It is bad news, because it is expected that such large wildfires – due to climate change – will occur much more often in the future.

You probably remember them: the Australian bushfires in 2019 and 2020. The images were dramatic, as were the consequences. Some 17 million hectares of nature went up in flames, dozens of people were killed and billions of animals were killed or, at best, lost their habitat. But it didn’t stop there, researchers say in the magazine Proceedings of the National Academy of Sciences† Because the intense fires also appear to have had an impact on the ozone layer.

What’s up with that?

Large wildfires generate huge clouds of smoke. And in the case of the Australian fires, they even reached the stratosphere (a layer in our atmosphere about 15 to 50 kilometers above the Earth’s surface). In 2021, researcher Pengfui Yu already showed that the smoke there caused parts of the stratosphere to warm up by two degrees Celsius. And it was during that investigation that he also came across indications that ozone was being destroyed following the fires in the southern hemisphere. It gave him the idea that the smoke clouds could have the same effect on ozone as particles pushed high into the atmosphere by volcanoes. As early as 1989, these particles were shown to initiate chemical reactions that lead to the destruction of ozone. Because when these particles end up in the atmosphere, they accumulate moisture on their surface. And once moist, they can react with chemicals circulating in the stratosphere. Consider, for example, the highly relevant dinitrogen pentoxide in the context of this research.

Nitrogen pentoxide

Normally, dinitrogen pentoxide reacts with sunlight, resulting in, among other things, nitrogen dioxide. And that resulting nitrogen dioxide then binds again in the stratosphere to chemicals that contain chlorine. But when volcanic aerosols enter the stratosphere, that all changes. Because those aerosols react with dinitrogen pentoxide to produce nitric acid instead of nitrogen dioxide. It means that the concentration of nitrogen dioxide decreases. And that forces chlorine-containing chemicals that normally bind to nitrogen dioxide to take a different approach: they are forced to transform into chlorine monoxide: a chemical that plays an important role in the breakdown of ozone. And so a volcanic eruption can lead to a depletion of the ozone layer.

Satellites

So there was already a suspicion that the Australian bushfires could damage the ozone layer in a similar way. But hard evidence for this was lacking. So scientists decided to look into it. They looked to satellite data to find out whether nitrogen dioxide concentrations fell sharply in the wake of the Australian fires. That turned out to be the case.

Fashion model

But was that really due to the smoke from the wildfires? To find out, the researchers used a model that can simulate the hundreds of chemical reactions that take place in our entire atmosphere. They introduced a huge cloud of smoke into the model, simulating the conditions at the time of the forest fires. And the model created a clear picture. As the smoke particles in the stratosphere increased, the nitrogen dioxide concentration dropped as sharply as the satellites had shown. “What we saw: more and more aerosols and less and less nitrogen dioxide in both our model and the data is a fantastic clue,” said study researcher Susan Solomon. “It tells us that these particles got wet and broke down some ozone.”

Battered ozone layer
Our ozone layer is already quite damaged. The main culprits are still the chlorofluorocarbon compounds that were used in the past as a refrigerant and propellant in, for example, refrigerators and freezers and aerosol cans. These compounds are broken down in the ozone layer by UV radiation, releasing chlorine radicals that break down the ozone molecules and reduce the ozone concentration. That is a problem, because the ozone layer is very important for life on Earth. For example, it blocks an important part of the dangerous UV radiation from the sun. In the 1980s it was therefore agreed to restrict the use of ozone-depleting substances. And now the ozone layer is slowly but surely recovering.

That the Australian bushfires had an impact on the ozone layer is bad news. But it is important to emphasize that the impact of the forest fires is much smaller than that of the infamous CFCs. What worries the researchers, however, is that these exceptional fires could take place much more frequently in the future – under the influence of, for example, higher temperatures and more drought. And with that, their impact on the ozone layer can also grow. “Australia’s bushfires appear to be the largest to date, but as global warming continues, there is reason to believe that such fires will become more frequent and more intense,” said Solomon. And it is feared that they could considerably delay the recovery of the ozone layer in the future. Maybe even in ways that we don’t all have as clear yet. “Smoke from wildfires is a poisonous concoction of organic yet complex substances. And I’m afraid that ozone will be pressured by a whole series of chemical reactions that we are now trying to unravel.”