WASHINGTON, DC - Earth's protective ozone layer above the Arctic was pierced by a hole of unprecedented size last winter and spring caused by a long cold period in the stratosphere, finds new research led by scientists at the National Aeronautics and Space Administration, NASA.
The hole covered 772,204 square miles (two million square kilometers) - about the size of Mexico - and allowed high levels of harmful ultraviolet radiation to strike northern Canada, Europe and Russia this spring, the researchers say.
The stratospheric ozone layer, extending from about 10 to 20 miles (15 to 35 kilometers) above the surface, protects life on Earth from the Sun's ultraviolet rays.
To investigate the 2011 Arctic ozone loss, 29 scientists from 19 institutions in nine countries analyzed measurements, including daily global observations of trace gases and clouds from NASA's Aura and CALIPSO spacecraft, ozone measured by instrumented balloons, meteorological data and atmospheric models.
The scientists - from the United States, Germany, The Netherlands, Canada, Russia, Finland, Denmark, Japan and Spain - found that at some altitudes, the Arctic cold period lasted more than 30 days longer in 2011 than in any previously studied Arctic winter, leading to the unprecedented ozone loss.
"Day-to-day temperatures in the 2010-11 Arctic winter did not reach lower values than in previous cold Arctic winters," said lead author Gloria Manney of NASA's Jet Propulsion Laboratory in Pasadena, California, and the New Mexico Institute of Mining and Technology in Socorro.
"The difference from previous winters is that temperatures were low enough to produce ozone-destroying forms of chlorine for a much longer time," Manney said.
"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," she said.
The study, published online Sunday in the journal "Nature," finds the amount of ozone destroyed in the Arctic in 2011 comparable to that seen in some years in the Antarctic, where an ozone hole has formed each spring since the mid-1980s.
The Arctic ozone loss "exceeded 80 percent over 18-20 kilometres altitude," the scientists said.
"Our results show that Arctic ozone holes are possible even with temperatures much milder than those in the Antarctic. We cannot at present predict when such severe Arctic ozone depletion may be matched or exceeded," they said.
Environment Canada scientist David Tarasick, whose research group played a key role in the report, is not being allowed to discuss the discovery with the media.
Environment Canada told reporter Margaret Munro of Postmedia News that an interview with Tarasick "cannot be granted."
"Tarasick is one of several Environment Canada ozone scientists who have received letters warning of possible 'discontinuance of job function' as part of the downsizing underway in the department," Munro reported Sunday.
Environment Canada and Health Canada now tightly control media access to researchers and orchestrate interviews that are approved, Munro reported in 2010.
University of Toronto physicist Kaley Walker is not bound by such restrictions.
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"In the 2010-11 Arctic winter, we did not have temperatures that were lower than in the previous cold Arctic winters," said Walker. "What was different about this year was that the temperatures were low enough to generate ozone-depleting forms of chlorine for a much longer period of time."
"Arctic ozone loss events such as those observed this year could become more frequent if winter Arctic stratospheric temperatures decrease in future as the Earth's climate changes," Walker said.
Stratospheric ozone has decreased three percent globally between 1980 and 2000 and thins by 50 percent over Antarctica in winter and spring.
The Antarctic ozone hole forms when extremely cold conditions trigger reactions that convert atmospheric chlorine from human-produced chemicals into forms that destroy ozone.
The same ozone-loss processes happen each winter in the Arctic. But the warmer stratospheric conditions there limit the area affected and the time frame during which the chemical reactions occur, resulting in far less ozone loss in most years in the Arctic than in the Antarctic.
The 2011 Arctic ozone loss occurred over an area considerably smaller than that of the Antarctic ozone holes.
The scientists explain that this happened because the Arctic polar vortex, a persistent large-scale cyclone within which the ozone loss takes place, was about 40 percent smaller than a typical Antarctic vortex.
While smaller and shorter-lived than its Antarctic counterpart, the Arctic polar vortex is more mobile, often moving over densely populated northern regions. Decreases in overhead ozone lead to increases in surface ultraviolet radiation, which are known to have adverse effects on humans and other life forms.
Although the total amount of Arctic ozone measured was more than twice that typically seen in an Antarctic spring, the amount destroyed was comparable to that in some previous Antarctic ozone holes.
This is because ozone levels at the beginning of Arctic winter are typically much greater than those at the beginning of Antarctic winter, the scientists say.
Manney said that without the 1989 Montreal Protocol, an international treaty limiting production of ozone-depleting substances, chlorine levels already would be so high that an Arctic ozone hole would form every spring.
Even with the limitations on emission of ozone-depleting chemicals, the long atmospheric lifetimes of substances already in the atmosphere mean that Antarctic ozone holes, and the possibility of future severe Arctic ozone loss, will continue for decades.
The scientists say further studies are needed to determine what factors caused the cold period to last so long.
"Our ability to quantify polar ozone loss and associated processes will be reduced in the future when NASA's Aura and CALIPSO spacecraft, whose trace gas and cloud measurements were central to this study, reach the end of their operational lifetimes," Manney said. "It is imperative that this capability be maintained if we are to reliably predict future ozone loss in a changing climate."
"Each of the balloon and satellite measurements included in this study were absolutely necessary to understand the ozone depletion we observed this past winter," said Walker at the University of Toronto. "To be able to predict future Arctic ozone loss reliably in a changing climate, it is crucial that we maintain our atmospheric measurement capabilities."
Other institutions participating in the study included: Alfred Wegener Institute for Polar and Marine Research, Potsdam, Germany; NASA Langley Research Center, Hampton, Virginia; Royal Netherlands Meteorological Institute, De Bilt, The Netherlands; Delft University of Technology, 2600 GA Delft, The Netherlands; Science Systems and Applications, Inc., Greenbelt, Maryland, and Hampton, Virginia; Science and Technology Corporation, Lanham, Maryland; Environment Canada, Toronto, Ontario, Canada; Central Aerological Observatory, Russia; NOAA Earth System Research Laboratory, Boulder, Colorado; Arctic Research Center, Finnish Meteorological Institute, Finland; Danish Climate Center, Danish Meteorological Institute, Denmark; Eindhoven University of Technology, Eindhoven, The Netherlands; Arctic and Antarctic Research Institute, St. Petersburg, Russia; National Institute for Environmental Studies, Japan; National Institute for Aerospace Technology, Spain; and University of Toronto, Ontario, Canada.