2015 Antarctic ozone hole 4th largest on record
The Antarctic ozone hole, acquired on Oct 2, 2015. Credit: NASA
Thursday, October 29, 2015, 7:47 AM - After apparently holding fairly steady with 2014 for most of the year, the Antarctic ozone hole in 2015 has now reached the fourth largest extent on record. Here's why.
According to NASA's Earth Observatory:
The ozone hole over Antarctica grew relatively large in 2015, according to data acquired by the Ozone Monitoring Instrument (OMI) on NASA’s Aura satellite and the Ozone Monitoring and Profiler Suite (OMPS) on the NASA-NOAA Suomi NPP satellite.
On October 2, 2015, OMI observed that the hole had reached its largest single-day area for the year. The image above, based on data acquired with OMI, shows the hole on that day. It spanned 28.2 million square kilometers — the fourth-largest area measured since the start of the satellite record in 1979. The largest single-day ozone hole recorded by satellite was 29.9 million square kilometers on September 9, 2000.
At a total extent of 28.2 million square kilometres, this year's ozone hole was surpassed by only by Sept 24, 2003 (28.4 million sq km), Sept 24, 2006 (29.6 million sq km) and September 9, 2000 (29.9 million sq km).
Why did the ozone hole grow so large this year? It was a combination of just how persistent ozone-depleting chemicals are in the atmosphere, and just how cold the atmosphere got over Antarctic during the past month.
A little background
The Montreal Protocol of 1989 was a world-wide agreement to phase out the use of chlorofluorocarbons (CFCs), chemicals mainly used as spray can propellants and refrigerants. The agreement was signed after intensive scientific research found that as CFCs spread through the atmosphere and made their way into the stratosphere, they were broken down into by-products that accelerated the destruction of ozone molecules. Since the stratosphere is the location of Earth's protective ozone layer, which absorbs and blocks harmful ultraviolet rays from the Sun, the loss of ozone there had serious ramifications for the future.
Ozone molecules, which are composed of three oxygen atoms joined together, are continuously formed and broken apart in the ozone layer. Under natural conditions, this continuous process of creation and destruction actually keeps a fairly stable concentration around the globe that has protected life at the surface for around 600 million years. The problem with the chlorine compounds that come from the breakdown of CFCs is that they accelerate ozone-destroying reactions and there is no acceleration of the ozone-forming reactions to compensate. Also, it just so happens that these ozone-destroying reactions need sunlight to occur, but they happen much more efficiently in colder temperatures. So, the Antarctic stratosphere, just getting the first sunlight of spring, while still extremely cold coming out of the dark winter, is an ideal time and place for these chemicals do to the most damage possible. Thus, the giant ozone "hole" develops over Antarctica each spring, which was first discovered in 1984.
Unfortunately, even though the source of CFCs has been greatly reduced or eliminated, due to the long-lived nature of these CFC by-products, there are still high concentrations of them in the Antarctic stratosphere. Thus, although it has been improving in recent years, giving some hope that it is now on the mend, the ozone hole still returns each southern spring, peaking in late September or early October.
Coupled with this, from late September through October of 2015, the stratosphere over the Antarctic was exceptionally cold, which produced a situation that was particularly favourable for the ozone-destroying reactions. This made the situation worse, causing the ozone hole to increase in size to the fourth largest extent on record.
In addition to the persistent CFC by-products and weather conditiions, NASA scientists have recently discovered that hydrofluorocarbons (HFCs), the chemicals produced to replace the phased out CFCs, are also having an impact on ozone concentrations.
Watch below: Widely used coolants contribute to ozone depletion, by NASA Goddard
According to this new study, several types of HFCs are potent greenhouse gases, so while their concentrations are quite low compared to other GHGs, they will cause some warming, and thus will indirectly add to the problem of ozone depletion.
The effect of HFCs on ozone depletion is two-fold. First, by warming the troposphere - the lowest level of the atmosphere - this causes stronger upwelling of ozone-poor air parcels into the stratosphere. Second, by helping to trap infrared radiation in the troposphere, that radiation is prevented from reaching the stratosphere. Thus, while the troposphere is gradually warming over time, the stratosphere is cooling, and a cooler stratosphere is more favourable for the chemical reactions that deplete ozone.
The researchers' model found that this will produce an additional 0.035 per cent decrease in ozone, over top of the decrease from existing CFC by-products, by 2050. While this effect is quite weak compared to the impact of CFCs, they have still been found to have a measurable contribution to ozone depletion that is much larger than previously thought. The effect of HFCs on global warming is also of concern, as they predict that, with continued production, these chemicals could contribute as much as 20 per cent of the effect of carbon dioxide by 2050.
"What the paper demonstrates is that when you put this much of an infrared radiation-absorbing material in the stratosphere, even though it nominally does not destroy ozone in the same way that mainline ODSs [ozone-depleting substances] do, it’s going to make a difference — it's going to start changing things," said David Fahey, director of NOAA’s Earth Systems Research Laboratory, who was not involved in the study. "It adds a new dimension of thinking that stratospheric scientists need to be aware of as they discuss these matters with policy makers."