The ozone in the stratosphere is essential to life on Earth

90% of the ozone (O3) in the atmosphere is in its upper layers (10 to 50 km above the surface), called the stratosphere. The ozone in this layer is essential to life on Earth as it absorbs a large proportion of ultra-violet (UV) radiation from the sun. How much radiation is absorbed depends on the wavelengths. The ozone in the upper atmosphere filters out 100% of UVc radiation, the most powerful, 90% of UVb and 50% of UVa.

It therefore protects human health from the harmful effects of UV radiation, such as cataracts, skin cancers and genetic mutations, and also protects the biosphere (plants and animals). Therefore, without ozone, life on Earth could not exist.

Figure 1 : absorption of UV radiation by stratospheric ozone according to wavelengths. Source [NASA 2012]



Origin of ozone in the upper atmosphere

Ozone concentrations are measured in Dobson units. These are used to measure the height of the integrated ozone column. One Dobson unit represents 0.01 mm of ozone depth. The average value for the integrated ozone column is about 300 Dobson units or 3 mm. Figure 2 shows how ozone concentrations vary with altitude. Maximum ozone concentrations are found at an altitude of 32 km.

Figure 2 : Ozone concentrations depending on altitude

Source [NASA 2012]


The ozone present in the atmosphere forms when molecular oxygen (O2) is destroyed by very short wavelength UV radiation (< 242 nm), leaving an oxygen atom (O), which in turn reacts with molecular oxygen to form ozone. Ozone is destroyed by reactions that involve atomic oxygen (in a cycle known as the Chapman mechanism), but also numerous other compounds that are naturally present in the atmosphere: OH and HO2 radicals, nitrogen oxides (NO and NO2) and halogenated radicals. These radicals are formed by photolysis and oxidation of compounds such as N2O, H2O, CH4 and a range of other halogenated compounds, such as bromine and chlorine (for example, oceans release emissions of methylene chloride and methylene bromide) [WMO – 2011a].


Physical and chemical phenomena involved in ozone depletion

In 1975, scientists demonstrated that human activities could disrupt the existing balance of chemicals forming and destroying stratospheric ozone [WMO, 1975] and that highly stable compounds such as chlorofluorocarbons (CFCs) released into the atmosphere were depleting the stratospheric ozone layer. This became evident in the early 1980s when an ozone "hole" was observed above the Antarctic at the end of the southern winter. Towards the end of the southern winter, when the sun reappears (September-October), ozone concentrations drop by 40 to 60%. The depth of the ozone layer then shrinks to 100 instead of 300 Dobson units. The maximum deficit occurs at an altitude of 20 to 25 km.

A high correlation has been demonstrated between the ozone deficit and concentrations of chlorinated radicals. Their presence in the atmosphere results from the natural release of methylene chloride from the oceans and especially from emissions of, CFCs, hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) released by human activities. CFCs are highly stable molecules with a long-term lifetime that are not chemically destroyed in the lower atmosphere, and are therefore transported into the stratosphere. In the stratosphere, their chlorine is released and disrupts the natural balance of ozone in these upper layers. These substances are recognised as ozone-depleting substances (ODS).

Measurements with both ground and satellite instruments have shown that the drop in ozone concentrations in the upper atmosphere is more pronounced at higher latitudes and in the Southern hemisphere. The annual drop in ozone concentrations is more pronounced at the South Pole than the North Pole because the prevailing meteorological conditions are different. At the South Pole, a vortex appears during the winter (with winds eddying at more than 300 km/hour). The Antarctic atmosphere is thus isolated from the rest of the Southern Hemisphere. Consequently, temperatures in this zone are in the range of -80 to -100 °C. This intense cold causes water vapour to condense and form clouds in the polar stratosphere. These clouds contain tiny ice crystals that fix chlorine as HCl and Cl2O2, thus forming a reservoir of these substances. As soon as the sun reappears, UV radiation releases the chlorine radicals, which quickly react with the ozone. At the time when the ozone layer is thinnest, the average value for the integrated ozone column is about 100 Dobson units or 1 mm.

In the Arctic (North Pole), meteorological conditions are much more variable from one year to the next and temperatures are never as low as in the Antarctic. This is why ozone depletion is virtually nil in some Arctic winters, while in others, persistent low temperatures in the stratosphere after the end of the polar night can sometimes cause severe ozone depletion.


Current status of ozone depletion

Figure 3 below shows ozone concentrations above the Antarctic measured at different times during 2011 and 2012 [NASA 2012].

Figure 3 : Changes in average ozone concentrations in the months of September 2001, October 2011 and May 2012, in Dobson units

Source [NASA 2012]

IV.1.2 Figure 3 Ozone EN

According to the World Meteorological Organisation (WMO), ozone destruction above the Arctic reached record levels in the spring of 2011 because of the persistence in the atmosphere of ozone-depleting substances and a very cold winter in the stratosphere [WMO, 2011b].

Observations from the ground, from weather balloons above the Arctic and by satellite show that the ozone column lost some 40% in depth in this region between the beginning of the winter and the end of March 2011. The previous record, in 2005, was a loss of about 30% over the entire winter.

The rate of stratospheric ozone depletion slowed considerably after the implementation of the Montreal Protocol (adopted in 1987, see below), banning the use of CFCs and replacement substances also recognised as ozone-depleting pollutants. Nevertheless, a return to the ozone concentrations of the 1970s is unlikely until halfway through the 21st century.

Today, scientists are reporting a slow decline in chlorine concentrations. The decline is slow because CFC concentrations are also declining more slowly than expected and the quantities of HCFCs that were used to replace them are still rising (despite bans on production and use, concentrations are still rising in the atmosphere because of the long lifetime of these molecules).

According to the assessment report issued in 2011 by the United Nations Environment Programme (UNEP) and the World Meteorological Organisation (WMO), the average stratospheric ozone deficit in 2008-2011 was as follows compared to the 1964-1980 period [WMO – 2014]:

90°S-90°N : -3.5%

60°S-60°N : -2.0%

35°S-60°S : -6.0%. A deficit of this order has been observed since 1996.

35°N-60°N : -3.5%. A minimum of -5.5% was observed in 1990.

20°S-20°N: almost stable.

The deficit differs with latitude and altitude. Since 1996, scientists have reported that the decrease in ozone concentrations has stopped, but it will take a long time before they are back to their 1970s levels. In the Antarctic, concentrations of ozone-depleting substances have been constant since 1996. The ozone deficit in October is stable at around 40% less than in 1980.

The table below shows trends in stratospheric O3 concentrations in more detail.

Table 1: Trends in stratospheric O3 concentrations [WMO – 2014]


The ozone in the stratosphere is also affected by climate change, which is tending to cool the stratosphere because the increase in CO2 concentrations is reducing infrared radiation back into space and may be increasing radiative cooling of the stratosphere. From 1980 to around 1995, average temperatures dropped by 1 to 2 K (degrees Kelvin) in the lower stratosphere and by 4 to 6 K in the upper stratosphere. There has been no significant change since then. The cooling trend is more pronounced over the Antarctic. These cooler temperatures have an effect on ozone depletion, which is intensifying in the lower stratosphere but less so in the upper stratosphere. At the same time, global warming is accelerating ozone formation in the troposphere. These changes in stratospheric temperatures are affecting the climate in the troposphere.


Effects of lower stratospheric ozone concentrations

These lower stratospheric ozone concentrations have climatic and biological repercussions (e.g. inhibition of photosynthesis in plants and an increase in skin cancers, cataracts and genetic mutations).

Recent satellite measurements show that annual exposure to UV radiation is rising. UV radiation is increasing in the middle latitudes but less so around the equator. In the Antarctic, UV radiation in the spring increased by 85% in the 1990-2006 period compared to the 1963-1980 period. In the middle latitudes, UV levels have remained constant since the 1990s [WMO – 2011a].


Action to limit ozone depletion

To address these phenomena, international agreements have been adopted and implemented to reduce the consumption and production of CFCs, halons and trichloroethane (T111), but also HCFCs, the first generation of CFC replacement gases.

The Vienna Convention on protection of the ozone layer was adopted on 22 March 1985 and came into force on 22 September 1988. It has been ratified by 197 countries (as of 1st June 2016).

In pursuance of the Vienna Convention, the Montreal Protocol on ozone-depleting substances was adopted on 16 September 1987. In 2012, the Protocol had been ratified by 197 Parties to the Convention. It regulates the use and production of certain halogenated compounds (CFC-11, CFC-12, CFC-113, CFC-114 and CFC-115), and several halons (1211, 1301, 2402). The Protocol places the Parties under a binding obligation to make their decisions on the basis of up-to-date scientific, environmental, technical and economic information, which must be assessed by expert panels of competent people working in an expert capacity in organisations across the world.

To support the decision-making process, advances in knowledge in these fields were assessed in 1989, 1991, 1994, 1998, 2002, 2006 and 2010 (this summary is based on the last of these reports). The information from these assessments has supported negotiations between the Parties that have resulted in successive amendments and changes to the 1987 Protocol. The four amendments to date (London amendment in 1990, Copenhagen in 1992, Montreal in 1997 and Beijing in 1999) include the regulation of HCFCs.

The crucial issue of the regulation of HFC gases (which are the second-generation CFC substitutes following the first-generation HCFCs) has been managed by the 28th Meeting of the Parties linked to the ozone-depleting substances [Kigali, Rwanda]. The 197 Parties reached on the 15th of October 2016, a compromise on an amendment of the Protocol. This involves the integration of HFCs as “regulated substances” for their productions and consumptions. Until now, HFCs were regulated for their emissions under the Kyoto Protocol (1st and 2nd periods). It is important to precise that HFCs do not act as ozone depleting substance.

This amendment will enter into force on the 1st of January 2019, if 20 Parties have ratified it. Otherwise, it will enter into force 90 days after the ratification by 20 Parties.


Will ozone concentrations return to normal?

Halogenated compounds are very stable and have unfortunately accumulated in the stratosphere in the last 50 years. They will continue to have an impact on ozone for many decades to come, even after their production has completely stopped.

According to the WMO-UNEP assessment report [WMO – 2011a and 2014], ozone concentrations could return to normal levels as follows:

    • outside the polar regions, ozone concentrations could return to their pre-1980 levels by about 2030-2040,
    • the ozone hole will continue to form every spring above the Antarctic until about 2045-2060,
    • above the Arctic, the situation will probably return to normal 10 to 20 years earlier.