“Scenarios for years in the distant future such as 2100 are illustrative of the scale of possible emissions due to large uncertainties related to the assumptions involved.”

Ozone treaties

1. The 1985 Vienna Convention for the Protection of the Ozone Layer and its 1987 Montreal Protocol on Substances that Deplete the Ozone Layer were established to protect the ozone layer and safeguard human health and the environment from overexposure to ultraviolet (UV) radiation due to ozone depletion. Both have achieved universal ratification, with 198 Parties.
2. The Vienna Convention calls on Parties to share scientific research, data, and observations. The Montreal Protocol controls nearly 100 manufactured ozone-depleting substances (ODSs), most of which are also potent greenhouse gases. The Kigali Amendment added 18 hydrofluorocarbons (HFCs) to the list of controlled substances. While these do not destroy ozone, they are powerful greenhouse gasses. The Amendment was adopted in 2016 and came into force on 1 January 2019.
3. Implementation of the Montreal Protocol has led to the phase-out of 99 per cent of ODSs which equates to roughly 1.8 million Ozone Depletion Potential (ODP*) tonnes, globally. The remaining 1 per cent is mainly hydrochlorofluorocarbons (HCFCs).
4. As of 9 June 2023, four amendments to the Montreal Protocol – the London, Copenhagen, Montreal and Beijing amendments – have been ratified by 197 out of 198 parties. The fifth and most recent, the Kigali Amendment, has been ratified by over 150 parties.

OZONE AND CLIMATE BENEFITS

5. Recent studies support evidence of a recovering ozone layer and the diminishing size and depth of the Antarctic hole since 2000, despite substantial interannual variability in the size, strength and longevity of the ozone hole which is driven by meteorological conditions.
6. Outside of the Antarctic region (90°N-60°S), there is limited evidence of total column ozone recovery since 1996.
7. Trends in stratospheric ozone vary with altitude. Measurements of upper stratospheric ozone at mid-latitudes in both the Northern and Southern hemispheres show increases of about 1.5-2.2% per decade and about 1-1.5% per decade in the tropics during the period 2000-2020.
8. There is evidence showing decrease of 1-2% per decade in the tropical lower stratospheric ozone from the period of 2000-2020.
9. Assuming continued compliance with the Montreal Protocol, total column ozone is expected to return to 1980 values by:
• Around 2066 in the Antarctic;
• Around 2045 in the Arctic;
• Around 2040 in the latitudinal belt (60°N-60°S).
10. Ozone protection efforts have contributed significantly to slowing climate change by avoiding an estimated 135 billion tonnes of carbon dioxide (CO₂) equivalent emissions from 1990 to 2010. This is five times larger than the annual emissions reduction target for the first commitment period (2008-2012) of the Kyoto Protocol, the predecessor to the Paris Agreement.
11. The decline of ODS emissions due to the implementation of the Montreal Protocol is estimated to avoid global warming of approximately 0.5-1 °C by mid-century compared to an extreme scenario with an uncontrolled increase in ODSs of 3-3.5% per year. Averaged over the globe (including the oceans), about 1°C avoided warming corresponds approximately to a 25% mitigation of global warming expected from all greenhouse gases.
12. According to a modelling study, without the Montreal Protocol, effective UV-B radiation could have increased by about a factor of five over the 21st century. Plants exposed to this extreme UV-B radiation would have experienced reduced photosynthesis and growth, which would have resulted in an estimated 325–690 billion tonnes less carbon held in terrestrial vegetation by the end of this century. This reduction in carbon sequestration would have resulted in an additional 115–235 parts per million of CO2 in the atmosphere, causing an additional rise of global-mean surface temperature of 0.5–1.0 °C.
13. Compliance with the 2016 Kigali Amendment to the Montreal Protocol, which requires a phase down of production and consumption of some HFCs is estimated to avoid 0.3-0.5°C of warming by 2100**. This contributes substantially to the achievability of the Paris Agreement on Climate Change to limit global warming to well below 2.0°C.
14. Combined with improvements in the energy efficiency of equipment in this sector during the transition to low-GWP alternative refrigerants could potentially double the direct climate benefits of the Kigali Amendment.
15. Assuming full compliance with the Montreal Protocol, surface solar UV radiation is expected to decrease 2-6% by 2090 at mid-latitudes with variable changes in the tropics of less than 3%.

* The ozone depletion potential of a chemical compound is the relative amount of degradation to the ozone layer it can cause, with CFC-11 as a benchmark at an ODP of 1.0.

** This estimate does not include contributions from HFC-23 emissions.

Health benefits

16.  The Montreal Protocol has prevented large increases in surface UV-B radiation, with greatest benefits at high latitudes. Modelling studies indicate that without the Montreal Protocol, at northern and southern latitudes of less than 50° the ultraviolet index (UVI), which indicates the intensity of UV radiation with respect to sunburn, would have increased by 10-20% between 1996 and 2020. This would have increased by 25% at the southern tip of South America and by more than 100% at the South Pole in springtime.

17. In the United States, full implementation of the Montreal Protocol is expected to prevent approximately 443 million cases of skin cancer, 2.3 million skin cancer deaths, and 63 million cases of cataracts for people born in the years 1890–2100, according to the US Environmental Protection Agency.

Economic benefits

18.  The Multilateral Fund for the Implementation of the Montreal Protocol has been assisting developing countries comply with their Protocol commitments. As of May 2023, contributions had reached nearly US$5.1 billion. The Fund has also received additional voluntary contributions of US$25.5 million from a group of donor countries to finance fast-track activities for the implementation of the HFC phase-down.

19. The Montreal Protocol will result in an estimated US$1.8 trillion in global health benefits (US$1.109 trillion for skin cancer alone) and almost US$460 billion in avoided damages to agriculture, fisheries, and materials from 1987 to 2060 (both cumulative estimates).

LOOKING TO THE FUTURE

20. The Montreal Protocol’s success inspired an effort to avoid HFCs, which have been used as replacements for many banned gases. The average global warming impact of 22 of the most used HFCs is about 2,500 times that of CO₂. Global CO₂-equivalent emissions of HFCs derived from observations increased by 18% from 2016 to 2020.

21. The Kigali Amendment requires a phasedown of 18 high global warming potential HFCs by more than 80 per cent (in CO₂-equivalent) from the baseline level over the next 30 years.

22. Estimates suggest that emissions avoided by 2100 could reach 420 gigatonnes of CO₂-equivalent, which is more than 10 years of present-day annual emissions of CO₂ due to human activities. The potential avoidance of up to 0.5°C of global warming by the end of the century would significantly add to global climate warming mitigation efforts.

23. Replacing HFCs also creates an opportunity to increase energy efficiency of cooling equipment by 10–50 per cent, significantly reducing energy costs to consumers and businesses and potentially doubling the climate impact of the Amendment.

24. Ozone recovery is a long-term mission. While the levels of ODSs are expected to decline throughout the world in the coming decades, the evolution of the ozone layer will be increasingly influenced by future abundances of greenhouse gases, natural events such as wildfires and volcanic eruptions, and even possibly geoengineering, that influence climate change and ozone recovery.