Last week, we learned that an ozone hole had formed above the North Pole for the first time ever. But what exactly is an ozone hole, why are they so dangerous, and what can we do to fix them?
To find out the answers to these questions, we went right to the source, and interviewed some of the researchers at NASA's Jet Propulsion Laboratory who discovered the ozone hole that formed last winter in the Arctic Circle. We asked them all our burning questions about the ozone layer and what's happening to it. Now, with some expert assistance, we've got the answers.
The ozone layer is located in the lower portion of the stratosphere, about 12 to 20 miles above our the Earth's surface. First discovered in 1913 by French physicists Charles Fabry and Henri Buisson, the ozone layer acts as a shield against the Sun's high frequency ultraviolet light, which in excess quantities can be damaging to life on Earth. The ozone layer absorbs about 97% to 99% of all UV radiation that enters Earth's atmosphere.
Ozone itself is a molecule composed of three oxygen atoms (O3), whereas the oxygen we breathe is a molecule composed of two oxygen atoms (O2). Together, these two molecules form what's known as the ozone-oxygen cycle. As UV light enters the stratosphere, oxygen molecules absorb the UV energy and use it to break apart into two separate oxygen atoms. These free atoms then combine with oxygen molecules to form ozone. The UV light then strikes the ozone molecules, which break apart into one oxygen molecule and a free oxygen atom, which keeps the process going, all the while absorbing the UV radiation before it can reach the surface.
The problems start when there's an influx of what's known as free radical catalysts, which includes nitric oxide, nitrous oxide, hydroxyl, chlorine, and bromine. All these are capable of breaking apart the ozone molecules so that they can no longer absorb UV light, allowing the radiation to reach Earth in far greater quantities. The use of chlorofluorocarbons, or CFCs, in the late 20th century caused massive damage to the ozone layer, as these human-made compounds are extremely stable and unusually capable of surviving the journey to the stratosphere.
These have been banned since 1987, but ozone levels have continued to drop by as much as 4% a year, and the polar regions have seen significant seasonal declines during the winter months. During these periods, certain regions see huge drops in the density of ozone molecules, which creates what's known as an ozone hole. We've known about an ozone hole over Antarctica since 1985, but it's only now that scientists have identified one over the Arctic as well.
There actually isn't a strict scientific definition for what constitutes an ozone hole. The term suggests a gap in the ozone layer - basically, an actual ozone-free hole in the atmosphere - but it's a bit more subtle than that. An ozone hole is really an area where ozone density has dropped below a certain critical baseline.
Ozone density in the atmosphere is measured in Dobson units (DU), named for Oxford meteorologist Gordon Dobson (pictured at left), who carried out pioneering research into the ozone layer in the first half of the 20th century. One DU is equivalent to a density of 269 quintillion ozone molecules per square meter.
In the Antarctic, scientists often use 220 DU as the baseline for what constitutes an ozone hole, as that was the minimum ozone density recorded before the ozone layer began to thin in the 1980s. Antarctic ozone density typically reaches around 350 DU during springtime, while the density inside the Antarctic ozone hole can reach as little as 100 DU.
That's all well and good for Antarctic, but that definition of an ozone hole can't simply be transplanted to the Arctic, because the two polar regions have very different baselines for ozone density. Michelle Santee, a researcher at NASA's Jet Propulsion Laboratory and a coauthor of the Nature paper, explained to us what the problems are:
The same "definition" of an ozone hole cannot be applied in the Arctic, where unperturbed springtime values are naturally much higher (about 450 DU) because of dynamical processes. In 2011, minimum total ozone dropped to about 220-230 DU for about a week in late March, and total ozone values were continuously below 250 DU (implying a larger decrease from "normal" than is needed to reach 220 DU in the Antarctic) for nearly a month.
Because scientists haven't seen an Arctic ozone hole before, and because what constitutes an Arctic ozone hole is necessarily different from what constitutes its Antarctic counterpart, there's been disagreement in the scientific community about whether the drops in density last winter really constituted an ozone hole. Santee suggests that this is, at least to some extent, a matter of semantics. She and her fellow researchers felt comfortable stating in their paper that last winter's ozone loss could "reasonably be described as an Arctic ozone hole", but even if one disagrees with that particular conclusion, this still represents a serious drop.
The ozone loss last winter represented a significantly greater drop in ozone density than had been observed previously in the Arctic. Michelle Santee gave us this useful overview of both how last winter fits in with past Arctic winters and how it lines up with the history of Antarctic ozone depletion:
In spring 2011 we observed stratospheric ozone destruction in the Arctic of unprecedented severity. Ozone loss took place at altitudes from about 14 to 23 km (9 to 14 miles) above the Earth's surface, and more than 80% of the ozone present in January at 18-20 km (~11-12 miles) had been chemically destroyed by late March. There have been other years with large ozone losses in the Arctic (e.g., 2005, 2000, 1996), but no previous year rivalled 2011, when the ozone loss over the Arctic was comparable to that in some early Antarctic ozone holes (e.g., 1985).
What does that mean going forward? It's hard to predict, because the level of ozone depletion is extremely dependent on larger weather patterns. The Arctic, in particular, is subject to massive year-to-year fluctuations in the severity of its winters. Warm winters mean very little ozone loss, while a particularly cold winter could create another ozone hole. Santee explains:
Winters in the Arctic stratosphere are highly variable. The past decade has included some of the warmest winters in the observational record, with minimal ozone loss, and also the two coldest winters with the largest ozone losses. At present we have no capability to predict, even at the start of the season, which winters will turn out to be exceptionally or persistently cold in the stratosphere. Thus next year could bring little or no ozone loss in the Arctic, or another record-setting ozone hole.
But these weather patterns don't just fluctuate randomly. It may seem paradoxical, but global warming tends to create colder conditions in stratosphere, which causes more rapid depletion of the ozone layer. Santee pointed out that scientists haven't yet conclusively shown that climate change causes cold Arctic winters to get even colder, but we have indeed seen generally more extreme Arctic winters in the last few years. The 2011 winter was only slightly more extreme than those before it, and yet we saw significant depletion of the ozone layer. That might well suggest that even moderate cooling of the stratosphere could trigger severe ozone depletion in future.
I've been referring to this as the Arctic ozone hole, but the "Arctic" part isn't strictly accurate. The location of the ozone hole constantly moves about due to changing weather conditions, and these can and did take the 2011 ozone hole outside the confines of the Arctic circle. Santee explained this process to us:
The chemical reactions that destroy ozone primarily take place within what is called the polar "vortex", a stratospheric wind pattern present over the polar regions during the winter months that acts to isolate the frigid air inside from the surrounding air. The Arctic vortex is highly mobile, moving around from day to day as the wind patterns change in space and time, often becoming distorted and elongated or shifting well off the pole. When that happens, the low-ozone region confined within it can drift over densely-populated northern regions. In 2011, the Arctic vortex did temporarily move over parts of northern Europe, Russia, and Mongolia.
To put that in perspective, Mongolia's northernmost point is at 52 degrees north latitude, a good fourteen degrees of latitude south of the Arctic Circle. There's far more landmass in the northern regions around the Arctic Circle than around Antarctica, which means the Arctic ozone hole has far more potential than its counterpart to drift over high population areas.
That isn't good news, obviously, but it also shouldn't be a cause of undue alarm. The constant changing wind patterns mean the ozone hole doesn't stay over any one area for very long, and the short-term effects of the heightened ultraviolet radiation in that particular area is likely to be marginal compared to the more general increase in global UV exposure. Santee explains:
We do not want to raise undue alarm about the health implications of the locally thinner stratospheric ozone layer during this specific episode. In 2011, overhead ozone was much lower than normal springtime levels in the Arctic, but only for a brief interval in late March.
As for the more general biological effects...well, the honest answer is that scientists aren't sure yet. Ozone depletion has definitely led to increased ultraviolet exposure, and this may well be responsible for greater incidence of certain forms of skin cancer and cataracts, not to mention damage to plants, plankton, and a range of other animals.
However, these are such big trends and there are so many variables to consider that it's difficult to establish clear and concrete links between these health risks and the depletion of the ozone layer. Taking general, common sense steps to prevent unnecessary exposure to ultraviolet radiation is always a good idea - such as wearing sunscreen and sunglasses - but the Arctic ozone hole probably doesn't drastically increase the need for these precautions. They're just things we really ought to be doing anyway.
It would be nice to have a way to fix the ozone holes, but it appears the damage has pretty much already been done, and all we can do now is try to not make things worse and give the ozone layer time to repair itself. Humans probably did the most we could do to protect the ozone layer way back in 1987 with the passage of the Montreal Protocol, which banned ozone-depleting substances like CFCs. The problem is that all the CFCs already released into the atmosphere are still up there, and will like remain in the atmosphere for decades to come. As long as that remains the case, we're probably going to remain vulnerable to severe ozone depletion.
Since we're always interested in futuristic solutions, we asked Santee whether an answer might be found in geo-engineering or other novel approaches. Unfortunately, she doesn't see much hope of finding an artificial way to restore the ozone hole:
As for the possibility of geoengineering a solution to the ozone hole, in addition to the risk of unintended consequences (remember that CFCs were thought to be completely benign when they were first invented!), there are insurmountable practical and technical difficulties. A one-time injection of manufactured ozone to the stratosphere would not restore the natural balance because the added ozone would simply be destroyed in the same chemical processes that cause the ozone hole; thus any additions to the ozone layer would need to be large and continuous so long as anthropogenic (human-caused) ozone-depleting substances persist in the stratosphere.
Moreover, the energy required to produce the needed amount of ozone would be far too costly (it would be a significant fraction of the electrical power generated annually in the U.S.). Finally, although the stratospheric ozone layer is essential for life on earth, ozone is explosive and highly toxic in large quantities, and suitable methods for storing and delivering it to the stratosphere have not been developed (and would have associated energy and environmental costs themselves).
So it looks like there's no quick fix to this problem. We're just going to have to do our best to look after the Earth and deal with the effects of ozone depletion as they come. We can at least try to proceed with as much knowledge about this ongoing issue as possible.
"2008 Scotland - Cairngorms arctic plateau near Lochnagar" by Space & Light on Flickr.
"Ozone Cycle" by NASA via Wikimedia.
George Dobson photo by Dr A. Dziewulska-Losiowa via.
"Clouds Clearing" by US Geological Survey on Flickr.
"2011 Arctice Ozone Loss" by NASA Goddard Photo and Video on Flickr.
arctic, iqaluit, fog, mist, night, city" by ascappaturai on Flickr.
"arctic sunset" by artic pj on Flickr.