|Dr. Gloria Manney is the lead author of an article in the current issue of Nature about the unexpectedly large loss of ozone over the Arctic during the winter of 2011.|
Manney’s study was published online Sunday in the journal Nature. She and her 28 co-authors have found that the amount of ozone destroyed in the Arctic in 2011 was comparable to that seen in some years in the Antarctic, where an ozone “hole” has formed each spring since the mid-1980s. The paper’s authors include scientists from 19 institutions in nine countries. Manney said she gets great personal satisfaction from the teamwork involved in this research project.
“I can’t emphasize this enough – this was a community effort,” Manney said. “When we started seeing this severe ozone loss, a lot of people in the ozone-research community got together. We all cooperated to write a very comprehensive paper, instead of competing with each other. It was a pleasure to be a part of this community effort.”
The stratospheric ozone layer extends from about 10 to 20 miles (15 to 35 kilometers) above the surface and protects life on Earth from the sun’s harmful ultraviolet rays.
The Antarctic ozone hole forms when extremely cold conditions, common in the winter Antarctic stratosphere, trigger reactions that convert atmospheric chlorine from human-produced chemicals into forms that destroy ozone. The same ozone-loss processes occur each winter in the Arctic. However, the generally warmer stratospheric conditions in the Arctic 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.
Manney and her collaborators studied three satellite data sets, including those from the Microwave Limb Sounder, or MLS, and the Ozone Measuring Instrument, or OMI, both on NASA’s Aura satellite. Molecules in the atmosphere emit radiation at a specific frequency. The satellite-borne MLS instrument is akin to a radio receiver, collecting signals that are later correlated to the amounts of specific compounds. The MLS data are used to calculate a vertical profile as a function of altitude, thus allowing the researchers to create a 3-D image of the ozone layer. Another NASA satellite data set provided information on clouds in the polar stratosphere that are important in converting chlorine into the forms that destroy ozone. Still other data sets were provided by balloon-borne instruments launched in the northern latitudes including Canada, Scandinavia and Russia.
The Aura Satellite has been collecting ozone data since 2004. The two satellites gather data as they orbit. For this study, Manney and her colleagues examined 3,500 measurements of the ozone layer and its chemical constituents. An older satellite no longer in service, the Upper Atmosphere Research Satellite, collected similar data sets for about 10 years beginning in the early 1990s. Manney and her team found that the ozone loss above the Arctic during the early months of 2011 was significantly higher than any previous year.
Shortly after the Upper Atmosphere Research Satellite was launched in 1994, Manney was the first author on a similar paper in Nature. She was among the first scientists to publish findings showing global satellite observations that pointed to ozone destruction caused by reactions with chlorine monoxide in the Arctic. .
“When I wrote that paper in 1994 in Nature, that was first time global measurements were used to demonstrate that some chemical ozone loss was happening in the Arctic,” Manney said. “I ended that paper by saying that since the temperatures in the Arctic stratosphere are so variable and we can’t predict what they’ll do from year to year, there is a possibility of seeing more severe chemical ozone loss. To me, in a sense, this paper is sort of the sequel to that 1994 paper.”
The 2011 Arctic ozone loss occurred over an area considerably smaller than that of the Antarctic ozone holes. This is 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.
“Day-to-day temperatures in the 2010-11 Arctic winter did not reach lower values than in previous cold Arctic winters,” Manney said. “The difference from previous winters is that temperatures were low enough to produce ozone-destroying forms of chlorine for a much longer time. 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.”
Although the total amount of Arctic ozone measured was much more than twice that typically seen during 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.
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. The long atmospheric lifetimes of ozone-depleting chemicals already in the atmosphere mean that Antarctic ozone holes, and the possibility of future severe Arctic ozone loss, will continue for decades.
She also said that researchers’ ability to monitor the ozone layer above the Arctic could be greatly diminished in coming years.
“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, and that for the balloon-based measurements spanning the Arctic that were also essential to our study, be maintained if we are to reliably predict future ozone loss in a changing climate.”
Manney said scientists can simulate the main features of current annual cycles of ozone loss, but the atmospheric processes are so complex and include so many variables that computer models don’t accurately predict what will happen.
“Current models that predict temperatures from year to year aren’t very good at getting the low values in the Arctic winter stratosphere,” she said. “We have limited ability to predict right now. From my point of view, that’s an important point. This paper shows that it really takes all these data sets – not just ozone as a function of altitude, but also measurements of the compounds involved in destroying it. We need to put together all the pieces that lead to chemical ozone destruction. This winter shows us that we don’t have a complete understanding of all of the atmospheric processes involved yet.”
For more information on NASA's Aura mission, visit: http://www.nasa.gov/aura
For more information on NASA's CALIPSO mission, visit: http://www.nasa.gov/calipso
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By Thomas Guengerich/New Mexico Tech
Steve Cole of NASA and Alan Buis of the Jet Propulsion Lab contributed to this article.