Last year the hole in the ozone layer above Antarctica was surprisingly small. But don't get used to that, say atmospheric scientists. Rather than signaling a speedy recovery, the shrinkage was likely a sign of variable weather.
The ozone hole still has decades of waxing and waning before it finally closes, suggests research presented this week at a American Geophysical Union meeting. (Also see "Ozone Depletion".)
“The hole should vanish entirely by 2070 and should start to recover in the next decade,” says one of the lead scientists on the research, Susan Strahan of NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
The size of the ozone hole above Antarctica, a region in the upper atmosphere characterized by very low ozone levels, has peaked in recent years, covering roughly 8.1 million square miles (21 million square kilometers) to 10.4 million sq. mi. (27 million sq. km.)—an area larger than South America.
More than 20 years ago the Montreal Protocol limited the use of ozone-depleting chemicals, which had left a visible wound in the Earth’s ozone layer. But since they have a life span of more than 100 years, these chemicals are still thinning the the upper atmosphere’s ozone layer.
This layer matters because it shields the Earth's surface from the sun's ultraviolet radiation, which can cause skin cancer and cataracts in people. That’s why the hole’s pattern of recovery and relapse has left scientists disappointed—and puzzled.
Two of the deepest and largest ozone holes of the past decade occurred in 2006 and 2011, for example, yet the hole in 2012 was the second smallest of the past 20 years.
Strahan and her colleagues Anne Douglass and Natalya Kramarova, also at NASA’s Goddard Space Flight Center, now think they know why. They say the hole contracts and expands not only in response to the level of ozone-depleting chemicals left in the atmosphere but also in response to wind.
“It’s very important to understand these year-to-year changes”, says Lorenzo Polvani, an atmospheric scientist at Columbia University in New York, who was not involved in the study.
Traditionally, scientists have measured the total amount of ozone in the atmosphere from top to bottom, then used those measurements to produce a single indicator of ozone recovery. But using newly available data from two satellites, Strahan and Kramarova were able to see what was happening inside the hole.
To their surprise, they found that in 2006 and 2011 the size of the holes were the same, but the total level of ozone inside them differed.
What they found next―with the help of an atmospheric model―was still more surprising. In 2006, strong winds blew more ozone over Antarctica, but there was also more ozone-depleting chlorine that year. In 2011, air masses blew in less ozone, but there was also less ozone-depleting chlorine.
So the wind effects balanced out to create a 2011 ozone hole of the same size as in 2006, but its overall amount of ozone was higher.
In 2012, the tale had another twist. Although severe ozone depletion took place at lower altitudes, strong winds brought lots of ozone to high altitudes over the Antarctic. As a result, the hole was smaller than average.
“Nobody has looked at this masking effect in detail before,” says Birgit Hassler, at NOAA’s Earth System Research Laboratory in Boulder, Colorado, who was not involved in the study. She adds, “Until now everyone assumed that one number was sufficient to explain the recovery of the ozone hole.”
Slow But Steady
The scientists still expect the ozone hole to show some signs of healing by 2025. Until then, we can expect more flip-flopping between good and bad years, the good ones bringing bringing less exposure to the sun’s harmful rays in the Southern Hemisphere—a small reprieve.
Douglass says this is “as good as can be expected. Some people would have hoped for a faster response, but we can expect a bumpy road to recovery."
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