The hole in the ozone layer hardly ever makes the headlines anymore, as climate change has pushed it out of the public eye. Wrongly so: This year is bigger and deeper than ever.
Every year, when eternal night falls over the South Pole, a disastrous process begins that was once considered one of mankind's greatest environmental concerns: the ozone hole begins to form. And despite all the countermeasures, such as the increasing ban on the notorious CFCs, it doesn't seem to want to close at the moment. It keeps growing even more. According to Nasa and NOAA, the American weather agency, the 2006 season broke all negative records of the past - since satellite measurements began in the 1970s, more extreme values have never been recorded.
At an altitude of 13 to 20 kilometers at the end of September, according to Paul Newman from Nasa, just 1.2 Dobsonian units (DU) of ozone were wafting through the atmosphere, where there were 125 DU in August. And over the entire column of air from the ground to the top of the stratosphere, the O3 molecules added up to 85 DU, which was also only a quarter of the August value and the previous lows from the end slightly undercut again in the 1990s. At the same time, the low-ozone area stretched over around 29.5 million square kilometers, which is slightly larger than in 2000, the previous record holder.
But why is the ozone hole expanding when the production and use of CFCs (chlorofluorocarbons) has been increasingly banned since the Montreal Convention was signed in 1987? One factor is the longevity of these chemicals, which were formerly used as propellants in aerosol cans and refrigerants in refrigeration systems: depending on the product, CFC compounds can survive in the atmosphere for up to 180 years, with each chlorine radical that is created having up to 100,000 ozone can crack molecules. On the other hand, it also takes at least a decade for the gas to rise to higher stratospheric layers. However, with very few exceptions, the industrialized countries did not completely stop production until 2000, and the developing countries are not expected to follow until 2010.
Nevertheless, harmless alternatives to CFCs were developed shortly after the agreement was passed, so that their concentration, according to NASA, reached its maximum in the troposphere in 1995 and in the stratosphere in 2001, where it exerts its ozone-damaging influence. However, the area of the ozone hole over the Antarctic will not be noticeably smaller again until 2024 at the earliest. Until then, an average annual decrease of 0.1 to 0.2 percent can be expected - larger fluctuations cannot be ruled out.
Ozone depletion is now being increasingly influenced by an ominous interaction between climate change and the formation of the ozone hole. In order for the chlorine to start reacting at all, certain climatic conditions must be met, which occur above all over the South Pole. During the wintry polar night, the temperatures in the atmosphere regularly fall below minus 80 degrees Celsius, which is why polar stratospheric clouds form, which contain chlorine from CFCs in easily cleavable compounds. Observations of the microwave radiation of various trace gases by NASA's Aura satellite show that chlorine levels are extremely high this year. At the same time, the so-called polar vortex always builds up in winter – a strong quasi-stationary wind system around Antarctica – which prevents air exchange with higher latitudes.
In the Antarctic spring, the onset of sunlight provides the energy needed to split the chlorine compounds, whose radicals go in search of the ozone molecules. Until the polar vortex weakens in November and December and more ozone-rich air flows in again, this process continues and deepens the ozone hole. However, the increased carbon dioxide levels in the atmosphere not only heat up the troposphere, but also allow the overlying stratosphere to cool down – they block heat radiation at higher altitudes. However, the colder it is there, the more clouds form and the more chlorine accumulates, which in turn drives degradation.
Using satellites and weather balloons, Newman's colleagues determined that at the end of September, temperatures in the lower stratosphere were five Kelvin below average, increasing the ozone hole by 3.1 to 3.9 million square kilometers. Because of global warming and the opposing trends in the stratosphere, the scientists therefore expect the recovery of the ozone layer to be delayed: The year 2068 instead of 2050 is now the new target.