Satellite monitoring suggests that the size of the hole has essentially stabilised, but according to the studies, which used satellite data to look inside the hole itself, any changes in size observed are being driven by temperature and winds, rather than a reduction in the amount of chlorine in the stratosphere.

Natalya Kramarova of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, mapped ozone concentration within the 2012 hole, which was the second-smallest since the mid-1980s. The map showed that the concentration varied with altitude; winds carried ozone into the upper altitudes in early October, masking the ozone destruction in the lower stratosphere and creating the impression that the ozone hole had improved. ‘Our work shows that the classic metrics based on the total ozone values have limitations – they don’t tell us the whole story,’ she said.

In a separate study, a team led by Susan Strahan, also of NASA Goddard, found that the holes of 2006 and 2011 contained different amounts of ozone-depleting chlorine, despite being the same size. They, too, found that ozone levels were affected by meteorological effects.

‘We’re still in the period where small changes in chlorine don’t affect the area of the ozone hole, which is why it’s too soon to say the ozone hole is recovering,’ Strahan said.

This story was published in the February 2014 edition of Geographical Magazine