For years, scientists have been baffled by the hole in the ozone layer and why the gap, which forms each year over Antarctica, continues to considerably change in size. While the area of the hole has ceased to grow much larger, the layer that shields the earth from the sun’s harmful ultraviolet radiation is far from fully recovered, December research from NASA shows.
The ozone hole peaked in the last decade, waxing and waning in an area between 8.2 million square miles and 10.6 million square miles—roughly the size of North America. Researchers are attributing the latest decrease in the hole’s area to changes in the weather.
A group of scientists from NASA’s Goddard Space Flight Center in Greenbelt, Md., studied the changing chemical makeup of the hole and discovered that the relatively small area of last year’s hole—the second smallest in the past 20 years—was due to a natural variation in wind patterns, not an international agreement to stop using ozone-depleting chemicals.
“We have identified another factor that wasn’t fully recognized before, and that is how much ozone gets brought to the polar regions in the first place, by the winds," Susan Strahan, a NASA scientist who coauthored the study, said during the annual meeting of the American Geophysical Union in San Francisco last week.
There are multiple holes in the earth's ozone layer, and scientists track the largest areas where concentrations of ozone—which has three atoms of the element oxygen in its molecule instead of the normal two atoms found in air we breathe—get thinner. Holes are where there are less than 220 Dobson units of ozone.
In 1987, one hundred ninety-seven countries signed the Montreal Protocol and phased out chlorofluorocarbons—better known as CFCs—as well as other substances that deplete the ozone layer and weaken the planet’s shield against destructive ultraviolet rays. The treaty has been slow to produce results; the chemicals can live up to 100 years and continue to damage the atmosphere, scientists say.
What does this new information mean for the atmospheric hole that the Environmental Protection Agency has linked to sunburn, skin cancer, and cataracts for decades?
While harmful chlorine levels found in CFCs have decreased in the atmosphere by about 3 percent in the Northern Hemisphere and 6 percent in the Southern Hemisphere, it’s too soon to tell if that has contributed to a stronger ozone layer, Strahan says. There’s a long way to go before a full recovery can be identified.
“Ozone holes with smaller areas and a larger total amount of ozone are not necessarily evidence of recovery attributable to the expected chlorine decline,” Strahan said in a statement. “That assumption is like trying to understand what's wrong with your car's engine without lifting the hood.”
In 2006, NASA reported the largest hole in the ozone layer on record, at 10.6 million square miles—large enough to cover almost all of Africa—with 2011's hole coming in a close second, at about 10 million square miles.
Last year the hole was significantly smaller, at 8.2 million square miles, about twice the size of the United States. Scientists believe this is due mostly to the strong winds that can shift the atmosphere and either quickly reduce the gap or make it worse.
Still, the atmosphere-chipping chlorine levels are expected to fall below 1990s levels between 2015 and 2030 because of the Montreal Protocol, according to Paul Newman, NASA’s chief scientist for atmospheric sciences.
At the current pace, the ozone hole is expected to recover by 2070, he says.
“We know the treaty is working. We fully expect to see recovery, but we are not there yet,” he says. “It’s going to be a bumpy road.”
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