ESS 6.4.2 Ozone Depletion

Learning Objectives

Describe the evidence of ozone depletion
Outline the causes of ozone depletion and mechanism of ozone depletion
Evaluate Montreal Protocol

Evidence and Causes of Ozone Depletion

Ozone-Depleting Substances (ODSs)

Substance Type	Contains Chlorine?	Contains Hydrogen?	Ozone-Depleting Substance (ODS)?	Greenhouse Gas (GHG)?	Environmental Impact / Notes
CFCs (Chlorofluorocarbons) – phased out	✅ Yes	❌ No	✅ Yes	✅ Yes	Strongly deplete the ozone layer; highly stable in the atmosphere; banned under the Montreal Protocol.
HCFCs (Hydrochlorofluorocarbons) – phased out	✅ Yes	✅ Yes	✅ Yes (less than CFCs)	✅ Yes	Transitional substitutes for CFCs; still deplete ozone but to a lesser extent; being phased out globally.
HFCs (Hydrofluorocarbons)	❌ No	✅ Yes	❌ No	✅ Yes	Do not harm the ozone layer, but are potent greenhouse gases that contribute to climate change; regulated under the Kigali Amendment.

The Action of ODSs

All ozone-depleting substances (ODSs) are human-made.
When chlorofluorocarbons (CFCs)—also known as freons—were first developed in the 1930s, they appeared to be ideal for industrial use due to their chemical stability and non-reactivity at ground level.

They were widely used as:

Propellants in aerosols
Blowing agents for gas-blown plastics (foams)
Pesticides
Flame retardants
Refrigerants, replacing earlier toxic and flammable chemicals

CFCs remain stable at ground level, but in the stratosphere, exposure to ultraviolet (UV) radiation breaks them down, releasing chlorine atoms.
These chlorine atoms:

React with ozone molecules, destroying them
React with oxygen atoms, preventing new ozone from forming

In both reactions, chlorine atoms are regenerated, enabling a chain reaction that allows a single chlorine atom to destroy many ozone molecules—a process with positive feedback that accelerates ozone loss.

Replacing CFCs in aerosols and foam production proved straightforward, but finding safe refrigerant alternatives has been more challenging. Earlier refrigerants were toxic or flammable, making CFCs appear safer at first.

Hydrochlorofluorocarbons (HCFCs) emerged as the most suitable replacements. They are non-toxic, non-flammable, and effective refrigerants, but still damage the ozone layer and act as greenhouse gases (GHGs).
Some HCFCs have a global warming potential (GWP) up to 2,000 times higher than CO₂, although their shorter atmospheric lifetime makes them less harmful than CFCs.

Because CFCs persist in the atmosphere for up to 100 years, it will take decades before bans translate into a significantly thicker ozone layer.

Actions on Ozone Depletions

Strategies for Reducing Ozone Depletion

Strategy for reducing pollution	Example of action
Altering the human activity producing pollution	Replace gas-blown plastics
	Replace CFCs and HCFCs with carbon dioxide, propane, or air as a propellant
	Replace aerosols with pump-action sprays
	Replace methyl bromide pesticides (but most gases that can be used to replace CFCs are GHGs)
Regulating and reducing the pollutants at the point of emission	Recover and recycle ODSs from refrigerators and AC units
	Legislate to have fridges returned to the manufacturer and coolants removed and stored
	Capture ODSs from scrap car AC units
Clean up and restoration	Add ozone to or remove chlorine from the stratosphere — not practical, but it was once suggested that ozone-filled balloons be released

Individual Action

The removal of CFCs from aerosols shows how public awareness can drive environmental change.
When the link between CFCs and ozone depletion became known, public campaigns encouraged consumers to avoid products containing CFCs.
This led to a major shift:

Consumers stopped buying CFC-based aerosols
Manufacturers adapted, producing CFC-free alternatives
In 1975, Johnson & Johnson became the first company to ban CFCs globally

This demonstrates how individual and corporate responsibility can influence environmental outcomes.

The Montreal Protocol (1987)

The United Nations Environment Programme (UNEP) coordinates global environmental protection efforts. It:

Negotiates international treaties (e.g., the Montreal Protocol)
Monitors and evaluates treaty implementation
Provides information to governments and the public

The Montreal Protocol is an international treaty regulating the production, trade, and use of CFCs and other ODSs.
All controlled ODSs contain chlorine or bromine, while compounds containing only fluorine (like HFCs) are not ODSs.

It is regarded as one of the most successful international environmental agreements ever established.
The discovery of the ozone hole triggered a swift global response. Even before legal bans, public pressure led to a voluntary industry phase-out of CFC-containing aerosols.

By 1987:

197 countries signed the Protocol
They agreed to freeze ODS production and consumption at 1986 levels by 1990, and to significantly reduce both by 2000
The Protocol was later strengthened through seven amendments
High-Income Countries (HICs) and Low-Income Countries (LICs) were given different phase-out schedules, giving LICs more time to comply

Timeline of CFC Reduction

Year / Period	Event
1970s	CFCs identified as ozone-depleting. USA and Sweden ban them in aerosols (1974).
1985	British Antarctic Survey reports the ozone hole.
1987	Montreal Protocol signed; over 30 countries agree to halve CFC emissions by 2000.
2006	NASA and NOAA record the largest ozone hole ever observed.
2007	Agreement reached to phase out HCFCs by 2030.
2012	25th anniversary of the Montreal Protocol.
2019	Kigali Amendment ratified to phase down HFC production by the late 2040s.

Significance and Success of the Montreal Protocol

Without the ban on ODSs, by 2050 the Earth would likely have faced global ozone hole conditions, rendering large parts of the planet uninhabitable.

Three key factors led to the Protocol’s rapid success:

The direct threat to human health made it a personal issue.
Satellite imagery vividly illustrated the ozone hole.
Practical substitutes for ODSs were available.

The Montreal Protocol stands as:

The most successful example of international environmental cooperation
A demonstration of the precautionary principle in science-based policymaking
A model of multidisciplinary collaboration
A treaty designed to account for economic differences among countries
The first environmental protocol with comprehensive monitoring and enforcement mechanisms

Current Challenges

Despite its success, challenges remain:

Due to the long atmospheric lifetime of CFCs, chlorine levels in the stratosphere peaked around 2005 and are not expected to return to pre-ODS levels before 2050.
Some LICs are still permitted to produce limited amounts of HFCs, and there is an illegal trade in ODS chemicals.
The ozone hole continues to form each year over Antarctica, closing during summer.
Large volcanic eruptions can cause temporary ozone loss.
Nitrous oxide (N₂O)—a powerful GHG and ODS from fertilizers—is not regulated under the Montreal Protocol and its emissions are rising.

Recent data (2020–2022) show that the ozone hole hasn’t significantly decreased, possibly due to:

Unexpected emissions of CFC-11
Stratospheric cooling caused by global warming, reducing air mixing
The Hunga Tonga–Hunga Ha’apai volcanic eruption (2022) injecting materials into the stratosphere

Looking Ahead

Continued monitoring of ozone levels and ODS production remains vital.
The Montreal Protocol demonstrates that global cooperation can address large-scale environmental problems, but climate change poses a greater challenge:

ODSs were limited to a few replaceable products.
Fossil fuels, which drive climate change, underpin the entire global economy—making the transition far more complex, though not impossible.