Ozone is a molecule composed of three oxygen atoms and has the chemical formula O3. It occurs naturally and can be found at varying levels in the Earth’s atmosphere. The majority of ozone (about 90%) is located in the stratosphere, between 15 and 30 kilometers above the Earth’s surface. At ground level, ozone is present in lower concentrations and is considered a pollutant that contributes to smog in urban areas. The discovery of the ozone layer dates back to 1913 when French physicists Charles Fabry and Henri Buisson first identified its existence.

The ozone layer refers to the region of the stratosphere, located between 15 and 30 kilometers above the Earth’s surface, where there is a high concentration of ozone. Its primary function is to absorb and filter out 97 to 99 percent of the Sun’s medium-frequency ultraviolet (UV) light, ranging from approximately 200 to 315 nanometers in wavelength. Without this protective layer, the exposed life forms on Earth’s surface would be at risk of damage from this harmful UV radiation.

The distribution of the ozone layer varies globally and is typically thinner near the equator, while it is thicker near the poles. The thickness of the ozone layer is determined by the amount of ozone present in a given column over a specific region, and it changes seasonally. The variations in thickness are attributed to atmospheric circulation patterns and the intensity of solar radiation. Most of the ozone is formed over the tropical regions and is then transported towards the poles by the prevailing wind patterns in the stratosphere.

Ozone layer depletion is a gradual process of thinning of the Earth’s ozone layer in the upper atmosphere. It is mainly caused by the release of chemical compounds containing gaseous bromine or chlorine from various human activities, including industrial processes. Ozone depletion occurs when chlorine and bromine atoms react with ozone molecules in the stratosphere, resulting in their destruction. One chlorine atom can break down over 100,000 ozone molecules before it is eliminated from the stratosphere. Compounds that release chlorine or bromine in the presence of intense UV light in the stratosphere are known as ozone-depleting substances (ODS). Examples of ODS that release chlorine include chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), carbon tetrachloride, and methyl chloroform. Halons and methyl bromide are examples of ODS that release bromine.

Ozone-depleting substances (ODS) are initially emitted at the Earth’s surface and eventually transported to the stratosphere through a process that can take up to two to five years. Besides human activities, natural phenomena like large volcanic eruptions can also indirectly affect ozone levels by producing tiny particles called aerosols. These aerosols increase the effectiveness of chlorine, derived from CFCs, in destroying ozone by providing a surface on which it can react. However, the effect of volcanic eruptions on ozone levels is temporary. Severe ozone depletion results in the formation of the so-called “ozone hole,” visible in satellite images of Antarctic ozone. Although ozone losses are not as drastic in the Northern Hemisphere, significant thinning of the ozone layer is observed over the Arctic and even continental Europe.

To protect the Earth’s ozone layer, world governments agreed in the late 1980s to phase out the production and consumption of ozone-depleting substances resulting from human activities. The Montreal Protocol, established in 1987, was the first international treaty to be signed by all countries of the world and is regarded as the greatest environmental success story in the history of the United Nations. The primary objective of the Montreal Protocol is to reduce the presence of ozone-depleting substances in the atmosphere. Developed countries were required to begin phasing out chlorofluorocarbons (CFCs) in 1993 under the original Montreal Protocol agreement. They were also required to achieve a 20% reduction relative to 1986 consumption levels by 1994 and a 50% reduction by 1998.

In the late 1980s, governments across the world agreed to protect the ozone layer by phasing out ozone-depleting substances (ODS) released by human activities. The Montreal Protocol, established in 1987, was the first international treaty signed by all countries and is considered a great success story in the history of the United Nations. The original agreement required developed countries to start phasing out chlorofluorocarbons (CFCs) in 1993 and achieve a 20% reduction by 1994 and a 50% reduction by 1998. The London Amendment in 1990 mandated complete phaseout of CFCs, halons, and carbon tetrachloride by 2000 in developed countries and by 2010 in developing countries. The Copenhagen Amendment in 1992 accelerated the phaseout of ODSs and included a phaseout of hydrochlorofluorocarbons (HCFC) for developed countries, beginning in 2004. The Montreal Amendment in 1997 included the phaseout of HCFCs in developing countries, as well as the phaseout of methyl bromide in developed and developing countries in 2005 and 2015, respectively. The Beijing Amendment in 1999 tightened controls on the production and trade of HCFCs and added bromochloromethane to the list of controlled substances with a phaseout targeted for 2004. The Kigali Amendment in 2016 extended controls to phase down the production and consumption of hydrofluorocarbons (HFCs) because they are potent greenhouse gases damaging to the earth’s climate and were adopted by industries in moving away from ozone-depleting substances.

The Vienna Convention, which was adopted in 1985, is considered the precursor to the Montreal Protocol. It is often referred to as a framework convention, as it established the framework for global efforts to protect the ozone layer. However, unlike the Montreal Protocol, the Vienna Convention did not mandate that countries take specific actions to control ozone-depleting substances. Rather, the countries of the world used the provisions of the Convention to create the Montreal Protocol, which established concrete measures to achieve the goal of protecting the ozone layer.