Claims of an ozone recovery are premature. Many expect to see such a recovery in the future, but it hasn’t been reliably seen just yet.
The ozone hole has become one of the great scientific stories of our time. In the mid-1980s it was realised that chlorofluorocarbons – released from such things as refrigerators – were making their way into the stratosphere, concentrating at the poles and destroying the UV-protective ozone.
The problem called for an international response. The subsequent 1987 Montreal Protocol banned ozone-destroying chemicals, which has led to the size of the ozone hole at the poles stabilising. But is it starting to reduce in size? Are we seeing the first signs of recovery and an eventual restoration of normal ozone levels in a few decades?
A recent study published in the journal Science says, ‘yes’. The researchers claim to have detected the first signs of healing in the southern hemisphere’s ozone hole. They conclude that the ozone concentration has increased since 2000, and that since then the size of the ozone hole has decreased by 4.5 m sq km.
The ozone hole appears in the austral spring – September and October. In the polar night, cfc’s wait for the sunlight when they release chlorine atoms, which then destroy the ozone.
The researchers used balloon observations taken from the south pole and from a Japanese research station on the coast of Maud Land. They say that October may not be the best month to see evidence of healing. October shows the deepest ozone depletion, but is subject to large fluctuations due to the state of the atmosphere.
They argue that signs of increasing ozone concentration is only visible in the September data since 2000.
The research was widely reported across the media, but the researchers’ own low estimates of confidence should have caused reporters to treat the story with more caution.
As the researchers themselves point out, data taken in October shows no improvement, and data from September shows an increase only at the 10% significance level, which is generally considered not reliable enough to make confident statements.
The problem is that there is too much variation on ozone from year to year to pick out, reliably, such a small trend in the data.
According to Susan Strathan of NASA’s Goddard Spaceflight Center, the paper’s results are, ‘a small piece of the puzzle, and we expect that in years to come we’ll see stronger evidence.’
Consequently, claims of an ozone recovery are premature. Many expect to see such a recovery in the future, but it hasn’t been reliably seen just yet.