Smoke from Australia's megafires has already traveled tens of thousands of miles around Earth. "It circles the globe in roughly a week," said Manvendra Dubey, who researches air pollution and wildfire smoke at Los Alamos National Laboratory.

Hundreds of blazes in Alaska, the Amazon, and the Arctic circle have also huffed thick plumes of smoke into the atmosphere over the past six months. These fumes — a mix of burnt particles and invisible gases — do gradually dissipate and eventually appear to vanish, of course. After all, after eons of wildfires and volcanic eruptions, the atmosphere isn't anything like the hazy air that amasses in a smoky bar.

Though, "it did feel like that in Canberra [Australia] for over two weeks," recalled Rebecca Buchholz, an air quality and wildfire smoke expert at the National Center for Atmospheric Research (NCAR), who recently returned from a visit to Australia's capital. It was a smoke-filled trip.

But, where does all the smoke eventually go?

Broadly, there are two components of wildfire smoke: the stuff you can see, and the stuff you can't. Each are bad for different reasons. Let's start with the visible stuff, like what the Australian smoke satellites have spied swirling over the Pacific Ocean.

Visible smoke

When bushfires burned gum trees and eucalyptus in vast swathes of Australia's treasured Wollemi National Park — which is presently closed due to fires — the charred remnants of those plants took to the air as tiny particles, known as particulates or aerosols.

This visible air pollution from Wollemi and many other parched bushlands in southeast Australia easily gets transported by prevailing winds. These particles have choked the air in the nation's three most populous cities of Sydney, Melbourne, and Brisbane, a combined population of some 12.5 million souls. The pollution, sometimes only traveling around 65 miles (in the case of Sydney), makes for a terrible breathing environment. "There are serious, direct impacts on air quality," said Mark Parrington, a senior scientist in the European Union's Copernicus Atmosphere Monitoring Service.

Inhaling particulates isn't just bad for your lungs. It's bad for your heart. Scientists have repeatedly shown breathing this stuff is linked to an acceleration of plaque build-up in arteries. And that's not all. There's mounting, troubling evidence that inhaling the smallest particles increases the risk for dementia and Alzheimer's disease.

Thankfully, winds (eventually) clear the air. Most of these particulates, which from space often appear as a brownish haze, get lofted some five to six miles into the troposphere (the lowest part of the atmosphere, where weather, like clouds, rain, cyclones, and beyond, occur).

And up here, most of this visible smoke soon meets its demise.

"The aerosol particles in the smoke will remain in the atmosphere for about five to seven days," said Faye McNeill, who researches atmospheric chemistry and air quality at Columbia University.

Many of these smoke particles will encounter rain, she said, and wash down to the surface with water droplets. Other particles, owing to gravity, will slowly fall to the ground. Some of the soot from Australia's fires, for example, blanketed glaciers in New Zealand. But most of these Australian-born particles fell into the sea, as the smoke had to cross the expansive Pacific Ocean, explained Neal Blair, an environmental engineer at Northwestern University.

(How particulates impact the sea is unknown. The particles could potentially fertilize the oceans with nutrients in the smoke, like nitrogen, noted Blair.)

Most visible smoke, then, returns relatively quickly to the surface world from whence it came. But the invisible components of smoke are a much bigger, and in many ways, more influential, story.

The effects of rising temperatures are felt around the world. Large and intense fires 🔥 burned in Alaska and Siberia and across Australia this year. pic.twitter.com/JIEd9v1hq7 — NASA Earth (@NASAEarth) January 15, 2020

Invisible smoke

When forests, woodlands, and grass burn, the vast majority of the resulting smoke is invisible and odorless to us. Most of the wood burned — on the order of 90 percent — is combusted into carbon dioxide, a potent greenhouse gas, explained Los Alamos' Dubey.

For Australia's current fire season, that means prodigious amounts of heat-trapping CO2 has been released into the atmosphere. From Sept. 1, 2019, through Jan.15, 2020, that number is some 400 megatons of carbon dioxide, said Copernicus' Parrington. That's equivalent to around 75 percent of all the carbon emissions Australia currently emits into the atmosphere each year.

Much of this carbon dioxide will likely stay in the atmosphere, where it will live for hundreds of years. Plants on land and plankton in the ocean will consume some of this carbon dioxide — though exactly how much is unknown.

"It will have climatic consequences," said Dubey.

Meanwhile, burned wood also releases other types of invisible gases. When sunlight reacts with some of these molecules, like carbon monoxide, the atmospheric chemical ozone is formed. Ozone is bad to breathe. It's linked to wheezing, lung irritation, and breathing pain.

"It inflames the tissues of the lungs," said NCAR's Buchholz, adding that Melbourne has seen high concentrations of ozone.

CO2 emissions from Australian wildfires Image: Copernicus Atmosphere Monitoring Service/ECMWF



This @CopernicusEU #Atmosphere Monitoring Service animation of high-resolution carbon monoxide & carbon dioxide forecasts highlights the detailed the structure of smoke transport from the #AustralianBushfires over the Southern Hemisphere.



More forecasts➡️https://t.co/ulxVuDL7n1 pic.twitter.com/clTn8Z0lsW — Copernicus ECMWF (@CopernicusECMWF) January 17, 2020

This ozone, combined with visible, sooty pollution (called particulates above) makes for some of the worst — and at times the worst — air quality on Earth. "Now you have Sydney with worse air pollution than Delhi," said Dubey, referencing India's abhorrent air quality.

Winds will gradually wash these noxious fumes away, but not before the toxins have entered millions of Australian lungs.

Other invisible gases burned off from Australian bushland, like carbon monoxide and related molecules, will get lofted into the air just like the visible particulates. Here, carbon monoxide lives in the atmosphere for some three months, noted Dubey, until naturally occurring chemical reactions in the atmosphere convert it into its final form — the heat-trapping greenhouse gas carbon dioxide.

"The atmosphere is often seen as an enormous chemical laboratory," said Parrington.

The stratosphere

While most of the sooty visible smoke lives, and soon dies, in the troposphere, a small fraction of Australia's smoke has been propelled even higher into the next level of the atmosphere, the stratosphere, noted Dubey. (The world's most famous guitar was named after this lofty region.)

The stratosphere starts at some 6 miles up in the atmosphere, and once particulates get up there, they can take longer to fall down to the surface — in large part because there's no weather to wash them back to earth.

"If [particulates] get there, then they can survive for a year," said Dubey.

Australian smoke back in Nov. 2019 Image: nasa

And so goes the tale of Australia's, and the world's, smoke.

It's terrible to breathe, and reacts with sunlight to become even more terrible to breathe.

And over the coming decades, the invisible smoke may be the most insidious of all air pollution: In a world with atmospheric carbon dioxide already amassing at historically and geologically unprecedented rates, equally unprecedented fires may keep adding even more heat-trapping gases to Earth's atmosphere, which continues to relentlessly warm.