Comparison images of the ozone hole over Antarctica in 2011 and 2012. 2012's was smaller so is it recovering?
The holes that formed in the ozone layer over Antarctica in 2011 and 2012 are a study in contrasts. The 2011 hole (top left) ranked among the ten largest recorded since the 1980s, while the 2012 hole (top right) was the second smallest. Why were they so different? Is it a sign that stratospheric ozone is recovering? These are the questions NASA scientists Anne Douglass, Natalya Kramarova, and Susan Strahan asked as they examined the holes using data from instruments on NASA’s Aura and NASA/NOAA’s Suomi NPP satellites. The images above represent the typical method of gauging the ozone hole. They show the extent (the geographic area covered) and the depth (the concentration of ozone from top to bottom in the atmosphere) as measured by Aura’s Ozone Monitoring Instrument. Blues and purples represent the lowest ozone levels. Each image shows the day of maximum extent—when the ozone hole was largest that year. But the view of area doesn’t tell the whole story, said Douglass. It says nothing about the chemistry or atmospheric dynamics that give the hole its shape. And if we don't know why the size and depth of the hole varies, it is impossible to know if policies meant to reduce ozone depletion (such as the Montreal Protocol) are having an impact. 2011 and 2012 offer prime examples. The Antarctic ozone hole forms in the southern spring when chlorine and other ozone depleting chemicals interact with sunlight to destroy ozone. It would be easy to assume that a larger ozone hole means more chemicals were present, but the real picture is more complicated. “2011 would have had less ozone even without ozone depleting chemicals,” said Strahan. Stratospheric ozone is naturally produced in the tropics and transported to the poles. In 2011, winds blew less ozone to Antarctica so there was less to destroy. Strahan also found less chlorine in the atmosphere over Antarctica in 2011 than in other years, but because there was less ozone, a large hole developed. In 2012, ozone depletion
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