9.2 - Reducing Ozone Depletion

Learning Objectives

I. The Montreal Protocol (Policy Solution)

• The main way to solve anthropogenic stratospheric ozone depletion is to limit the release of CFCs.

• The Montreal Protocol is a global agreement signed in 1987 (in force since 1989) to phase out CFCs and other ozone-depleting chemicals. It was strengthened several times as the science improved, and in 2009 it became the first UN treaty ratified by every country in the world (198 parties as of 2026). It's often called the most successful environmental treaty ever.

• Results: CFC production ended in developed countries in 1996 and worldwide in 2010. Chlorine in the stratosphere peaked around 2000 and is slowly falling. The 2025 Antarctic ozone hole was the fifth smallest since 1992 (NASA and NOAA). The last full UN assessment (2022) projects that ozone will return to 1980 levels around 2040 over most of the world, around 2045 over the Arctic, and around 2066 over Antarctica; the next assessment is due at the end of 2026. The hole's size still changes from year to year with stratospheric temperatures, so scientists judge recovery by the long-term trend.

II. CFC Substitutes and the Phase-Out

• Two numbers for comparing substitutes: ozone-depleting potential (ODP) measures how much ozone a chemical destroys compared with CFC-11 (ODP = 1). Global warming potential (GWP) measures how much heat it traps compared with the same mass of $\text{CO}_2$ (GWP = 1, see 9.3). The ideal substitute has an ODP of 0 and a low GWP.

III. The Kigali Amendment (Phasing Down HFCs)

• Why it was needed: HFCs solved the ozone problem but created a climate problem. That's the CED's point that some replacements are strong greenhouse gases: HFC-134a traps about 1,530 times as much heat as the same mass of $\text{CO}_2$ over 100 years, and HFC use was growing fast as air conditioning spread in hot, developing countries.

• What it does: the Kigali Amendment to the Montreal Protocol (adopted in Kigali, Rwanda, in October 2016, in force since 2019) phases HFCs down by about 80 to 85% by the late 2040s. Developed countries began cutting in 2019 and must reach an 85% cut by 2036. Most developing countries froze their HFC use in 2024, and a few very hot countries, like India, freeze in 2028.

• Who's in (as of 2026): more than 170 parties have ratified it. The US ratified it in 2022 and carries out its phasedown through the EPA under the AIM Act (2020).

• Payoff: the 2022 UN ozone assessment estimated that following the Kigali Amendment avoids about 0.3 to 0.5°C of warming by 2100.

IV. Other Substitutes: Natural Refrigerants

• Some uses skip fluorinated chemicals entirely. Most home refrigerators sold in Europe use isobutane (a hydrocarbon with a GWP of about 3), and many supermarket and industrial freezers use ammonia or $\text{CO}_2$.

• Trade-offs: hydrocarbons are flammable, ammonia is toxic, and $\text{CO}_2$ systems have to run at very high pressure, so each needs special equipment and safety rules.

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