OBSERVER: Antarctic ozone hole closes early, indicating signs of recovery
enora@ALSOcons…
Data from the Copernicus Atmosphere Monitoring Service (CAMS) confirmed the closure of the 2025 Antarctic ozone hole on 1 December, marking the weakest and shortest‑lived ozone hole in five years. For the second year in a row, the average extent and duration of the ozone hole were notably less compared to the very large, long‑lasting holes observed between 2020 and 2023. These recent developments indicate global progress on ozone layer recovery efforts under the Montreal Protocol and its amendments. Since it entered into force in January 1989, the Protocol has led to the phase out of 99% of ozone‑depleting substances (ODS)–synthetic compounds used in refrigeration, air conditioning, foams, and aerosol sprays which significantly accelerate the depletion of the ozone layer when they reach the stratosphere. By tracking ODS and monitoring the ozone hole, CAMS plays a critical role in the ongoing recovery efforts, turning scientific evidence into coordinated action to protect the ozone layer. In this Observer, we examine how this year’s ozone hole developed, what recent variability tells us about long-term recovery, and how new observational capabilities such as Sentinel-5 contribute to understanding these trends.
A smaller and shorter‑lived ozone hole in 2025
In 2025, the Antarctic ozone hole developed relatively early in mid‑August and reached a maximum area of around 21.08 million km² in early September, significantly below the 26.1 million km² recorded in 2023. It gradually reduced to an area between 15 and 20 million km² in September and remained fairly stable until the end of October, followed by a rapid decline in November. Indicators pointed to the possibility of a very early closure in November, but a persistent small area of low ozone delayed the closure until 1 December. Nevertheless, the 1 December closure marked the earliest since 2019.
Alongside the reduced area and duration, CAMS analyses show higher‑than‑usual minimum values of the ozone column and lower ozone mass deficit values compared with recent years.
The ozone hole is generally defined as the area where total column ozone, the integrated amount of ozone in a vertical column of air above the surface, falls below 220 Dobson Units (DU). One DU is a unit used to describe total column ozone and corresponds to a 0.01-millimetre-thick layer of pure ozone at the Earth’s surface under standard conditions. This threshold allows scientists to compare the size and severity of the ozone hole consistently from year to year.
CAMS monitoring critical to ozone surveillance
CAMS, implemented by the European Centre for Medium‑Range Weather Forecasts (ECMWF), plays a central role in monitoring the ozone layer. The Service maintains a dedicated ozone monitoring page which provides daily maps, charts, and animations showing the evolution of the Antarctic ozone hole every year. These products include forecasts and analyses of total column ozone, as well as ozone concentrations at different altitudes in the atmosphere, both globally and over the polar regions, up to five days ahead.
CAMS combines a state‑of‑the‑art numerical model of the atmosphere with a wide range of satellite observations. By assimilating these observations into the ECMWF Integrated Forecasting System (IFS), CAMS generates a consistent, high‑resolution picture of ozone and related variables. These forecasts and analyses are regularly evaluated against independent in situ observations, such as ozonesondes launched on weather balloons from several Antarctic stations, to ensure accuracy and reliability.
In 2023, CAMS further increased its capabilities by incorporating a detailed representation of stratospheric chemistry in its global modelling system based on the Belgian Assimilation System for Chemical Observations (BASCOE). Developed by the Royal Belgian Institute for Space Aeronomy, BASCOE introduces 57 additional chemical species into the ECMWF IFS, allowing CAMS to forecast key species which influence ozone depletion.
Sentinel-5: a new view of the 2025 ozone hole
The publication of the first Copernicus Sentinel‑5A images at the end of November marked a new milestone for European atmospheric monitoring. One of the images shows the 2025 ozone hole on 13 October, marking a clearly defined region with total column ozone values below 220 DU.
The Sentinel‑5 instrument, part of the Copernicus Sentinel family, was launched aboard the first EUMETSAT Meteorological Operational Second Generation (MetOp) satellite in August. Developed by a consortium of companies from 15 European countries led by Airbus in Germany, Sentinel‑5 is an Ultraviolet Visible Near‑infrared Short‑wave infrared (UVNS) spectrometer designed to strengthen Europe’s capabilities in atmospheric composition monitoring, weather forecasting, and climate services. Sentinel‑5 feeds data into the Copernicus Data Space Ecosystem, strengthening the information base available to CAMS and the Copernicus Climate Change Service (C3S).
Following a polar, sun‑synchronous orbit at an altitude of about 832 kilometres, Sentinel‑5 completes 14 to 16 orbits per day and provides daily global coverage with high spatial resolution, including detailed information over the polar region where the Antarctic ozone hole develops. It also measures a range of other trace gases including nitrogen dioxide, formaldehyde and methane, as well as aerosols. These measurements are important for monitoring air pollution, solar energy potential, and the radiative forcing of the atmosphere.
The science behind the ozone hole and why variability persists
The ozone layer is located in the stratosphere, which extends from roughly 7 to 20 kilometres altitude, depending on latitude, season, and weather, and up to around 50 kilometres. It acts as a protective shield by absorbing most of the Sun’s harmful ultraviolet radiation. In the 1970s and 1980s, scientists discovered that certain widely used synthetic compounds containing chlorine, bromine, and fluorine were depleting this layer. The effect was most dramatic over Antarctica, where severe ozone loss led to the emergence of the ozone hole, triggering urgent international action.
The onset of the Antarctic ozone hole each year is driven by a combination of meteorological conditions and chemical processes over the polar stratosphere. During the Southern Hemisphere winter, strong stratospheric winds form a stable polar vortex over Antarctica. Within this vortex, temperatures fall to extremely low values, allowing polar stratospheric clouds to form.
When sunlight returns in spring (from August onwards), chemical reactions on the surface of these clouds release highly reactive forms of chlorine and bromine from ozone‑depleting substances, which rapidly destroy ozone molecules. This triggered a substantial reduction in ozone concentrations and a marked drop in total column ozone below the 220 DU threshold. As temperatures rise and the polar vortex breaks down towards the austral summer, the summer in the Southern Hemisphere, which typically occurs between December and February, ozone‑rich air from lower latitudes mixes into the polar stratosphere and the ozone hole closes.
The amount of ozone‑depleting substances in the stratosphere is slowly declining thanks to the Montreal Protocol. At the same time, the extent and duration of the ozone hole can vary from year to year due to variability in stratospheric winds and temperatures as well as exceptional events such as major volcanic eruptions. Climate change also affects the stratosphere. As the lower atmosphere warms, the stratosphere tends to cool, which can favour conditions for ozone‑destroying chemical reactions.
This phenomenon explains why the very large and long‑lasting ozone holes from 2020 to 2023 can coexist with the more moderate seasons of 2024 and 2025 when the overall amount of ozone‑depleting substances is declining. Persistent monitoring is essential to distinguish natural variability and exceptional events from the underlying long‑term recovery.
Global progress and EU environmental policy objectives
In March 2025, a new study reported that the observed recovery of the Antarctic ozone hole is primarily attributable to the reduction in ozone‑depleting substances following the implementation of the Montreal Protocol. Based on scientific assessments, if current trends continue, ozone over Antarctica is likely to return to a level close to its 1980 values by about 2066.
For the EU, progress on ozone recovery provides important lessons for broader climate and environmental policy. It shows that early, science‑based and cooperative international agreements can reverse environmental damage and that robust observation and monitoring systems are essential both to trigger action and to verify that actions taken are effective. High-quality and reliable data delivered by CAMS, which is part of Copernicus under the EU Space Programme, together with observations from Copernicus satellite missions such as Sentinel‑5A, are crucial to continue tracking the ozone layer’s recovery and monitoring greenhouse gases and air pollutants. This sustained flow of transparent, robust information underpins EU climate and environmental objectives and helps demonstrate, in a measurable way, the impact of coordinated international policy.
To learn more about the Antarctic ozone hole development, watch this video featuring ozone hole expert Johannes Flemming.
All CAMS ozone‑related data products, including forecasts, analyses of ozone concentrations, stratospheric temperatures and ozone‑depleting substances, are freely available via the Atmosphere Data Store.
