Two European scientific campaigns have made it possible to verify the state of the ozone layer in the Northern hemisphere. Even if researchers refer to a "bite" rather than a "hole", the concentration of this protective element has fallen by more than 30% above Scandinavia and Siberia in recent years. Apart from a number of transient causes which can partially explain this decline, the extent of the problem must be attributed to global change.
The ozone layer is situated in the stratosphere
at an altitude of between 12 and 50 kilometres. It surrounds the
planet and is indispensable for life on earth as it absorbs the
harmful ultraviolet rays emitted by the sun. But for the last two
decades we have been aware of the fact that this protective shield
is deteriorating. Emissions attributable to human activities are
suspected of being to a large extent responsible for this fundamental
attack on the terrestrial environment, particularly chlorofluorocarbons
(CFCs), which were extensively used in aerosols, refrigerators and
air conditioning systems until the Montreal Protocol, signed in
1987 by the United Nations, which prohibited their use as from 1996.
Basing their theories on the fact that the stratospheric temperature has to be particularly low for chlorine to be able to react to the contact of ozone, until the beginning of the 1990s scientists thought that the influence of chlorinated products was particularly significant above the Antarctic, in the southern hemisphere, where warning signals about the "hole in the ozone layer" seemed to be most acute.
Monitoring the Arctic with the EASOE
The situation had not been so well understood in
the northern hemisphere, however. So a vast campaign to measure
the ozone in the region of direct concern to Europe was launched
in 1991 with the support of the European Commission in the framework
of the Environment Programme. The European Arctic Stratospheric
Ozone Experiment was a project which lasted five months (November
1991-March 1992) and which mobilised 250 scientists from 18 countries.
A thousand sounding balloons were dispatched into the atmosphere
and three airliners collected their data. The results were perturbing,
since they revealed that ozone quantities had never been so low
in Northern Europe.
Researchers put forward two theories as natural explanations for this discovery, however. The first concerns the eruption of the Pinatubo volcano in the Philippines, which lasted for several days in June 1991, despite the fact that the volcano had been inactive for more than 600 years. Several thousand tonnes of ash and sulphur dioxide were projected into the atmosphere. Specialists believe that stratospheric clouds of volcanic origin have widely contributed to lowering the stratospheric temperature in that area. In addition, the ozone was quite seriously depleted by sulphurous emissions.
The second theory is also based on natural causes: a particularly tenacious anticyclone which settled above the Arctic during the winter of the same year. This prevented masses of ozone-rich air from penetrating the stratosphere; as a result, the ozone was gradually depleted and could not be renewed.
SESAME and Northern Europe
Similar, but more extensive research was pursued in the winters of 93-94 and 94-95. The Second European Stratospheric Arctic and Mid-latitude Experiment campaign (SESAME), also supported by the Environment Programme, this time concentrated more on the situation in the middle latitudes. Given the interest raised by the EASOE, 300 scientists from all over the world took part in SESAME. Enormous transparent balloons filled with 100000 m3 of hydrogen were released into the Siberian and Scandinavian skies from where they collected data at an altitude of 30km. Transmitted to some 30 ground stations, the results were once again highly significant: in the winter of 1994-95, the thickness of the ozone layer fell by 30%.
"This can no doubt be largely explained by the fact that the stratosphere was much colder that year with atmospheric conditions similar to those which we find above the Antarctic," explains John Pyle, one of those responsible for SESAME. With records of -90°C in the stratosphere of the northern hemisphere, the temperature fell to levels which had not been recorded for 29 years.
The researchers also observed that the polar vortex - a complex of whirlwinds isolated from any air exchange with the outside - had moved on various occasions towards the north of Europe, from Scandinavia to Siberia. The drop in the ozone content of the vortex was approximately 0.7% every day from mid-January onwards and 50% compared with values for previous years for the whole of the winter.
The tenacity of CFCs
"We cannot get away from the fact that the ozone level has declined steeply and that this phenomenon is widespread," concludes John Pyle. "I would not, however, refer to this so much as a hole, but as a significant bite taken out of the ozone layer. What we are observing is confined to a high latitude and it is not very likely that it will provoke serious health problems in the longer term. The phenomenon nevertheless represents an unmistakable sign of a global change."
The Treaty of Montreal which now prohibits the use of CFCs does at least exist. John Pyle considers that "it is a pity that this agreement was not instituted earlier." CFCs are substances capable of surviving in the atmosphere for several years. They are likely to contribute to a thinning of the ozone layer for part of the next century.