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The ozone layer shields us and other living things from the sun’s harmful ultraviolet radiation. Ozone depletion is the gradual thinning of the ozone layer. Ozone depletion leads to the formation of an ozone hole. The depletion of the ozone layer could have serious consequences for human health and the environment. Before we talk about ozone depletion, let’s first understand what the ozone layer is.
Several layers make up the Earth’s atmosphere. The troposphere, the lowest layer, extends from the surface up to 10 km. For comparison, Mount Everest, the tallest mountain in the world, is only 9 km high. The stratosphere is the next layer. It occupies space from 10 km high to 50 km. Airplanes operate in the lower stratosphere.
The ozone layer occupies a space in the stratosphere, between 15 km and 30 km above the Earth. Ozone is a molecule. It contains three oxygen atoms. At every point in time, ozone molecules are constantly forming and getting destroyed in the stratosphere. The ozone layer absorbs the sun’s ultraviolet (UV) radiation, preventing it from reaching the Earth. Exposure to UV radiation can cause skin cancers and cataracts in humans. It can also harm the plant, animal, and marine life.
Ozone concentrations in the atmosphere have natural variations. The concentrations change with seasons, latitudes, and sunspots. Recovery has followed every natural ozone depletion. However, since 1970, the ozone layer has depleted well beyond natural variations.
The leading cause of ozone depletion is chlorofluorocarbons (CFCs). Chlorine and bromine molecules destroy ozone molecules when they come into contact with it in the stratosphere. They can destroy ozone much more quickly than nature can create new ones. Other than CFCs, substances that destroy ozone include hydrofluorocarbons (HCFCs), methyl chloroform, carbon tetrachloride, halons, and methyl bromide.
In the 1970s, refrigerators, air conditioners, fire suppressants, foam insulation, etc., contained CFCs as aerosol propellants. However, concerns about ozone depletion led to many countries banning the use of CFCs.
However, not all sources of chlorine and bromine contribute to the depletion of the ozone layer. For example, the chlorine from sea salt, volcanoes, and swimming pools does not reach the ozone layer.
The stratospheric ozone layer is getting thinner over both hemispheres. However, ozone depletion is significantly less severe over the northern hemisphere than in the southern hemisphere. Antarctica has recorded an ozone hole every year since the 1980s during September, springtime. The ozone hole is not really a hole in the ozone layer. Rather, it is an area of the stratosphere with extremely low ozone concentrations.
Scientists measure ozone depletion by Dobson Units (DU). Concentration levels of 220 DU or less represent ozone depletion. This constitutes what we call the ‘ozone hole’. The ozone hole is only apparent in the southern hemisphere. The ozone hole achieved its largest historical extent in September 2000. The hole occupied an area of 28.4 million sq. km. This area is equal to seven times that of the EU.
Since 2000, scientists have noted that the hole has shown signs of healing. This is predominantly due to the phasing out of ozone-depleting gases under the Montreal Protocol. In 1987, the United Nations Environment Programme adopted the Montreal Protocol. The Protocol led to reductions in the release of ozone-depleting substances since 1986. However, variations in temperatures also drive the extent of the ozone hole. Warmer temperatures lead to a smaller hole, such as in 2019.
But we cannot directly attribute warmer temperatures and ozone recovery to anthropogenic climate change. Greenhouse gases cause warming in the troposphere while they have a cooling effect in the stratosphere. The cooling in the stratosphere has a positive impact on ozone recovery, except in the polar regions. In the polar regions, very low temperatures can contribute to the formation of polar stratospheric clouds. These clouds facilitate ozone depletion.
Even periodic volcanic eruptions can influence the depletion of the ozone layer. Volcanic eruptions increase particle load in the stratosphere, leading to ozone depletion. This offers a partial explanation for the years we’ve recorded comparatively large ozone holes.
In 2021, the ozone hole over the southern hemisphere covered a maximum area of 24.8 million sq. km. It resembled the ozone hole from 2019. The 2021 ozone hole was one of the largest and deepest recorded recently. According to the Copernicus Atmosphere Monitoring Service, lower than average temperatures and strong winds circling Antarctica in the stratosphere contributed to the large 2021 ozone hole.
The 2019 ozone hole was a tiny and short-lived one. Scientists say that special meteorological conditions drove the 2019 ozone depletion. August and September 2019 in Antarctica recorded exceptionally high temperatures around 20-30 km above the ground. The warm temperatures stopped the formation of icy clouds. These clouds usually trap molecules that cause ozone depletion.
Scientific evidence suggests that we still need more action to mitigate the depletion of the ozone layer. Measures that governments can take to continue ozone recovery include:
1. Ensuring that authorities correctly and strictly implement restrictions on ozone-depleting substances.
2. Ensuring that we deal with sources of ozone-depleting substances in an environmentally-friendly manner and replace them with sustainable, climate-friendly alternatives.
3. Ensuring that no one illegally uses ozone-depleting substances.
4. Abiding by the Montreal Protocol’s rule to reduce the use of ozone-depleting substances in applications it does not consider consumption.
5. Ensuring that we do not develop any new technology or chemical that will threaten the ozone layer.
Reducing and phasing out CFCs and HCFCs contribute significantly to the fight against climate change. However, phasing out these substances has led businesses to use other types of gases to replace them. These gases do not harm the ozone layer, but they do have a significant warming effect. Therefore, in 2016, parties amended the Montreal Protocol to include hydrofluorocarbons (HFCs) on the list of controlled substances.
Governments must regularly come together to assess and monitor the developments of new gases harmful to the ozone layer. The list of controlled substances must be periodically reassessed to ensure that the ozone layer can fully recover.