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France and the nine planetary boundaries
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Stratospheric ozone depletion

A planetary boundary not crossed, thanks to the commitment of the international community

Ozone is a naturally occurring trace gas in the atmosphere, formed by the action of solar radiation on oxygen. Its concentration is highest in the stratosphere, the layer of the atmosphere located between 10 and 50 km above the earth's surface, with a maximum between 20 and 30 km altitude. This part of the stratosphere, with its higher concentration of ozone, is commonly referred to as the "ozone layer".

The ozone layer plays an essential role for living organisms. It acts as a protective filter against the sun's ultraviolet rays (UVB and UVC), which are particularly harmful to human health (risk of skin cancer, eye disease, weakening of the immune system) and ecosystems (slower photosynthesis, plant and crop growth, lower productivity of phytoplankton).

As a result of complex chemical reactions, the thickness of the ozone layer naturally varies according to seasonal variations in temperature and sunlight: it decreases in winter and spring in the polar regions. In the 1980s, researchers identified a noticeable thinning, or even destruction, of the ozone layer ("hole in the ozone layer"), particularly pronounced at the South Pole above Antarctica, but also at the North Pole (Arctic). This phenomenon is caused by the introduction into the atmosphere of chemical substances containing chlorinated and brominated molecules. Ozone-depleting substances (ODS) are mainly:

  • Chlorofluorocarbons (CFCs), used in refrigeration systems and air conditioners, as propellants in aerosol cans, and as solvents;
  • Hydrochlorofluorocarbons (HCFCs), used as substitutes for CFCs due to their shorter atmospheric lifetime;
  • halons, used in fire extinguishing equipment;
  • carbon tetrachloride (CTC), used to produce CFCs, for nylon synthesis, as a solvent and as a cleaning agent;
  • trichloroethane (TCA), used mainly to manufacture fluorinated polymers, which are used as insulators in the manufacture of lithium batteries;
  • methyl bromide, used to disinfect soil against pests, and to disinsectize agricultural storage facilities and industrial infrastructures.

As part of the work on planetary boundaries, researchers have chosen the "ozone concentration in the stratosphere", measured in Dobson units, as a control variable. A "Dobson Unit" ( DU) is defined as an ozone layer 0.01 mm thick under normal conditions of atmospheric temperature and pressure. The average value of the ozone column is 300 DU, so researchers have set the global limit at 275 DU, or 95% of the pre-industrial value (290 DU) - (Table 9).

Table 9: Control variable and global boundary for stratospheric ozone depletion

Control variable

Planetary boundary

Global value

Ozone concentration in the stratosphere measured in Dobson units (DU)

275 DU, or 95% of pre-industrial value (290 DU)

285 DU

Source: based on Steffen et al., 2015

Since the 2000s, thanks to strong international mobilization, the ozone layer situation has stabilized. Since then, ozone concentration in the stratosphere has been estimated at 285 DU on average, which means that the global boundary is respected. However, values of around 200 DU are still recorded locally in spring over the Antarctic.

MEASURES TAKEN TO RESTORE THE OZONE LAYER

In response to the destruction of the ozone layer, the international community adopted the Montreal Protocol in 1987, with the aim of drastically reducing the production and consumption of ozone-depleting substances (initially CFCs and halons). It was the first international treaty to be signed by all UN-recognized states. Following its implementation, CFC production and consumption were drastically reduced. Since 1987, the protocol has been amended five times, notably to take account of other ozone-depleting substances.

At European level, the Protocol was supplemented in 2009 by the Regulation31 on substances that deplete the ozone layer (Chart 16 ). While the Montreal Protocol regulates the production and trade of these substances, the 2009 regulation bans most of their uses. It also defines a system of quotas and licenses for the production, import, export and use of these substances and products containing or relying on them, as well as mandatory reporting by the companies concerned.

31 Regulation (EC) No 1005/2009 of the European Parliament and of the Council of 16 September 2009 on substances that deplete the ozone layer

Figure 16: Consumption of controlled substances that deplete the ozone layer

Notes: consumption corresponds to production plus imports, minus exports and minus destruction; gases taken into account: CFCs, halons, CTC (carbon tetrachloride), TCA (trichloroethane), HCFCs, HBFCs (hydrobromofluorocarbons), BCM (bromochloromethane), MB (methyl bromide).
Sources: European Environment Agency; United Nations Environment Programme, 2022

France ceased production of halons in 1994 and CFCs in 1995. However, in accordance with the ODS regulation, derogations are provided for certain uses: as synthesis intermediates, as manufacturing agents, or for essential laboratory and analytical uses. The rise in TCA and CTC production from 2018 onwards is explained by growing demand (Figure 17).

Figure 17: Trends in the production of ozone-depleting substances* (ODS) in France

*CFCs, halons, CTCs, TCAs, HCFCs.
Note: CFCs include CFC113, CFC114, CFC115. HCFCs include HCFC 22, HCFC 141B and HCFC 142 B. CTC refers to carbon tetrachloride and TCA to trichloroethane (methyl chloroform).
Source: Ministry of the Environment, 2022. Data reported under the Montreal Protocol on ozone-depleting substances

CFCs were replaced by HCFCs, then by hydrofluorocarbons (HFCs), which do not contain the chlorine atom responsible for stratospheric ozone depletion. Unfortunately, HFCs are powerful greenhouse gases. For example, releasing 1 kg of HFC-134 into the atmosphere will have the same impact on the climate as 1,300 kg of CO2 or a 10,000 km journey in a sedan.

The Montreal Protocol was therefore supplemented by the Kigali Amendment in 2016, which provides for the phasing out of the production and consumption of certain HFCs. European regulation no. 2037/2000 provides for the cessation of HCFC production by January 1, 2026. Regulation no. 517/2014 on fluorinated greenhouse gases requires a 79% reduction in HFC fluids between 2015 and 2030.

In France, regulations provide for a set of certification and monitoring rules for equipment dependent on fluorinated greenhouse gases (refrigerators, air-conditioning systems, etc.). The French Ministry of the Environment has also set up aid schemes to support companies wishing to use alternatives to HFCs, whose prices are set to rise as a result of the reduction in European quotas.

French HFC emissions have fallen by 40% in ten years, from an estimated 17,629 kilotonnes of CO2 equivalent per year (kt CO2 e/yr) in 2011 to 10,695 kt CO2 e/yr in 202132.

The production of CFCs, HCFCs and halons is reported by all countries to the United Nations Environment Programme (UNEP)33. Since 2012, scientists have observed an unexpected increase in trichlorofluoromethane (CFC-11) emissions in Asia, probably due to illegal production. Vigilance is therefore still called for.

The outlook for the ozone layer

Regular monitoring of the ozone layer is based on measurements taken by satellites, balloons, aircraft and ground observation stations. According to the latest scientific assessment of ozone layer depletion (2022) carried out jointly by the World Meteorological Organization (WMO) and UNEP, the ozone hole is in the process of being closed. In some parts of the stratosphere, it has been recovering at a rate of between 1.5% and 2.2% per decade at mid-latitudes in the northern and southern hemispheres, and between 1.1% and 1.6% per decade in the tropics, since the year 2000.

Restoring the ozone layer remains a very slow process, as chemicals have a long atmospheric lifetime. Based on current policies, models predict a return to 1980 levels by approximately 2066 over the Antarctic, 2045 over the Arctic and 2040 in the rest of the world.

The Montreal Protocol is thus considered a major success in terms of global environmental protection. What's more, according to the Scientific Assessment Panel34 of the Montreal Protocol, the Kigali Amendment should make it possible to avoid global warming of 0.3 to 0.5°C by 2100.

The challenge today is to find substitute products with the least possible impact, and to limit our need for air conditioning by using nature-based or less energy-intensive solutions, in order to both guarantee the integrity of the ozone layer and preserve the global climate system. This first universal treaty, the Montreal Protocol, could then serve as an example and pave the way for future climate negotiations.

32 Citepa, April 2022 - Secten format.
33 Data available at ozone.unep.org/countries.
34 Author of a four-yearly scientific assessment of ozone depletion in accordance with Article 6 of the Montreal Protocol.

Further information