Ozone Depletion and the Montreal Protocol

High in the atmosphere lies an invisible shield that protects all life from the Sun's most dangerous rays: the ozone layer. In the twentieth century, humans discovered they were accidentally destroying it. The story of how the world found the problem and then fixed it through the Montreal Protocol is the great success story of global environmental action. It is also a popular exam topic.

Ozone is a special form of oxygen. A concentration of it in the stratosphere (the ozone layer) performs a vital job: it absorbs most of the Sun's harmful ultraviolet (UV) radiation before it reaches the ground.

This protection is essential. Without the ozone layer, the intense UV radiation would cause skin cancer, eye damage and harm to crops and sea life. The ozone layer is, in effect, the Earth's natural sunscreen.

In the 1970s and 1980s scientists made an alarming discovery: the ozone layer was thinning, and a large "ozone hole" had appeared over Antarctica.

The cause was human-made chemicals, above all chlorofluorocarbons (CFCs). CFCs had four main uses worth remembering:

• Refrigerants: the cooling fluid in refrigerators and air conditioners.
• Propellants: the gas that pushes the spray out of aerosol cans.
• Foam production: blowing agents used in making plastic foams.
• Solvents: cleaning agents for delicate electronic components.

CFCs were not used in making tubeless tyres. Exam questions often slip that in as a false option.

When CFCs drift up to the stratosphere, they release chlorine. That chlorine destroys ozone molecules. As the ozone thinned, more harmful UV radiation could reach the Earth's surface. This threatened health and ecosystems.

CFCs are released worldwide, yet the hole forms over Antarctica. The reason lies in Antarctic weather. In winter, a ring of fierce winds called the polar vortex (bounded by the polar front) seals off the air above Antarctica. The trapped air becomes extremely cold. In that cold, polar stratospheric clouds form. The surfaces of these ice clouds host chemical reactions that convert chlorine into its most ozone-destroying forms. When sunlight returns in the Antarctic spring, this stored chlorine destroys ozone very rapidly. The result is the seasonal ozone hole over Antarctica specifically.

Industry replaced CFCs with hydrofluorocarbons (HFCs), which contain no chlorine and so do not harm the ozone layer. HFCs took over the old CFC roles. Their main uses:

• Refrigerants: cooling systems in refrigerators and air conditioners.
• Aerosol propellants: the push gas in spray products.
• Foam blowing agents: making plastic foams.
• Fire retardants: fire-suppression systems.

HFCs are not used in lubricants. But the fix created a new problem: HFCs are potent greenhouse gases. The Kigali Amendment (2016) to the Montreal Protocol therefore commits countries to phase down HFCs as well.

Faced with this danger, the world acted with rare speed and unity. In 1987, nations signed the Montreal Protocol, an international treaty to phase out CFCs and other ozone-destroying chemicals.

The Montreal Protocol worked. Almost every country joined, the harmful chemicals were replaced with safer alternatives, and the ozone layer has begun to heal, expected to recover over the coming decades. It is widely regarded as the most successful environmental treaty in history. It is a model for how global cooperation can solve a planet-wide problem. This offers an encouraging contrast to the slower progress on climate change.