Wildfires do more than char the ground. They spit particles into the air. Normally, smoke particles from even the extreme wildfires that are coming to characterize global heating don’t get far: they stay in the lower atmosphere and settle to the ground after a day to a week.
Now, two Israeli scientists from the Weizmann Institute investigating an enigmatic extreme haze detected over the southern hemisphere in 2020, which persisted for months, have deduced its unexpected origin. Such global hazes are usually associated with volcanoes.
Not this time. The haze was caused by the extreme fires in southeastern Australia that began in 2019 and burned into 2020 and a sort of perfect storm of circumstances, says Prof. Ilan Koren of Rehovot’s Weizmann Institute of Science. He researched the conundrum with Dr. Eitan Hirsch, then his student and now at the Israel Institute for Biological Research in Nes Tziona.
The sheer heat of the fire lifted the particles into the stratosphere, and once particles reach that high, they can stay there for months or even years.
Smoke from wildfires is normally trapped in the lower atmosphere, aka the troposphere. The smoke particles hit an inversion layer called the tropopause – a sort of boundary between the troposphere and the stratosphere. But the Australian fires were energetic enough to lift the particles through that boundary, according to the study published last week in Science.
Satellite data showed the haze blanketed the southern hemisphere especially from January to March 2020, but persisted through July – reaching all the way around and back to Australia’s west coast, says the Weizmann Institute.
A mysterious haze
Our story begins in January-February 2020, when record-breaking spikes of aerosols were noticed over the southern hemisphere. The haze was akin to that produced by violent volcanic eruptions, except there hadn’t been any.
- We’re No. 1: Global warming has surpassed global cycle
- Climate change briefs: Earth passed temperature tipping point decades ago
- We’re giving coral cancer too: The climate change stories on our radar
Volcanic eruptions violent enough to blacken the skies far and wide happened three times in the last 70 years: the 1974 eruption of Fuego in Guatemala; Mexico’s El Chichón in 1982; and Pinatubo’s explosion in 1991 in the Philippines.
Eruptions are anything but rare, especially if we factor in undersea ones. But another key source of aerosols in the air is burning biomass, and extreme wildfires are becoming more frequent in some parts.
Like all things climatic, the causes are myriad and interconnected. Roaring fires in Australia and the western United States, for example, are driven by rising average temperatures – the global average has already risen by about 1 degree Celsius – and droughts and aridification, which create more plant mass liable to burn. All it takes for a dry forest to burn is a spark.
In 2017, it was shown that emissions from extreme fires in Canada had reached the lower atmosphere, in amounts akin to a moderate volcanic eruption. But the fires in Australia were of another magnitude altogether.
“The Australian bushfire season of 2019 to 2020 was record-breaking in several ways,” Koren and Hirsch write: in intensity, in duration and in spread. Key to the muck in the sky were five massive fiery outbreaks in the continent’s southeast. The fires produced were so intense that the particles passed the troposphere and rose into the stratosphere, over time penetrating as high as 35 kilometers (nearly 22 miles).
Not all wildfires are fated to cast a pall over half the planet. The circumstances surrounding the southeastern Australian conflagrations were unusual, Koren and Hirsch explain. The tropopause is lower over that region.
Over the tropics, the troposphere’s top can be 18 kilometers above the Earth’s surface. Far north and south, it drops to around 8 to 10 kilometers in height. The first element enabling the particles’ trans-layer flight was simply having less atmosphere to cross, the Weizmann Institute explains.
Also, the regions were near or within the mid-latitude cyclones belt, where wind conditions are frisky enough to transport smoke particles to the stratosphere. Once the particles reach the boundary between the lower atmosphere and the stratosphere, the brisk winds spread them around the hemisphere.
And there we have it. Global warming is contributing to the frequency and intensity of wildfires which, under appropriate circumstances, can have practically global impact. Hirsch and Koren note that the aerosol readings from these Australian fires were among the highest readings ever obtained, surpassing even the figures produced by Mount Pinatubo in 1991.
Horrendous events such as the mega-fires in California, Australia, the Amazons and even the Arctic – especially the Arctic – seem likely to become even more frequent as climate change continues apace.
How did these smoke particles penetrate through the boundary to the stratosphere? How was this fire different? Well, actually a distant fire in Canada had done that too - and both fires occurred in non-equatorial, high latitudes.
Pyrocumulus clouds, i.e., clouds fueled by the energy of fires, were thought to convey the smoke particles upward but Hirsch and Koren realized such clouds were only present for a small fraction of the fires’ duration. They couldn't explain the sheer amounts of particulate matter that reached the stratosphere, which brings in the cyclone belt.
“People in Chile were breathing particles from the Australian fires,” Hirsch says.
The effect went beyond breathing. “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,” Koren explains. “The ultimate effect of that smoke on the atmosphere was cooling."