The Incredible Shrinking Ozone Hole
The ozone hole waxes and wanes with the seasons every year, slowly vanishing as the Southern Hemisphere reaches the peak of its summer. But this year the hole closed up earlier than in recent years; for the last three years the hole has lingered on well into December, according to Dr. Richard McPeters, principal investigator for NASA's Total Ozone Mapping Spectrometer (TOMS) at the NASA Goddard Space Flight Center (GSFC).
Right: Ozone concentrations over the Southern Hemisphere just after the disappearance of the ozone hole, which would have appeared as purple or pink. This image was constructed from data from NASA's Total Ozone Mapping Spectrometer (TOMS).
Dissipating earlier than expected so soon after widening to a record size may seem to send a mixed message about whether the hole is improving or worsening. Either interpretation would be unjustified, McPeters said.
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Long-term trends cannot be drawn from a single year's ozone hole because its size and duration hinge on that year's weather. Because of this, the hole's behavior shows the same kind of random variation from one year to the next as weather factors like temperature and precipitation. [more information]
"Any particular year, there's just too much randomness in the weather to put your finger on an ultimate explanation for why it happened this way," said Dr. Paul Newman, atmospheric physicist at GSFC.
The details of a particular year's weather may be unexplainable, but the influence of the weather on the ozone hole is well understood.
The attention-grabbing behavior of this year's hole -- both the record size and the quick disappearance -- can be largely attributed to the influence of an atmospheric phenomenon known as "planetary-scale waves," Newman said.
"Just think of (a planetary-scale wave) as being a big low pressure system that almost straddles the entire Southern Hemisphere," Newman said. "These lows and highs ... are so big that you can't see it on a regular weather chart. That's why we call them planetary-scale waves."
Above: A size comparison between this year's ozone hole (pluses) and last year's (line). Note this year's high peak in mid-September, followed by a rapid decline. The shaded region and white line represent the range and mean between 1979 and 1992.
This year, these planetary waves of air pressure were unusually weak in the Southern Hemisphere while the ozone hole was forming during August and early September.
Roughly speaking, planetary waves exert an influence that works against the destruction of ozone by CFCs. So this lull in planetary wave activity allowed the hole to grow to its record-breaking size.
Then around mid-September when the size of the ozone hole peaked, the strength of these planetary waves grew dramatically, which hastened the demise of the hole, Newman said.
"The key ingredient here is this almost random strength of these large-scale weather systems. Even though you've got lots of chlorine (CFCs) in our atmosphere, and it's always going to get cold over Antarctica every year (which exacerbates ozone destruction), the day-to-day size of the ozone hole is really controlled by the fine details (of weather)," Newman said.
The story of how planetary waves work against CFC-induced ozone destruction is rather complicated.
These vast pressure waves influence ozone destruction in several ways, but for explaining this year's ozone hole, the most relevant impact of the waves is on the size and stability of the massive jet stream encircling Antarctica called the "Antarctic vortex."
The isolation provided by the vortex prevents warmer, ozone-rich air surrounding Antarctica from flowing toward the pole, which would help replace the destroyed ozone and raise temperatures over the continent. Instead, the ozone-rich air -- which is carried toward the pole by the action of the planetary waves -- builds up at the edge of the vortex, forming a "ring" of high ozone concentrations around the continent that can be seen in the satellite images.
Left: Image of the record-size ozone hole taken by NASA satellites on September 9, 2000. Blue denotes low ozone concentrations and yellow and red denote higher levels of ozone. Notice the ring of high ozone concentrations formed when the Antarctic vortex blocks the southerly migration of ozone formed in the tropics. [More images and credits]
Without the warming effect of these waves, the air inside the vortex drops to extremely cold temperatures during the winter's perpetual night. These low temperatures set the stage for ozone destruction, since the chemical reactions that lead to ozone destruction are catalyzed by icy clouds that only form in very cold air.
This year's unusually weak planetary waves allowed the vortex to expand to a greater size. The larger vortex amounted to a larger arena for the destruction of ozone, resulting in the record-size hole.
When the strength of these waves picked up in mid-September, they exerted a force on the vortex which blew it apart earlier than usual. As the vortex broke down, the surrounding warm, ozone-rich air mixed with the air over Antarctica, raising ozone concentrations above the threshold for an ozone "hole."
So this year's headline-generating ozone hole is a reflection of the unusual behavior of the planetary waves in the Southern Hemisphere, while this behavior itself can't be easily explained.
"Do we understand why these (planetary waves) were weaker this year? Well, no, we don't," Newman said.
"It's unexplained in the same sense that we can't really explain ... why you get an unusually cold winter this year and not last year," McPeters said. "Long-term weather is intrinsically unpredictable."
Above: A graph showing the concentrations of one type of CFC over time. Notice the steady rise until about 1990 -- three years after the Montreal Protocol established a phase-out program for CFCs. Concentrations of CFCs have started to decline. (Note that in the graph, "ppt" stands for parts per trillion, not parts per thousand.) Image courtesy of the National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory.
This year's ozone hole doesn't by itself give any indication of the long-term trend, but measurements show that CFC concentrations in the stratosphere have leveled off and in the lowest layer of the atmosphere, the troposphere, CFC concentrations have started to decline.
These measurements indicate that the ozone hole is not worsening, and may soon start to improve. But this improvement is going to come very slowly, Newman said.
"The ozone hole isn't going to go away for a long time," he said. "This is because the lifetimes of CFCs and HCFCs and halons are so long. We might be back to 1979 levels sometime around 2050 or so."
Peering Into the Ozone Hole -- Science@NASA article about September's record-breaking ozone hole, including a discussion of the hole's dependence on weather
NASA's Total Ozone Mapping Spectrometer -- Home page for the instrument, which takes daily snapshots of ozone concentrations and UV levels around the Earth
Stratospheric Ozone - an electronic textbook
Glossary of ozone-related terms
The Montreal Protocol of 1987 -- Text of The Montreal Protocol, which set provisions for phasing out the use of chemicals determined to hasten ozone destruction
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|For lesson plans and educational activities related to breaking science news, please visit Thursday's Classroom||Author: Patrick L. Barry
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