Ozonloch schließt sich: Spielen natürliche Faktoren eine größere Rolle als gedacht?

Das Ozonloch gehört nicht unbedingt zum klassischen Programm unseres Blogs. Zuletzt haben wir im September 2017 darüber berichtet: “Ozonloch über dem Südpol im September 2017 unerwartet stark geschrumpft“. Im November 2017 hatte dann die NASA weitere gute Nachrichten. Im Jahr 2017 schrumpfte das Loch auf ein Rekordminimum, das es so seit 1988 nicht gegeben hat. Grund: Ein unstabiler und wärmerer antarktischer Vortex:

Warm Air Helped Make 2017 Ozone Hole Smallest Since 1988

Measurements from satellites this year showed the hole in Earth’s ozone layer that forms over Antarctica each September was the smallest observed since 1988, scientists from NASA and NOAA announced today.

According to NASA, the ozone hole reached its peak extent on Sept. 11 (2017), covering an area about two and a half times the size of the United States — 7.6 million square miles in extent — and then declined through the remainder of September and into October. NOAA ground- and balloon-based measurements also showed the least amount of ozone depletion above the continent during the peak of the ozone depletion cycle since 1988. NOAA and NASA collaborate to monitor the growth and recovery of the ozone hole every year. “The Antarctic ozone hole was exceptionally weak this year,” said Paul A. Newman, chief scientist for Earth Sciences at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “This is what we would expect to see given the weather conditions in the Antarctic stratosphere.”

The smaller ozone hole in 2017 was strongly influenced by an unstable and warmer Antarctic vortex — the stratospheric low pressure system that rotates clockwise in the atmosphere above Antarctica. This helped minimize polar stratospheric cloud formation in the lower stratosphere. The formation and persistence of these clouds are important first steps leading to the chlorine- and bromine-catalyzed reactions that destroy ozone, scientists said. These Antarctic conditions resemble those found in the Arctic, where ozone depletion is much less severe. In 2016, warmer stratospheric temperatures also constrained the growth of the ozone hole. Last year, the ozone hole reached a maximum 8.9 million square miles, 2 million square miles less than in 2015. The average area of these daily ozone hole maximums observed since 1991 has been roughly 10 million square miles.

Although warmer-than-average stratospheric weather conditions have reduced ozone depletion during the past two years, the current ozone hole area is still large because levels of ozone-depleting substances like chlorine and bromine remain high enough to produce significant ozone loss. Scientists said the smaller ozone hole extent in 2016 and 2017 is due to natural variability and not a signal of rapid healing.

First detected in 1985, the Antarctic ozone hole forms during the Southern Hemisphere’s late winter as the returning sun’s rays catalyze reactions involving human-made, chemically active forms of chlorine and bromine. These reactions destroy ozone molecules. Thirty years ago, the international community signed the Montreal Protocol on Substances that Deplete the Ozone Layer and began regulating ozone-depleting compounds. The ozone hole over Antarctica is expected to gradually become less severe as chlorofluorocarbons — chlorine-containing synthetic compounds once frequently used as refrigerants — continue to decline. Scientists expect the Antarctic ozone hole to recover back to 1980 levels around 2070.

Ozone is a molecule composed of three oxygen atoms that occurs naturally in small amounts. In the stratosphere, roughly 7 to 25 miles above Earth’s surface, the ozone layer acts like sunscreen, shielding the planet from potentially harmful ultraviolet radiation that can cause skin cancer and cataracts, suppress immune systems and also damage plants. Closer to the ground, ozone can also be created by photochemical reactions between the sun and pollution from vehicle emissions and other sources, forming harmful smog. Although warmer-than-average stratospheric weather conditions have reduced ozone depletion during the past two years, the current ozone hole area is still large compared to the 1980s, when the depletion of the ozone layer above Antarctica was first detected. This is because levels of ozone-depleting substances like chlorine and bromine remain high enough to produce significant ozone loss.

NASA and NOAA monitor the ozone hole via three complementary instrumental methods. Satellites, like NASA’s Aura satellite and NASA-NOAA Suomi National Polar-orbiting Partnership satellite measure ozone from space. The Aura satellite’s Microwave Limb Sounder also measures certain chlorine-containing gases, providing estimates of total chlorine levels. NOAA scientists monitor the thickness of the ozone layer and its vertical distribution above the South Pole station by regularly releasing weather balloons carrying ozone-measuring “sondes” up to 21 miles in altitude, and with a ground-based instrument called a Dobson spectrophotometer.

The Dobson spectrophotometer measures the total amount of ozone in a column extending from Earth’s surface to the edge of space in Dobson Units, defined as the number of ozone molecules that would be required to create a layer of pure ozone 0.01 millimeters thick at a temperature of 32 degrees Fahrenheit at an atmospheric pressure equivalent to Earth’s surface. This year, the ozone concentration reached a minimum over the South Pole of 136 Dobson Units on September 25 — the highest minimum seen since 1988. During the 1960s, before the Antarctic ozone hole occurred, average ozone concentrations above the South Pole ranged from 250 to 350 Dobson units. Earth’s ozone layer averages 300 to 500 Dobson units, which is equivalent to about 3 millimeters, or about the same as two pennies stacked one on top of the other. “In the past, we’ve always seen ozone at some stratospheric altitudes go to zero by the end of September,” said Bryan Johnson, NOAA atmospheric chemist. “This year our balloon measurements showed the ozone loss rate stalled by the middle of September and ozone levels never reached zero.”

Siehe auch Beitrag auf The Daily Caller.

So viel natürliche Variabilität war der NASA dann aber offenbar ungeheuer. Wenige Monate später, im Januar 2018 legte die NASA daher noch schnell eine andere Pressemitteilung nach, die die FCKWs wieder in den Vordergrund rückte:

NASA Study: First Direct Proof of Ozone Hole Recovery Due to Chemicals Ban

For the first time, scientists have shown through direct satellite observations of the ozone hole that levels of ozone-destroying chlorine are declining, resulting in less ozone depletion.

Measurements show that the decline in chlorine, resulting from an international ban on chlorine-containing manmade chemicals called chlorofluorocarbons (CFCs), has resulted in about 20 percent less ozone depletion during the Antarctic winter than there was in 2005 — the first year that measurements of chlorine and ozone during the Antarctic winter were made by NASA’s Aura satellite. “We see very clearly that chlorine from CFCs is going down in the ozone hole, and that less ozone depletion is occurring because of it,” said lead author Susan Strahan, an atmospheric scientist from NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

CFCs are long-lived chemical compounds that eventually rise into the stratosphere, where they are broken apart by the Sun’s ultraviolet radiation, releasing chlorine atoms that go on to destroy ozone molecules. Stratospheric ozone protects life on the planet by absorbing potentially harmful ultraviolet radiation that can cause skin cancer and cataracts, suppress immune systems and damage plant life. Two years after the discovery of the Antarctic ozone hole in 1985, nations of the world signed the Montreal Protocol on Substances that Deplete the Ozone Layer, which regulated ozone-depleting compounds. Later amendments to the Montreal Protocol completely phased out production of CFCs.

Past studies have used statistical analyses of changes in the ozone hole’s size to argue that ozone depletion is decreasing. This study is the first to use measurements of the chemical composition inside the ozone hole to confirm that not only is ozone depletion decreasing, but that the decrease is caused by the decline in CFCs. The study was published Jan. 4 in the journal Geophysical Research Letters.

The Antarctic ozone hole forms during September in the Southern Hemisphere’s winter as the returning sun’s rays catalyze ozone destruction cycles involving chlorine and bromine that come primarily from CFCs. To determine how ozone and other chemicals have changed year to year, scientists used data from the Microwave Limb Sounder (MLS) aboard the Aura satellite, which has been making measurements continuously around the globe since mid-2004. While many satellite instruments require sunlight to measure atmospheric trace gases, MLS measures microwave emissions and, as a result, can measure trace gases over Antarctica during the key time of year: the dark southern winter, when the stratospheric weather is quiet and temperatures are low and stable.

The change in ozone levels above Antarctica from the beginning to the end of southern winter —  early July to mid-September — was computed daily from MLS measurements every year from 2005 to 2016. “During this period, Antarctic temperatures are always very low, so the rate of ozone destruction depends mostly on how much chlorine there is,” Strahan said. “This is when we want to measure ozone loss.” They found that ozone loss is decreasing, but they needed to know whether a decrease in CFCs was responsible. When ozone destruction is ongoing, chlorine is found in many molecular forms, most of which are not measured. But after chlorine has destroyed nearly all the available ozone, it reacts instead with methane to form hydrochloric acid, a gas measured by MLS. “By around mid-October, all the chlorine compounds are conveniently converted into one gas, so by measuring hydrochloric acid we have a good measurement of the total chlorine,” Strahan said.  

Nitrous oxide is a long-lived gas that behaves just like CFCs in much of the stratosphere. The CFCs are declining at the surface but nitrous oxide is not.  If CFCs in the stratosphere are decreasing, then over time, less chlorine should be measured for a given value of nitrous oxide. By comparing MLS measurements of hydrochloric acid and nitrous oxide each year, they determined that the total chlorine levels were declining on average by about 0.8 percent annually. The 20 percent decrease in ozone depletion during the winter months from 2005 to 2016 as determined from MLS ozone measurements was expected. “This is very close to what our model predicts we should see for this amount of chlorine decline,” Strahan said. “This gives us confidence that the decrease in ozone depletion through mid-September shown by MLS data is due to declining levels of chlorine coming from CFCs. But we’re not yet seeing a clear decrease in the size of the ozone hole because that’s controlled mainly by temperature after mid-September, which varies a lot from year to year.”

Looking forward, the Antarctic ozone hole should continue to recover gradually as CFCs leave the atmosphere, but complete recovery will take decades. “CFCs have lifetimes from 50 to 100 years, so they linger in the atmosphere for a very long time,” said Anne Douglass, a fellow atmospheric scientist at Goddard and the study’s co-author. “As far as the ozone hole being gone, we’re looking at 2060 or 2080. And even then there might still be a small hole.”

To read the study, visit: http://onlinelibrary.wiley.com/doi/10.1002/2017GL074830/abstract


Währenddessen spielt die taz das Gegenteil-Spiel. Hat man dort vielleicht Probleme mit der englischen Sprache und kann die NASA-Pressemitteilung nicht verstehen? Taz am vom 4.3.2018:

Neue Studie zur Ozonschicht: Das Ozonloch ist zurück
Die Schutzschicht dünnt sich aus. Forscher suchen noch nach einer Erklärung. Die US-Regierung zögert ihre Zustimmung für ein Abkommen hinaus.

Weiterlesen in der taz.