. (LIDAR – Light Detection and Ranging – is a remote-sensing method that uses laser pulses to measure ranges, or distances, from a particular target to the Earth.)
How did these smoke particles penetrate the tropopause ceiling? Why did they come from these particular fires? One clue, says Dr. Hirsch, lay in another, distant forest fire that occurred several years ago in Canada. Then, too, high AOD levels had been recorded. Both the Canadian and Southeastern Australia fires occurred in high latitudes, away from the equator.
The height of the troposphere shrinks at such high latitudes: Over the tropics its upper ceiling can reach up to 18 km (a little over 11 mi) above the surface, while somewhere above the 45th parallel, in both the Northern and Southern hemispheres, it takes a sudden step down to around 8-10 km (5-6.2 mi) in height. So the first element enabling the particles’ trans-layer movement was simply having less atmosphere to cross.
Prof. Koren and Dr. Hirsch considered pyrocumulus clouds – clouds fueled by the energy from fires – as a means of transporting smoke to the stratosphere. However, when inspecting the satellite data, the scientists noticed that pyrocumulus clouds formed only over a small fraction of the fires’ duration and were mostly seen over fires burning on the central part of the coast. In other words, these clouds could not explain the large amounts of particles that were transported to the stratosphere; furthermore, an additional mechanism for lifting smoke downwind from the sources was missing.
This brings up the second element: the weather patterns in the strip known as the mid-latitude cyclone belt. Running through the southern end of Australia, this is one of the stormiest regions on Earth. The smoke was first advected (moved horizontally) to the Pacific Ocean by the prevailing winds in the lower atmosphere, where some of it converged into deep convective clouds and was lifted in the clouds’ core into the stratosphere. An interesting feedback mechanism known as “cloud invigoration by aerosols” can further deepen the clouds.
In a previous study, the authors had shown that in conditions such as the pristine environment over the Southern Ocean, the convective clouds are “aerosol limited.” The elevated smoke levels could thus act as cloud condensation nuclei, allowing the clouds to develop deeper and increasing the number of clouds that are able to penetrate the tropopause and inject the smoke into the stratosphere.
Once in the stratosphere, the particles found themselves in a different world than the one they had just left. When below, they were at the mercy of mixing and churning air currents. On top, the air moves in a steady, linear fashion; specifically, one strong current was moving them east, over the ocean to South America to the Indian Ocean and back to Australia. Along the way, the particles slowly settled around the entire hemisphere. “People in Chile were breathing particles from the Australian fires,” says Dr. Hirsch. By sailing on an endless air current, these smoke particles remained airborne for much longer than those in the lower atmosphere.
“For people on the ground, the air may have just seemed a bit hazier or the sunsets a bit redder. But such a high AOD – much, much higher than normal – means sunlight was getting blocked, just as it does after volcanic eruptions,” says Prof. Koren. “So the ultimate effect of that smoke on the atmosphere was cooling, though we still do not know how much influence that cooling and dimming may have had on the marine environment or weather patterns.”
“There are always fires burning in California, in Australia, and in the tropics,” Prof. Koren adds. “We might not be able to stop all of the burning, but we do need an understanding that the precise locations of those fires may grant them very different effects on our atmosphere.”
Prof. Ilan Koren’s research is supported by the de Botton Center for Marine Science; the Sussman Family Center for the Study of Environmental Sciences; the Dr. Scholl Foundation Center for Water and Climate Research; the Ben May Center for Chemical Theory and Computation; Scott Eric Jordan; the Yotam Project; the estate of Emile Mimran; and the European Research Council. Prof. Koren is the incumbent of the Beck/Lebovic Chair for Research in Climate Change.
The Weizmann Institute of Science in Rehovot, Israel, is one of the world’s top-ranking multidisciplinary research institutions. The Institute’s 3,800-strong scientific community engages in research addressing crucial problems in medicine and health, energy, technology, agriculture, and the environment. Outstanding young scientists from around the world pursue advanced degrees at the Weizmann Institute’s Feinberg Graduate School. The discoveries and theories of Weizmann Institute scientists have had a major impact on the wider scientific community, as well as on the quality of life of millions of people worldwide.