But, as a NASA team reported at the AGU Fall Meeting in mid-December, the extremely small ozone hole in 2012 – the second smallest since the mid 1980s – is not yet a sign of recovery.

"Ozone holes with smaller areas and a larger total amount of ozone are not necessarily evidence of recovery attributable to the expected chlorine decline," said Susan Strahan of NASA’s Goddard Space Flight Center. "That assumption is like trying to understand what’s wrong with your car’s engine without lifting the hood.

Natalya Kramarova, also at NASA, used data from the NASA-NOAA Suomi National Polar-orbiting Partnership satellite to obtain "the first look inside the ozone hole". In this way, she created a map of ozone concentration with altitude in the middle of the hole from September to November 2012.

The measurements revealed that previous data for total column ozone in 2012 had made the hole appear smaller than it actually was, as winds blew additional ozone into higher altitudes in early October, above the ozone destruction in the lower stratosphere. This meteorological effect masked the damage to the ozone hole and gave the false impression that it could be recovering.

"Our work shows that the classic metrics based on the total ozone values have limitations – they don’t tell us the whole story," said Kramarova.

The hole truth

Similarly, Strahan found that the two large ozone holes of 2006 and 2011, two of the biggest and deepest for the last 10 years, formed for different reasons. While the 2006 hole contained more ozone-depleting chlorine, Strahan believes that in 2011 winds transported less ozone to Antarctica. Again, total column measurements of ozone had not revealed variations in two key factors – the winds that bring ozone to the region and chemical loss due to chlorine.

To obtain the result, Strahan used data from the NASA Aura satellite’s Microwave Limb Sounder to track nitrous oxide concentrations, which vary inversely with chlorine.

The team says that stratospheric chlorine was expected to drop by around 5% from 2004–2013 as a result of the Montreal Protocol, but their measurements indicate variations of ±5% each year.

So what does this mean for the recovery of the ozone hole? Since the early 1990s there have been more than enough ozone-depleting substances above Antarctica to destroy almost all the ozone in the lower stratosphere in spring and early summer. This means that the size of the seasonal ozone hole has largely been controlled by the temperature in the lower stratosphere, which affects the speed of ozone-destroying chemical reactions, and by winds that transport in different amounts of ozone.

As concentrations of ozone-depleting substances start to drop, the ozone holes in some years between 2015 and 2033 are likely to have a smaller area due to chlorine decline. And once ozone-depleters reach pre-1990 levels, which is likely to happen in the mid-2030s or so, the hole will start to recover, with full recovery predicted for around 2070.

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