How can we repair the ozone layer

It was one of the few success stories in international environmental policy. With the Montreal Protocol at the end of the 1980s, the political course was set to initiate a recovery of the ozone layer. This restricted the production of long-lived chlorofluorocarbons (CFCs). At the time, these substances were used as refrigerants in refrigerators and as propellants for spray cans. They destroy ozone in high layers of the earth's atmosphere and thus the protective shield against life-threatening UV-B radiation. Thanks to the agreement, the production and consumption of harmful chemicals have since fallen by more than 90 percent.

The World Meteorological Organization (WMO) has long been dampening hopes of a quick recovery because CFCs only degrade slowly. At the same time, it also spread optimism, as the total ozone content of the atmosphere has stabilized since 2000. That optimism is now called into question. As an international study led by scientists from ETH Zurich and the Physical-Meteorological Observatory in Davos shows, the ozone layer is by no means recovering over the tropics and in mid-latitudes - on the contrary, it is continuing to thin out there.

ETH researcher William Ball is the main author of the study. "I'm concerned," he says. To understand his concern, it takes a more detailed look at the studied data, which has been measured by various independent satellite systems for decades. About 90 percent of the ozone in the earth's atmosphere hovers at a height of about twelve to 48 kilometers, in the so-called stratosphere. The remaining ten percent is in the lowest level of the atmosphere, the troposphere, which extends from sea level to an average of twelve kilometers.

The data of the current study confirm earlier studies: As a result of the Montreal Protocol, the ozone content in the upper stratosphere, at an altitude of 32 to 48 kilometers, is increasing, as hoped. There are also signs of the recovery of the ozone hole over Antarctica. "This is clear evidence that the Montreal Accords are working," says William Ball.

Since the start of the measurements, the layer has never been as thin as it is now

But in the lower stratosphere - at an altitude of around 15 to 24 kilometers - ozone has been continuously and apparently inexorably lost since 1987. In the study, the authors emphasized the term "continuous" with an exclamation mark and in italics, which is unusual. A good 40 percent of global ozone is distributed at this altitude. It is therefore crucial how the recovery takes place in this layer. The ozone depletion at this altitude more than makes up for the success in the upper air layers. "Overall, the vital ozone layer has never been so thin since measurements were taken," warns atmospheric researcher William Ball.

"It is not the development that we expected," says Johannes Staehelin, professor emeritus at the ETH Institute for Atmosphere and Climate in Zurich and co-author of the study. The atmospheric chemist has been working on the long-term development of the ozone layer since the late 1980s. He is convinced: "Without the Montreal Protocol, two thirds of the global ozone layer would probably be destroyed in the long term."

Nevertheless, the question arises: How do the new findings fit in with the previous assessments by the WMO and the researcher reports that the ozone hole is closing? The answer is relatively simple: So far, the main focus has been on the so-called total ozone in the atmosphere, that is the amount of ozone measured from the earth's surface to the upper limit of the stratosphere. That means: This calculation also includes the ozone in the troposphere, the lowest part of the atmosphere. Since there is no significant exchange of air at the boundary between the troposphere and the stratosphere, the loss above is compensated for by the growth below, as the authors interpret in their study.

Humans are responsible for the increased ozone levels below. It produces nitrogen oxides in internal combustion engines and hydrocarbons in industry. Both substances promote the formation of ozone. The paradox is: The massive thinning of the vital ozone layer in the stratosphere is compensated for by the ozone that humans cause and which is a harmful irritant gas. For decades, authorities have been trying to reduce ozone concentrations and summer ozone peaks through appropriate regulations - so far with mixed success. Viewed cynically, one can currently be grateful that humans produce ozone and thus prevent the protective shield against UV rays from being further weakened.

Global warming, as climate models show, is likely to change the distribution of air in the stratosphere

But that cannot be the solution. The authors of the study therefore call for everything to be done to find the causes of ozone depletion in the lower stratosphere. But the atmosphere is not a machine whose mechanism is known down to the last detail. Too many factors play a role in the breakdown of ozone: the intensity of the sun, for example, and the exchange of air between layers of the atmosphere.

Nevertheless, there are hypotheses as to what causes the ozone depletion could be: One concerns the very short-lived halogen compounds, so-called Very Short Lived Substances (VSLS), with shelf lives of less than six months. A study at the German Aerospace Center near Munich shows that these substances can reach the stratosphere and can damage the ozone layer. Most of the emissions come from industrial processes. The substance dichloromethane belongs to this substance category. This is a substance that occurs in solvents and has a strong ozone depletion potential. Consumption has risen sharply in recent years. Researchers see another explanation for ozone depletion in climate change, the influence of which on the ozone layer has not yet been studied very well: climate models show that global warming is likely to change the distribution of air in the stratosphere. Ozone is basically formed over the tropics, and winds then transport it to medium latitudes.

And then there is the Montreal Protocol itself: Prohibited substances are still being released into the atmosphere. In addition, the industry continues to produce ozone-depleting substances, the production of which is not regulated in the environmental agreement.