Stratospheric ozone above the Arctic suffered an “unprecedented” loss last winter and spring, reaching its lowest levels on record. According to researchers, the amount of ozone destroyed in the region was comparable to that seen in some years in the Antarctic, where an ozone hole has formed every spring since the mid-1980s.
The Antarctic ozone hole was first reported in 1985 in the journal Nature, in a paper by a team of researchers with the British Antarctic Survey, who found that stratospheric ozone levels above the South Pole had plunged precipitously. The BAS scientists ascribed the hole to a theory, first postulated during the 1970s by chemists Mario Molina and F. Sherwood Rowland, that chlorine from chlorofluorocarbons, or CFCs (chemicals used in refrigerants and aerosol propellants), was entering the atmosphere and destroying the ozone.
The ozone layer over Antarctica was being depleted because the extreme winter cold above the southern continent leads to the formation of polar stratospheric clouds, the icy surfaces of which form platforms for chemical reactions that separate chlorine from CFCs. Chlorine builds up over the course of the long polar winter, and the return of sunlight in the spring starts the process that causes that chlorine to attack ozone.
The Montreal Protocol has led to the phasing out of CFCs, but the compounds persist in the atmosphere and so does the Antarctic ozone hole, although it is expected to diminish in size and, according to some researchers, may disappear entirely by 2050.
In contrast, ozone loss above the Arctic has been far more limited, because while the region may be frigid relative to much of the rest of the planet, it’s positively balmy compared with Antarctic. Conditions have not permitted the buildup of free chlorine in the way that they have over Antarctica.
Writing in Nature, a research team led by Gloria Manney of NASA’s Jet Propulsion Laboratory reported on Sunday that more than 80 percent of the ozone present in the Arctic in January 2011 had been destroyed by late March at altitudes of about 11 to 12 miles. At its largest, the ozone hole covered an area of approximately 772,000 square miles, about five times the size of Germany or California.
The reason the hole appeared, the authors said, was not that the stratosphere above the Arctic was significantly colder than it had been during previous Arctic winters, but because it stayed cold for longer.
“The difference from previous winters is that temperatures were low enough to produce ozone-destroying forms of chlorine for a much longer time,” said Manney in a NASA press release.”This implies that if winter Arctic stratospheric temperatures drop just slightly in the future, for example as a result of climate change, then severe Arctic ozone loss may occur more frequently.”
The reference to climate change alludes to the fact that, as increasing levels of greenhouse gases trap heat in the lower atmosphere, the stratosphere is expected to become colder. Indeed, the Intergovernmental Panel on Climate Change noted in 2007 that “there has been global stratospheric cooling since 1979.”
However, although the link between climate change and Arctic ozone loss has been established before, the emergence of this year’s hole has caught researchers by surprise and left them scrambling to determine whether it is an anomaly or the beginning of a trend.
“Our results show that Arctic ozone holes are possible even with temperatures much milder than those in the Antarctic,” Manney and her fellow authors point out. “We cannot at present predict when such severe Arctic ozone depletion may be matched or exceeded.”
Image at Top: Left: Ozone in Earth’s stratosphere at an altitude of approximately 12 miles in mid-March 2011, near the peak of the 2011 Arctic ozone loss. Right: chlorine monoxide –- the primary agent of chemical ozone destruction in the cold polar lower stratosphere –- for the same day and altitude. Credit: NASA/JPL-Caltech.