What’s going on with the hole in the ozone layer?

Scientists first sounded the alarm about human-caused destruction of the ozone layer in 1974, but it took several more years before the international community reached an agreement to ban chemicals that damage the ozone layer.

Paul Crutzen and his team played a pivotal role in bringing this issue to the world’s attention at the time. Today, the ozone hole is slowly healing, but progress is being hindered by the growing challenges of climate change.

In late November 1987, the German news magazine Der Spiegel shocked the nation when, in large letters, it blared the headline “Ozone Hole” across its cover, with a gaping black hole and a giant spray can cutting through the blue sky in the background. For the first time, Germans truly understood the gravity of the situation: the planet was on the brink of environmental catastrophe.

Just a few weeks ago, a large area of ​​the ozone layer over Antarctica collapsed for the first time. The ozone layer is a key layer that acts as a protective shield in the atmosphere 15-25 kilometers above the Earth’s surface, blocking harmful ultraviolet (UV) radiation from space. According to Der Spiegel, a survey flight revealed that the ozone hole has expanded to an area the size of the United States.

In South America, shepherds reported that their livestock were going blind due to excessive exposure to UV rays, and in Australia, television stations began warning sunbathers of their “tanning time” — the number of hours they had left before sunburn occurred.

The planet’s fate seemed sealed: Soon, unblocked ultraviolet radiation would destroy plants and animals and destroy humanity’s food supply. Incidence of skin cancer was predicted to skyrocket to unprecedented levels.

But scientists were already sounding the alarm about ozone depletion as early as 1974. Chemists Mario Molina and Frank Sherwood Rowland published a hypothesis in Nature magazine that chemicals used as propellants in aerosol cans and related compounds (known as chlorofluorocarbons (CFCs), or halons), were accumulating in the stratosphere and damaging the ozone layer.

They proposed that this destruction was happening in the upper stratosphere, about 40 kilometers above Earth. Their estimate was off by about 20 kilometers, but their core idea was correct. But it took years for conclusive evidence to emerge and for a global effort to combat CFCs to gain momentum. The first ozone hole was detected in Antarctica in 1985, but it wasn’t until 2020 that a similar hole was observed in the Arctic.

NASA research flights

Molina and Rowland’s Nature article inspired research teams around the world, including the group of Paul Crutzen at the Max Planck Institute for Chemistry in Mainz, to focus on the problem of ozone depletion. Crutzen, who passed away in January 2021, shared the 1995 Nobel Prize in Chemistry with Molina and Rowland for their contributions to understanding the chemistry of the ozone layer.

Ozone has long been a subject of meteorological research because its concentrations naturally fluctuate in the atmosphere, but the chemical processes that cause these variations were not well understood.

Chemist Christoph Brühl, who joined Crutzen’s group as a doctoral student in the early 1980s, recalls a period of intensive research: “My colleague Susan Solomon worked with NASA to carry out extensive research flights that measured ozone, CFCs, and other gases in situ. These provided essential and unique data for our models.”

These field measurements allowed researchers to test Crutzen’s theory that CFCs were causing ozone layer destruction through the catalytic cycle – a chemical reaction in which ozone molecules are continually broken down in a chain reaction. At the time, scientists were already well aware of the natural catalytic cycles that contribute to ozone layer destruction, in particular the cycle driven by nitric oxide (NO) and nitrogen dioxide (NO₂).

NO and NO₂ can reach the stratosphere through aircraft emissions or are produced there through chemical reactions such as nitrous oxide in agriculture. In these cycles, NO reacts with ozone (O₃) to destroy it and produce nitrogen dioxide (NO₂) and oxygen (O₂). Sunlight then breaks the NO₂ back down into NO and oxygen (O), continuing the cycle and further depleting ozone.

Crutzen’s key contribution was to apply this catalytic cycle model to CFCs, with chlorine atoms splitting ozone molecules one by one.

The role of clouds

Interestingly, ozone depletion pauses during the polar night, which lasts for several months. But once the night ends in both Antarctica and the Arctic, the depletion process resumes with full force. Crutzen’s team discovered why: Polar stratospheric clouds (PSCs), which form in the extreme cold of the polar night, are a key factor.

The first rays of sunlight in spring set off a chain reaction on the surfaces of these clouds that leads to the destruction of the ozone layer by chlorine. This problem is further exacerbated by the unique weather conditions in the polar regions which prevent the mixing of different air masses, allowing the ozone-depleting reactions to proceed largely unchecked.

Initially, the chemical industry denied the link between CFCs and the ozone hole. But as Crutzen’s work and the work of other scientists were published, the connection became undeniable. This growing evidence spurred one of the most successful global environmental movements in history.

In 1987, countries around the world signed the Montreal Protocol, which came into force in 1989 and banned the production and use of ozone-depleting substances. Over the next few years, these harmful chemicals were gradually phased out and replaced with more environmentally friendly alternatives.

Wildfires destroy the ozone layer

But that’s not all: Brühl’s former colleague Susan Solomon, now at the Massachusetts Institute of Technology, took a closer look at the impact of Australia’s devastating wildfires in 2019 and 2020. Her team found that hydrochloric acid molecules, residues of CFCs, attach to the surface of wildfire smoke particles.

These particles react with other materials and release chlorine molecules. When exposed to sunlight, these chlorine molecules break down into highly reactive chlorine radicals, which rapidly destroy ozone molecules. “After these fires, the stratosphere looked like another planet,” Solomon said in 2023.

The power of trace gases

“We’ve known for a long time that the ozone layer is sensitive to a variety of trace gases, even at low concentrations,” Brühl says. For example, in the 1970s, Crutzen warned that supersonic aircraft like the Concorde could damage the ozone layer by emitting nitrogen oxides. At the time, the possibility of frequent intercontinental supersonic flights was imminent. Because these aircraft flew directly into the stratosphere, this could have devastating effects.

Similarly, when NASA’s space shuttle began launching with its big boosters in the 1980s, data showed a significant drop in ozone levels within a few hundred kilometers of its flight path. Given these precedents, Bruhl remains cautious about the current recovery of the ozone layer.

Large volcanic eruptions also contribute to ozone depletion. These eruptions release large amounts of sulfur and other compounds into the stratosphere, which negatively impacts the ozone layer. For example, the eruption of Hunga Tonga Hunga Ha’apai volcano in Tonga in January 2022 released a large amount of material into the stratosphere.

“The impact on the ozone layer is still measurable,” Brühl says. Despite these ongoing challenges, there is some good news: The Antarctic and Arctic ozone holes, which sometimes extend all the way to northwestern Europe, are gradually shrinking. The only question that remains is how long it will take for the effects of CFCs to disappear completely.