The ozone hole

The Copernicus Programme supports international efforts to preserve ozone layer

The 16th of September is marked by the United Nations as the International Day for the Preservation of the Ozone Layer. The aim of this occasion is to raise awareness, on the anniversary of the signing of the Montreal Protocol (signed in 1987), about the impact of human activities on the ozone layer and to highlight how the Protocol and subsequent amendments has tackled its depletion. Through continuous monitoring of the ozone layer and delivering high quality data on its current state, the Copernicus Atmosphere Monitoring Service (CAMS), one of the six services of the European Union’s Copernicus Programme, supports the Montreal Protocol and contributes to international efforts to halt ozone depletion.

Due to the particular meteorological and chemical conditions that exist in the stratosphere over the southern polar region during late winter and spring, human-made chemicals reduce ozone concentrations causing the ozone layer to become much thinner and the so-called ozone hole to form over the Antarctic. Signed in 1987, the Montreal Protocol, and its subsequent amendments, is aimed at protecting the ozone layer by phasing out the production of these chemicals. Thanks to these efforts the ozone layer is slowly recovering but, because of the long atmospheric lifetimes of ozone-depleting chemicals, it will still take at least 40 years for the ozone layer to recover in full.

What is the ozone layer? 

Ozone is a colourless and very reactive gas that can be found throughout all layers of our atmosphere. Most ozone (about 90%) is found in the stratosphere, which begins at about 10–16 kilometres above Earth’s surface, depending on latitude, and extends to an altitude of about 50 kilometres. Ozone in the stratosphere protects life on Earth from harmful ultraviolet (UV) radiation.

In contrast, the remaining approximately 10% of ozone can mostly be found in the troposphere, the lowest layer of the atmosphere, and plays different roles in the atmosphere. On the one hand, it is a surface air pollutant which can cause poor air quality and can be harmful to humans, animals and vegetation. On the other hand, the mid-troposphere it plays a key role in atmospheric chemistry as the main source of the hydroxyl radical which regulates the concentrations of pollutants and greenhouse gases. Additionally, it is considered a greenhouse gas (GHG) when found in the upper troposphere due to the absorption and re-emission of infrared radiation by ozone molecules.

The Ozone Hole

Over the last few decades, emissions of human-made chemicals such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and others, have resulted in the thinning of the ozone layer. These chemicals are often referred as ozone-depleting substances (ODSs) due to their impact on the ozone layer. This is most obvious over the Antarctic, where the chemical destruction of ozone, constrained by the strong winds of the stratospheric polar vortex, results in the ‘ozone hole’. Despite the fact that ODSs have the most significant impact on ozone in the Antarctic, and occasionally the Arctic, they have some influence on stratospheric ozone concentrations worldwide. On 1 January 1989, the Montreal Protocol on Substances that Deplete the Ozone Layer (a protocol to the Vienna Convention for the Protection of the Ozone Layer) entered into force as an international treaty designed to protect the ozone layer by phasing out the production of numerous human-made ODSs responsible for ozone depletion. As a result of this first universally ratified treaty in the history of the United Nations the ozone layer is slowly recovering. To ensure the treaty is successfully executed over this long time period, policy makers need information about the amounts of ozone and related chemical species in the stratosphere.

The ozone hole in the last years 

What exactly do we mean by the term ‘ozone hole’? The ozone hole is an area in the polar regions in which total column ozone values have been reduced to less than 220 Dobson Units (DU). Total column ozone corresponds to the total amount of ozone in a one-square-metre column of air extending from Earth's surface to the top of the atmosphere. A DU corresponds to the number of molecules of ozone in a layer of pure ozone 0.01 millimeters thick at 0°C and a pressure of 1013.25 Hectopascal (hPa) over Earth’s surface. The average total column ozone is about 300 DU.

CAMS scientists closely monitor the evolution and development of the Antarctic ozone hole between August and December each year. The ozone hole typically grows rapidly in area from the second half of August to its maximum extent between the middle and end of September. Historical data from CAMS shows that the average maximum spatial extent is 20 million square kilometres in the range of 15 and 22 million square kilometres. From the beginning of October through to the end of November the size of the ozone hole reduces until the breakup of the polar vortex in early austral summer. However, the reduction of the ozone hole each year depends on the meteorological conditions. The Antarctic polar vortex of 2020, 2021 and 2022 were especially strong which has resulted in the ozone hole persisting with a large area through to the end of October and into November. This is in contrast to one of the smallest ozone holes since the mid-1980s in 20191 when a sudden stratospheric warming in mid-September, in which the temperature in the upper stratosphere increased to 40 degrees above normal, caused the polar vortex to weaken and split into two before dissipating.

The Ozone Hole

Why the Antarctic? 

The large-scale dynamics of the atmosphere, for example the Brewer-Dobson circulation, distribute ODSs throughout the stratosphere, so why does the hole form over the Antarctic most of the time? There are several reasons for this. Firstly, during the polar night, temperatures in the Antarctic stratosphere fall to values below -78°C, and the difference between the cold pole and warmer extra-tropics allows the so-called stratospheric polar vortex to form. The vortex itself consists of strong winds circulating high in the atmosphere around the Southern Ocean and Antarctica, isolating cold air over the South Pole and allowing ozone-depleting chemicals to accumulate. The low winter temperatures in the Antarctic stratosphere also allow polar stratospheric clouds (PSCs) to form. As the sun rises in the spring, these clouds are conducive to chemical reactions, including chlorine and bromine, that destroy ozone molecules in the region.

This is in contrast to the Arctic where mountainous landmasses at high latitudes around the Northern Hemisphere perturb the dynamics of the atmosphere more than in the Southern Hemisphere, with the result that the Arctic polar vortex is weaker and warmer than its southern counterpart. This doesn’t mean that Arctic PSCs cannot form. In the first months of 2020, for example, the Arctic polar vortex was exceptionally strong and long-lived, and temperatures in the Arctic stratosphere were low enough for long enough to allow the formation of PSCs, resulting in significant ozone losses over the Arctic.

How Copernicus monitors the ozone hole

Copernicus is a European Union programme that provides environmental observations with the Sentinel satellites and information services based on Earth Observation and in situ (non-space) data. Six Copernicus data services utilise Earth Observation data to create value-added information by processing and analysing the data and transforming them into freely available open access products and datasets such as air pollution forecasts, climate reports, early warmings, and maps for emergency management, among other products.

“Copernicus provides environmental observations with the Sentinel satellites and in situ (non-space) data”

In conjunction with the work of partners, Copernicus transform data into action: support the development of clean energy infrastructure projects, climate applications and other environmental and climate monitoring efforts, that have relevant impacts across different sectors all across the world. One of these services is the Copernicus Atmosphere Monitoring Service (CAMS), whose mission is to monitor and forecast the state of the environment in the atmosphere.

The Ozone Hole

CAMS provides consistent and quality-controlled information related to air pollution and health, solar energy, greenhouse gases and climate forcing, everywhere in the world. CAMS builds on many years of European research and development, and on existing European and national capacities, experience and know-how. The current portfolio of mature operational products was designed in close consultation with the users and developed through a series of EU-funded precursor projects starting in 2005.

It is operated by the European Centre for Medium-Range Weather Forecasts (ECMWF) on behalf of the European Commission. ECMWF is an independent intergovernmental organisation supported by 35 states. It is both a research institute and a 24/7 operational service, producing and disseminating numerical weather predictions to its member states.

To provide and further develop the CAMS, ECMWF works with many service providers around Europe. By doing so, CAMS combines the expertise and infrastructure that exist in Europe to provide a range of services that are unequalled by any other organisation in the world.

At the core of the CAMS global data products are operational 5-day forecasts of atmospheric composition, including ozone, which are produced twice a day at 00 hr and 12 hr UTC. The forecasts are initialised from analyses which combine all available satellite observations of ozone in the atmosphere with the CAMS model system to provide an accurate representation of the global atmosphere.

The CAMS system uses the ECMWF Integrated Forecast System (IFS), which is a state-of the art global weather forecasting model, including the sources, sinks and chemical reactions, to represent atmospheric composition. Furthermore, it provides a reanalysis of global atmospheric composition which is a self-consistent and quality assured historical dataset based on satellite observations covering a two decade period starting in 2003. When used in conjunction with the ERA-5 meteorological reanalysis, which also includes total column ozone data, from the Copernicus Climate Change Service (C3S), the long-term dataset is further extended back in time to cover the period of 1979 to the present.

“when assessing the ozone layer, CAMS provides three-dimensional data, rather than just ‘total column’ ozone analyses and forecasts”

Quality assurance of the CAMS ozone data is provided by routine comparisons against in situ measurements made by ozonesondes launched on balloons. Ozonesondes provide an independent dataset (i.e. not used in the CAMS system) which are crucial for assessing and validating the quality of the CAMS data products.

The Ozone Hole

When assessing the ozone layer, CAMS provides three-dimensional data, rather than just ‘total column’ ozone analyses and forecasts. This model is based on a combination of satellite and in-situ observations with numerical modelling to provide reliable data that makes it possible to accurately monitor the creation, development and closure of the ozone hole.

Monitoring UV radiation

CAMS data turned into action

As the ozone hole can reach populated regions in the Southern Hemisphere, bringing with it the risk of exposure to unhealthy levels of ultraviolet (UV) radiation, it is important to track its formation and monitor related radiation. This is one area where CAMS observations are crucial.

CAMS monitors and forecasts the UV Index, a measure of the amount of UV radiation reaching the surface of the Earth taking into account the effect of ozone, clouds and aerosol particles in the atmosphere, on a daily basis. CAMS data cover past trends, the current situation, and forecasts of ozone concentrations for several days in advance, enabling authorities and the public to make informed decisions.

The scale of the UV Index goes from zero up to 11 with higher numbers signifying an increasing risk of exposure, potential damage to the skin and eyes, and less time for damage to occur. More information can be found on the World Health Organization website.

While UV from the sun is the best natural source of Vitamin D, it is also one of the major causes of skin cancer. Globally, it is estimated that over 1.5 million cases of skin cancer (melanoma and non-melanoma combined) were diagnosed in 2020. Additionally, the sun’s UV rays can cause DNA damage and certain eye diseases, such as cataracts. Nonetheless, the UV Index is often not well understood or is often misinterpreted. To address the high incidence of skin cancer, the Cancer Council Victoria in Australia established the SunSmart programme in the 1980s. Since its inception, the SunSmart campaign has encouraged people to be "sun smart" and "slip (on a shirt), slop (on sunscreen), slap (on a hat)" to mitigate their UV exposure risk.

The SunSmart app and widget were first developed in 2010 with the aim of translating the UV index into an easy, useful tool that would help Australians make informed decisions about sun protection. SunSmart has made it easy for users to maintain safe levels of UV exposure by checking the ‘UV index’ at their location at any time and uses this data to generate UV index and sun protection times. Sun protection times are the times of the day when UV is forecast to reach 3 or higher. These are used alongside the forecast UV levels, as research has shown that details regarding time are the most helpful for users in making decisions about their sun protection.

The Ozone Hole

What’s next for the ozone layer? 

What started as cause for alarm in the late 20th century has developed into predictions that the ozone layer will completely recover between 2060 and 2080 due to the implementation of the Montreal Protocol and further amendments supporting its recovery. The fact that the Antarctic ozone hole is expected to gradually close, with springtime total column ozone returning to 1980 values sometime in the 2060s, is down to the continuance of this international effort. This is a great example of how damage can be repaired with the correct intervention.

The long-term monitoring service that CAMS is providing is one of the best tools at our disposal, not only to realistically assess the progress of the efforts in protecting the ozone layer, but also in helping to mitigate the negative effects that the depletion of the ozone layer has in our lives.

*CAMS is implemented by the European Centre for Medium-Range Weather Forecasts (ECMWF) on behalf of the European Commission


1 cf. Rao et al., 2020 or Saffieddine et al.,2020.