From Science to Global Action on World Ozone Day
Today, on World Ozone Day, we celebrate the achievements of the world’s first universally ratified and most successful environmental treaty, under which countries have phased out 99% of controlled ozone-destroying chemicals. The Montreal Protocol’s legacy is one of responding to science and taking action to protect human health, the environment, and the ozone layer.
Today is also a day to recognize that the work of the Montreal Protocol and Vienna Convention is far from done. Science must continue to drive timely and effective action on important topics and ensure continued protection against ongoing threats related to ozone and climate.
Forty years ago, the Vienna Convention for the Protection of the Ozone Layer was established in response to scientific evidence that man-made chemicals were severely depleting the Earth’s stratospheric ozone layer – an invisible shield that filters out the Sun’s harmful ultraviolet (UV) radiation. Without this barrier, UV radiation poses serious risks to human health and causes widespread damage to plants, animals, and microbes, making the ozone layer essential to protecting all life on Earth.
In the mid-70s, scientists confirmed that man-made chemicals were the culprits behind the depletion in ozone – particularly chlorofluorocarbons (CFCs), primarily used in refrigeration, air conditioning, aerosol propellants, and insulation. There are many other ozone-depleting substances (ODS), including hydrochlorofluorocarbons (HCFCs), halons, and methyl bromide.
The false-color view of total ozone over the Antarctic pole from 1979 to 2024. The purple and blue colors show where there is the least ozone, and the yellows and reds the most ozone.
The international community came together to address this global crisis to our health and safety, leading to the adoption of the Montreal Protocol on Substances that Deplete the Ozone Layer on September 16th, 1987, which set the first legally binding targets for phasing out these ODS. The Protocol continues to expand its scope, notably with the 2016 Kigali Amendment that phases down hydrofluorocarbons (HFCs), the fluorochemical industry’s super pollutant replacement for CFCs and HCFCs.
Scientific studies provide robust evidence of a recovering ozone layer as a direct result of actions taken under the Montreal Protocol. However, science continues to show that there are ongoing threats and ozone recovery is not guaranteed.
Nitrous Oxide (N2O)
Nitrous oxide (N2O) is the most significant remaining ODS and the third most significant greenhouse gas (GHG) being emitted into the atmosphere today, 237 times more potent than CO2. Mostly unchecked and ignored in environmental policy discussions, total annual anthropogenic N2O emissions have increased 40% since 1980, undermining efforts to heal the ozone layer and meet climate emissions targets.
The 2024 United Nations Global Nitrous Oxide Assessment is clear: emissions of this super pollutant are rising faster than expected. Proactively taking ambitious action on N2O is critical to limiting global warming to 1.5°C, and avoiding the equivalent of up to 235 billion tonnes of CO2 emissions by 2100. This action would support continued ozone layer recovery, and improve air and water quality, preventing up to 20 million premature deaths globally by 2050.
While most human-caused N2O emissions originate from agriculture, about 5% are released as by-products of the industrial production of nitric acid, adipic acid, and caprolactam – chemicals used in fertilizers, textiles, and other products. Low-cost abatement strategies for these industrial sources already exist. Employed in many facilities and mandated in some regions, these technologies can reduce industrial N2O emissions to nearly zero.
Universal abatement of industrial N2O emissions is a low hanging fruit for climate mitigation given the low-cost and widely available technologies, and industry and policymakers must take the necessary steps towards putting a stop to these avoidable emissions.
Hydrofluoroolefins (HFOs) and Per- and Polyfluoroalkyl Substances (PFAS)
As laid out in EIA’s Persistent Problems report, hydrofluoroolefins (HFOs), the latest generation of fluorinated chemicals used as refrigerants, aerosols, and more, are problematic for human health and for the planet as a source of per- and polyfluoroalkyl substances (PFAS), known as ‘forever chemicals’. While HFOs have lower GWPs, this does not reflect their true climate impact, as emissions from their production and atmospheric breakdown contribute indirectly to climate change and ozone depletion. Some common HFOs degrade into HFC-23, the HFC with the longest atmospheric lifetime and the highest 100-year GWP. Many HFOs also degrade into PFAS, such as trifluoroacetic acid (TFA), a persistent ultra-short-chain PFAS. A wide body of data documents TFA contamination in water bodies, remote ice cores, drinking water, food, beverages, and even human bodies. Alarmingly, TFA concentrations in the environment are rapidly increasing.
While research continues to understand the full extent of PFAS toxicity, clear cases of harm are documented. For example, perfluorooctanoic acid (PFOA) was phased out after its serious health and environmental impacts were revealed. Recently, a group of scientists published a statement supporting a PFAS definition that includes fluorinated chemicals like HFCs, HFOs, and by-products like TFA.
The transition away from HCFCs and HFCs is of utmost importance, but does not require HFOs. Instead, HFCs can be replaced with non-fluorinated, ultra-low GWP natural alternatives or not-in-kind technologies, already widely available and in use for most applications.
Monitoring Emissions, Including HFC-23
Time and again, science aimed at monitoring concentrations of substances in the atmosphere and their emissions has proven central to successful Montreal Protocol implementation and safeguarding the health of our planet. Even today ongoing unexpected emissions continue to pose a threat to ozone and climate. It is imperative that we preserve and strengthen global scientific capacity for atmospheric and emissions monitoring.
Atmospheric monitoring was essential for making the connection between CFCs and other ODS and the depletion of the ozone layer. And in 2018, when scientists detected unexpectedly high atmospheric concentrations of banned CFC-11, EIA investigations uncovered its widespread illegal production and use in China. Recent global air measurements indicate CFC-11 emissions are again in decline, highlighting the importance of early warnings from atmospheric monitoring systems and strengthened compliance and enforcement mechanisms.
Global atmospheric monitoring also reveals a substantial increase in the atmospheric abundance of potent super pollutant HFC-23 over recent decades, reaching a record high of 17,300 tonnes per year in 2019, more than a quarter of a billion tonnes of CO2 equivalent. The majority of HFC-23 emissions are as an unwanted by-product during production of some fluorinated chemicals, like HCFC-22. Since 2020, the Kigali Amendment requires Parties to destroy HFC-23 by-product emissions. Despite a decline in emissions between 2019-2022, atmospheric data show levels remain substantially higher than expected or officially reported. A 2024 study using updated atmospheric estimates concluded that HFC-23 emissions have not been destroyed to the extent reported by Parties.
It is vital to maintain and enhance atmospheric monitoring capacity globally and in regions lacking coverage. Closing the existing gaps through a comprehensive global monitoring network will aid in addressing the range of ongoing threats to the Protocol’s success, especially the significant unreported, unaccounted for and unexpected emissions linked to fluorochemical production, including HFC-23.
Very Short-Lived Substances (VSLS)
A recent study compared ground-based measurements of chemicals regulated under the Montreal Protocol to satellite observations of stratospheric chlorine concentrations – the key driver of ozone loss. The results were surprising: stratospheric chlorine was not decreasing as quickly as expected based on ground-based measurements.
The scientists explained the slower rate of decline by including chlorine-containing substances that are not currently banned under the Protocol, called chlorinated very short-lived substances (VSLS). Although VSLS typically remain in the atmosphere for less than six months, a portion of their chlorine still reaches the stratosphere and contributes to ozone depletion. This study found that the increases in chlorinated VSLS slow the recovery of stratospheric ozone, partially offsetting the benefits of long-lived ODS reductions through the Montreal Protocol.
Given the significant threat posed by chlorinated VSLS, best practices and alternatives must be explored, so that these uncontrolled emissions can be mitigated at national and global levels.
From Science to Global Action
As we celebrate the Protocol’s achievements, and stakeholders who continue to support its implementation and legacy, we must also face the urgent need for more ambitious action to address ongoing threats.
It is crucial to strengthen controls under the Montreal Protocol to reduce emissions by strengthening monitoring, reporting, verification, and enforcement to enhance compliance and prevent illegal production and use. Persistent threats must be examined and addressed, and the global network of scientists and policymakers can support ambitious and accelerated action by filling existing knowledge and policy gaps. Continued engagement with and investment in the Montreal Protocol is essential for ensuring that the fragile ozone recovery stays on course, and protecting the myriad health, economic, and environmental benefits it provides.
And while there remains a lot of work to be done to protect the ozone, our health and the planet’s, the Montreal Protocol is a reminder of all we can accomplish through global collaboration.