The ozone hole over Antarctica is the smallest observed since 1982, due to abnormally warm temperatures in the upper atmosphere that dramatically limited ozone depletion in September and October this year, according to NASA and NOAA satellite measurements.
However, the researchers at NASA and The National Oceanic and Atmospheric Administration (NOAA) cautioned that this is due to warmer stratospheric temperatures and not a sign that atmospheric ozone is suddenly on a fast track to recovery.
The annual ozone hole reached its peak extent of 16.4 million square kilometres on September 8, and then shrank to less than 10 million square kilometres for the remainder of September and October, NASA said in a statement.
During years with normal weather conditions, the ozone hole typically grows to a maximum area of about eight million square miles in late September or early October, it said.
"It's great news for ozone in the Southern Hemisphere," said Paul Newman, chief scientist for Earth Sciences at NASA's Goddard Space Flight Center in the US.
"But it's important to recognise that what we're seeing this year is due to warmer stratospheric temperatures. It's not a sign that atmospheric ozone is suddenly on a fast track to recovery," Newman said.
Ozone is a highly reactive molecule comprised of three oxygen atoms that occurs naturally in small amounts.
Roughly seven to 25 miles above Earth's surface, in a layer of the atmosphere called the stratosphere, the ozone layer is a sunscreen, shielding the planet from potentially harmful ultraviolet radiation that can cause skin cancer and cataracts, suppress immune systems and also damage plants.
The Antarctic ozone hole forms during the Southern Hemisphere's late winter as the returning Sun's rays start ozone-depleting reactions.
These reactions involve chemically active forms of chlorine and bromine derived from man-made compounds.
The chemistry that leads to their formation involves chemical reactions that occur on the surfaces of cloud particles that form in cold stratospheric layers, leading ultimately to runaway reactions that destroy ozone molecules.
In warmer temperatures fewer polar stratospheric clouds form and they don't persist as long, limiting the ozone-depletion process.
This is the third time in the last 40 years that weather systems have caused warm temperatures that limit ozone depletion, said Susan Strahan, an atmospheric scientist with Universities Space Research Association, who works at NASA.
Similar weather patterns in the Antarctic stratosphere in September 1988 and 2002 also produced atypically small ozone holes, she said.
"It's a rare event that we are still trying to understand. If the warming hadn't happened, we would likely be looking at a much more typical ozone hole,” said Strahan.
There is no identified connection between the occurrence of these unique patterns and changes in climate.
NASA and NOAA monitor the ozone hole via complementary instrumental methods.
Satellites, including NASA's Aura satellite, the NASA-NOAA Suomi National Polar-orbiting Partnership satellite and NOAA's Joint Polar Satellite System NOAA-20 satellite, measure ozone from space.
The Aura satellite's Microwave Limb Sounder also estimates levels of ozone-destroying chlorine in the stratosphere. — PTI
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