Firemen prepare as a bushfire approaches homes on the outskirts of the town of Bargo on December 21, 2019 in Sydney.

Fires in southeastern Australia in 2019–20 sent plumes of smoke into the stratosphere.Credit: David Gray/Getty

Smoke from intense wildfires that raged across southeastern Australia in 2019–20 caused atmospheric temperatures to spike and probably made the hole in the ozone layer bigger, finds a study1.

Extreme drought in 2019 gave rise to bush fires of unprecedented intensity, which burnt more than 5.8 million hectares. In addition to causing catastrophic damage, the fires generated plumes of smoke that rose into the atmosphere and bumped up temperatures in the lower stratosphere over Australia by 3 °C. Globally, temperatures in the lower stratosphere rose by 0.7 °C — the biggest increase since the eruption of Mount Pinatubo in the Philippines in 1991 sent a plume of ash into the atmosphere, says the study, which was published in Scientific Reports on 25 August. The temperature boost lasted for around four months.

The effect “was equivalent to the type of impact that we’d see from a moderate volcanic eruption”, says palaeoclimate scientist Nerilie Abram at the Australian National University in Canberra. “What we’re discovering about the scale of these fires is remarkable.”

Smoke in the stratosphere

The stratosphere sits between roughly 10 and 50 kilometres above Earth’s surface. Smoke particles don’t typically get to the stratosphere, but smoke from the Australian fires reached heights of more than 35 kilometres owing to unusual, fire-induced pyrocumulonimbus clouds. These smoke-infused thunder clouds hold lots of black carbon, which absorbs heat and rises into the lower stratosphere like a hot-air balloon, says study co-author Jim Haywood, an atmospheric scientist at the University of Exeter, UK. Once there, the black carbon continues to absorb sunlight and warm the air.

“There were parts of the coastline that were under a haze of smoke for months on end,” says Abram. “The scale of that bush-fire season was just off the charts.”

Haywood and his team used data from polar-orbiting and remote-sensing satellites to observe changes in the distribution of smoke particles in the stratosphere, and combined this information with climate models. They found that the impact of smoke particles on stratospheric temperatures that was predicted by the models matched observed temperature spikes. Previous studies2 have used models to simulate the length and degree of warming in the aftermath of the fires, but this study incorporates a global analysis, definitively attributing the rise in temperatures to the bush-fire smoke, says Haywood.

“This is really putting another nail in the coffin,” says Clare Murphy, an atmospheric chemist at the University of Wollongong in Australia. She says the work builds on earlier studies, “extending the evidence”.

Damaged ozone layer

The models also indicated that chemical reactions between the smoke and ozone in the atmosphere exacerbated the Antarctic ozone hole, making it bigger. “The year before the fires, we had a puny little ozone hole,” says Haywood. “In 2020, we were taken rather aback because there was a very, very deep ozone hole.” He says the hole lasted for around five months.

Depletion of the ozone layer strengthens the southern polar vortex, a pocket of low pressure and cool air over the South Pole. That creates a feedback loop: the stronger the polar vortex is, the more it depletes the surrounding ozone and the longer it keeps the hole open for. When the ozone layer is damaged, more radiation from the Sun gets through to Earth, causing harm to the environment and human health. Warming in the stratosphere can also lead to ozone-layer damage, by altering the atmosphere’s dynamics.

Exactly how wildfire smoke and ozone interact is still a puzzle, owing to the complex chemical cocktail in smoke. Climate change is expected to increase the frequency and severity of wildfires, so Haywood emphasizes that it is important to nail down how smoke and fires will affect the ozone layer.