An Arctic Ozone 'Hole' - a new threat?

I was looking at the news today and came across a very short article from the Independent, which, despite it's size, had been the sixth 'most-read' Environmental article in the Independent during 2011. I'm not sure whether it's the topic that drew the most readers, or the title: 'GIANT ozone HOLE found above Arctic' (own capitalisation!). Due to this likely exaggeration and the fact that I hadn't heard this apparently popular news story, I decided to have a look at the original paper the article was based on. The paper was published in Nature as is titled 'Unprecidented Arctic Ozone Loss in 2011' by Manney et al 2011.* They reported a record low ozone level of 220-230 Dobson Units (DU) recorded in March 2011, but could you day this was a 'huge hole'? Also, was this anything to do with climate change or this just a random and freak occurrence?

Polar Stratospheric Clouds (PSCs) photographed in Sweden
http://earthobservatory.nasa.gov/IOTD/view.php?id=622

According to the authors, the possibility of significant ozone 'holes' in the Arctic has been debated for decades, with some arguing that higher temperatures in the Arctic than the Antarctic prevented such formations. The basic
mechanisms of chemical ozone depletion are that low temperatures in the stratosphere (high in the atmosphere!) in the Arctic encourage the formation of Polar Stratospheric Clouds (see picture) made up of water vapour and Nitric Acid (HNO3). These clouds and other aerosols provide surfaces for the conversion of Chlorine into reactive forms such as Chlorine Monoxide (ClO) which act as catalysts for Ozone removal. It's this chemical process that is thought to be behind the large reduction in Ozone concentrations in the Stratosphere during the 2010-11 winter.

This process usually only lasts around 2-3 months in the Arctic when the temperatures are cold enough. During the 2010-11 winter however, an unusually strong polar vortex in the Stratosphere meant colder conditions lasted for much longer, around 4 months, a similar pattern to the Antarctic which has a large Ozone 'hole' during the winter. This led to an unprecedented level of 'Chlorine activation' as described above, including Chlorine Monoxide. Shown in the graph below as the thick red line, 2011 high ClO levels stuck around long after levels declined in previous 'cold' years (eg 2004-5 shown in Orange) and chlorine was only 'deactivated' in mid-March compared to February in 'normal' years. It seems to follow patterns of typical Antarctic ClO levels (shown in dark grey) more closely than Arctic patterns (light grey), strongly suggesting it was this chemical process that caused the low levels of ozone in 2011.

       1 Dec                               1 Jan                                1 Feb                          1 Mar                             1 Apr              _
Stratospheric Chlorine Monoxide levels during the Arctic winter 2010-11 shown in Red. Previous 'cold' years are
shown as Orange (2004-5) lines and Blue (1995-6) and Purple (1996-7) trianges. Direct from Manney et al 2011

Also, the levels of Ozone itself (O3) followed a typical Antarctic pattern more closely than an Arctic one, as seen in the graph below:

Stratospheric Ozone (O3) levels during the Arctic 2010-11 winter. Key as above. Direct from Manney et al 2011.

Perhaps a better indicator, which allows direct Antarctic/Arctic comparison, is the fraction of the vortex area with ozone concentration of <275 Dobson Units (DU). As can be seen below, during March, the area of the vortex where DU was <275 was approaching the proportion seen in the Antarctic, which occur with much lower temperatures and cause a severe and prolonged Ozone 'hole' every year.

Direct from Manney et al 2011

It's clear then that the levels of Ozone depletion seen during the 2010-11 winter was significant, and if repeated could cause significant biological impacts through UV levels. In my opinion, however, to call it a 'Giant hole' is stretching it somewhat! Although the levels of ozone were reduced, the area affected was around 60% less than in the Antarctic, and the minimum levels of 220 DU as we've seen above are still nowhere near typical Antarctic Levels.

Is it likely to happen again though? It's possible that what happened this year was a 'perfect storm', when all the factors combined to cause this anomaly. There are suggestions that greenhouse gas emissions may be causing a rise in these events, firstly by altering atmospheric composition and therefore it's radiative properties which affect Stratopheric vortex formation, and secondly by cooling the stratosphere itself. Stratospheric temperatures have been dropping over the last 30 years or so on average, a trend I found surprising. Also, the fact that this ozone event happened with temperatures not much colder than previous years makes a repeat more likely. That being said, temperature is not the only factor, as we've seen; duration of temperature is likely just as important.

It's difficult to say what really caused this anomaly to be so much worse than during other 'cold' winters, and it's even more difficult to model and predict, so it's likely we'll only know of such an event after it happens. After looking into it, though, I'm not convinced it's a great threat, and is unlikely to be on the same scale as other climate change induced threats. As always, more study is required!
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*On a side note, I'd really encourage you to follow the link and check out the interactive pdf version available under 'pdf options', it looks like a great way to include supplementary info and links to related articles