Despite a global ban in place since 2010, atmospheric concentrations of five ozone-depleting chemicals have reached a record high.

Chlorofluorocarbons are entirely man-made gases used in a variety of applications, including refrigeration, air conditioning or as chemical solvents. They have been increasingly regulated by a series of international treaties since the 1980s. The 1987 Montreal protocol, which has been unanimously ratified, restricted the release of Chlorofluorocarbons to the atmosphere where they contribute to the destruction of the ozone layer: a region high up in the stratosphere which absorbs harmful ultraviolet radiation and protects life below.

The goal of the Montreal protocol was to induce a decline in the atmospheric Chlorofluorocarbons concentration through controlling and increasingly restricting, the production of these chemicals. This has worked well for many ozone-depleting substances, which is why the ozone layer is slowly recovering. And so the recent increase in atmospheric concentrations of five Chlorofluorocarbons is quite surprising.

The Montreal protocol has succeeded in eliminating the biggest sources of Chlorofluorocarbons. Credit: Sebastian Ganso/Pixabay (Content License)

Our findings, while worrying, should be considered an early warning. The impact of all five Chlorofluorocarbons on the recovery of the ozone layer is still small. Nevertheless, we do not fully understand where they are coming from, so this could change in the future, and we should not ignore the cumulative effect of these emissions on human health and the environment.

Global picture

Our team has been analysing air samples from all over the world, focusing on so-called “background” sites that are far away from the sources of these Chlorofluorocarbons, or in fact any industrial emissions. An example is the Cape Grim observatory on the remote west coast of Tasmania. This is the basis for our assessment of the threat these chemicals pose, as it reveals global trends in their atmospheric concentration.

Our main findings for the period 2010-2020 were twofold. First, concentrations of Chlorofluorocarbon-13 and Chlorofluorocarbon-113a continued their previously observed – and puzzling – increase. Rising concentrations of Chlorofluorocarbon-113a even accelerated around 2016. Second, concentrations of Chlorofluorocarbon-114a and Chlorofluorocarbon-115 were stable since the 2000s, while those of Chlorofluorocarbon-112a had even started to decrease. However, all of them began increasing around 2013-2014.

Global emissions of the five Chlorofluorocarbons weighted by their impact on ozone depletion (a) and the climate (b). Credit: Western et al. (2023)/Nature Geoscience

These observations, combined with additional knowledge about atmospheric circulation and how Chlorofluorocarbons are removed from the atmosphere through chemical reactions, allowed us to estimate the global emissions of these five gases. Their damage to the ozone layer can be expressed through their ozone depletion potential, which states how much ozone would be destroyed compared to the same quantity of Chlorofluorocarbon-11, which is different for each Chlorofluorocarbon.

The result is a relief. Emissions between 2010 and 2020 only resulted in a very small loss of around 0.002% of global stratospheric ozone.

There is no time to relax, though, for two reasons. All five Chlorofluorocarbons are also potent greenhouse gases and, once emitted, will remain in the atmosphere for decades to centuries. Their warming effect in 2020 was already approximately that of Switzerland’s total CO₂ emissions. And if those emissions continue on their upwards trajectory, their contribution to climate change will expand too. The persistence of these gases in the atmosphere must be taken seriously: all emissions are a legacy for future generations to contend with.

Finding the sources

The first step towards avoiding future emissions is to find out where the current ones are coming from. There were already some hints in previous studies, which we gathered and combined with our own information, such as on the exact timing of when emissions started accelerating.

We found that three of the five Tracking down the sources (113a, 114a and 115) can be produced during the manufacture of other chemicals, which is allowed under the Montreal protocol, most notably hydrofluorocarbons. The hydrofluorocarbons have replaced Chlorofluorocarbons for many applications as an ozone-friendly alternative. However, like Chlorofluorocarbons, they are greenhouse gases and their production is now being reduced in many countries under the 2016 Kigali Amendment to the Montreal protocol, which could reduce climate-related warming by 0.5 degree Celsius.

It’s likely that the Chlorofluorocarbons are leaking out during the production process, where they are either used as a feedstock (a chemical ingredient to make another chemical) or as a result of incomplete conversion of the feedstock to the target chemical. The production of hydrofluorocarbons really took off in developing countries after Chlorofluorocarbons were banned in 2010, which is around the same time as the increase in emissions of these five Chlorofluorocarbons.

The production of hydrofluorocarbons is predicted to further increase over the next few years, which could result in increasing emissions of these hydrofluorocarbons. The hydrofluorocarbon-113a is used to make at least one hydrofluoroolefin which are alternatives to hydrofluorocarbons that don’t heat the climate and may be used long into the future. Despite hydrofluorocarbons and hydrofluoroolefins being more benign alternatives to Chlorofluorocarbons, there may still be some cost to ozone during their production if Chlorofluorocarbons continue to leak into the atmosphere.

We were unable to find a plausible source for the other two, Chlorofluorocarbons-13 and 112a. The fact that their emissions are increasing and we don’t know why is a concern in itself.

Return to Montreal?

The Montreal protocol has been a huge success in mitigating emissions of ozone-depleting substances. Total Chlorofluorocarbon emissions are now only around 5% of their peak in the late 1980s. Yet an increase in the atmospheric abundance of some Chlorofluorocarbons is still at odds with the treaty’s goals – and their elimination, by clogging leaks in industrial processes, could present an easy win to reduce these country-sized emissions of ozone-depleting and climate-warming gases.

It will take careful consideration by countries signed up to the protocol to find the necessary controls for quashing these trend-bucking emissions. In the meantime, we will continue to use our eyes in the sky to monitor the progress of a whole host of Earth-damaging gases.

Luke Western is Research Associate in Atmospheric Science, University of Bristol. Johannes Laube is Honorary Lecturer, Centre for Ocean and Atmospheric Sciences, University of East Anglia.

This article first appeared on The Conversation.