The hole in the earth’s ozone, the protective layer which filters out harmful ultraviolet radiation, appears to be smaller this year, New Zealand scientists say.
The scientists from the National Institute of Water & Atmospheric Research (NIWA) say that the area of the hole has peaked at about 24 million square kilometres, an area which is about three times the size of Australia but which is 20 per cent smaller than 29 million square kilometres the hole peaked at in 2003.
“Measurements from the ground at Scott Base suggest that there is slightly more ozone this year than the average for recent years,” says NIWA atmospheric scientist Dr Stephen Wood. “But ozone levels are still lower than before the ozone hole started forming in the early 1980s.”
Before the early 1980s, ozone levels in Antarctica never went below 220 Dobson units (DU), but in the past couple of weeks the Scott Base instruments have measured the level of ozone at 165 DU and satellite measurements show parts of the hole as low as 110 DU.
The depletion in the ozone layer, or the ozone hole, occurs each austral spring over Antarctica and is triggered by a combination of chlorine pollutants in the atmosphere, extremely cold winter temperatures, and the return of sunlight in spring. The polar vortex, a band of strong winds that effectively keeps the atmosphere above Antarctica isolated during winter, is a key part of the recipe.
Before they were banned, chlorofluorocarbons (CFCs), which produce pollutants such as chlorine, were once widely used in aerosol cans and refrigerators. Now thanks to the ban, these chemical pollutants are starting to decline. “We are starting to see the chlorine decline in the stratosphere — that’s where the ozone is — but we haven’t yet seen is a consistent decrease in the severity of ozone depletion,” says Dr Wood.
So Dr Wood cautions against reading too much into the smaller size of the hole this year. “We need to see smaller or less severe ozone holes over a number of years before we can say for certain that ozone is recovering,” he says.
Typically the ozone hole lasts until November or December, when increasing temperatures cause the winds surrounding the South Pole to weaken, and ozone-depleted air inside the polar vortex to mix with ozone-rich air outside it. It is then that the ozone depletion impacts New Zealand.
“When the ozone hole breaks up in November or early December, ozone-depleted air moves into surrounding areas in the southern hemisphere, including New Zealand,” says Dr Wood. (The only inhabited area affected by the ozone hole at the moment is the southern tip of South America.)
“The later the ozone hole breaks up, the higher the sun is in the sky over New Zealand and the larger the effect on UV levels. If New Zealand experiences a combination of lower ozone with high sun and few clouds, then skin-damaging UV levels can be extreme,” Dr Wood says.
About half the long-term ozone decline at mid-latitudes in the southern hemisphere, which includes New Zealand, has been caused by the export of ozone-depleted air from Antarctica, the NIWA scientists believe.
In recent years, there has been about ten percent less ozone over New Zealand in the summer than 30 years ago.