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Ozone Depletion in the Atmosphere

By Martin Britton
July 21, 2014


Table of Contents:

- The Twist in the Climate Tale – Ozone, its Rise, and its Fall
- Ozone: the other oxygen
- Connecting the dots
- Losing the ozone – losing our cool?
- Ozone's long-winded reach
- Down below – the insidious effects of tropospheric ozone
- A tale of two tropospheric worlds

The destruction of stratospheric layer of ozone by chlorofluorocarbons or CFC's and the increase of tropospheric ozone

The Twist in the Climate Tale – Ozone, its Rise, and its Fall

Ozone is a chemical that's probably most famous for not being there at all. Mention ozone, and the the 'ozone hole', a worrisome gap in our planet's protective shield of stratospheric ozone, springs immediately to mind. Concerns about a hole in the ozone layer (or holes – there are actually two, one at each pole) first hit the headlines back in the 1980's.
 
 
Figure 1. Annual records of ozone hole size and minimum stratospheric ozone levels (Source NASA)
 
The ozone layer, found between 20km and 50km above the Earth (in what is called the stratosphere) is one of the atmosphere's good guys. There, it helps protects us from the Sun's ultraviolet (UV) rays. A hole in that layer is decidedly bad news for sun-lovers and outdoor workers alike – too much UV can cause skin cancer.
 
But this molecule isn't only found in the stratosphere's ozone layer. And it's not just the lack of it that's causing us problems. Man's activities are also increasing ozone in other places, with serious effects on health. It may even be (worryingly) topping up global warming's growing heat load.
 
So the ozone story is one that needs a careful telling, so as to fully understand all its ramifications. But what exactly is this mysterious, and apparently influential, compound?
 
Ozone: the other oxygen
 
The answer is simple. Ozone is oxygen. Or to be more precise, oxygen that travels around in threes, rather than the pairs of the more familiar oxygen we breathe.  While that is referred to as O2, ozone, with its 3 atoms of oxygen locked tightly together, is jotted down as O3.  In high concentrations, ozone is a pale blue gas, and one that's very dangerous to life, as it can be very reactive.
 
It can be concentrated locally to high values. Then it can be smelt, reeking strongly of chlorine. But for the most part, it is found in low concentrations, usually less than 100 ppb. It readily breaks down, especially in the presence of certain compounds. The most notorious of these are the chlorofluorocarbons, or CFCs.
 
These man-made compounds, once commonly used as refrigerants and solvents (and in aerosols) triggered the 'ozone hole' problem some thirty years ago. Made up of carbon, fluorine and chlorine, when lofted high into the stratosphere CFCs are broken down into their elements, by UV light. The chlorine so-produced is a real 'ozone destroyer', acting as a catalyst to turn O3 back into O2.
 
Connecting the dots
 
After scientists noticed our planet's ozone layer was thinning dramatically – especially over the poles – they made the connection to man-made CFCs. They also discovered why the poles are affected the worst. Icy clouds, called the 'polar stratospheric clouds', form extensively there. The ice crystals in these clouds act as accelerants for the ozone destruction process, by helping to make those catalyst chlorine compounds.
 
The damage is worst in the spring months in Arctic and Antarctic, as the Sun brightens. Strong UV light is needed to complete the destruction of the ozone. Once scientists had pin-pointed CFCs as the cause, such compounds were swiftly banned across the globe. But because CFCs are so long-lived, damage to the ozone layer has continued – and is still far from being undone.
 
Losing the ozone – losing our cool?
 
As a result, we are still living with the effects of the ozone depletion in the stratosphere. But scientists now think those effects extend far beyond worries about UV (and the need to slap on more sun-cream). The most obvious of these is that the loss of stratospheric ozone at the poles seems to be having a mild cooling effect there. That's because ozone, like CO2, is a greenhouse gas. It absorbs the sun's energy, and then radiates out heat.
 
So with less ozone floating around in the stratosphere, there should be a cooling at the top of the atmosphere – something scientists from NASA have observed (see Figure 2 below) . But this is a very slight effect. The IPCC have placed[i] this cooling at just -0.05 W/square meter. That's an awful lot less than the +1.6 W/sq. M of warming that increased CO2 levels have produced.

 
Figure 2: Changes in stratospheric temperature, 1979-2014. Anomalies ( departure from average). Source: National Climatic Data Center.
 
Ozone's long-winded reach
 
It turns out, though, that changes in stratospheric ozone are having an even more complex, and significant, effect than that of the small-scale cooling from its depletion. It seems that lower ozone levels also impact another part of our climate system – the global  wind fields, and in particular those blowing around Antarctica. Scientists from Australia[ii] have recently show the winds circulating around this icy continent  are stronger now than they have been for a 1,000 years.
 
And while much of the blame has been pointed at increased levels of CO2, some of it has also landed at the foot of lower stratospheric ozone. Those more powerful winds are having odd effects, too, such as making Australia drier. And they are also bringing a little packet of extra planetary warming in their wake. That's because the strengthened winds that are circling more tightly around the south pole are shifting major cloud formations further south too.
 
Recent work in the Geophysical Research Letters[iii] show that this actually allows more of the sun's energy to hit the planet's surface. So while less stratospheric ozone means a cooler stratosphere, the effect on the winds and clouds produces the opposite – a warmer surface, by as much +0.2 W/sq. m.
 
Down below – the insidious effects of tropospheric ozone
 
The complexity doesn't stop there. Ozone is also being affected by man's activities much further down in the atmosphere – in the troposphere. That's thanks to the malign influence of our burning of fossil fuels. The combustion gases, including nitrogen oxide, carbon monoxide and volatile organic compounds, react with water and CO2 in the presence of UV light. And one of the products of these reactions is our friend ozone. But at these lower levels, it is anything but friendly.
 
It causes the respiratory system to be irritated. Breathing can be affected, while asthma is made worse. In the long-term, the ill-effects of tropospheric ozone can be life-threatening. The World Health Organization reckoned, in a 2008 study[iv], that up to 21,000 people die prematurely each year in Europe alone, thanks to increased tropospheric ozone.
 
Higher levels of this ozone are also causing local warming, thanks to ozone's strong greenhouse gas property. It is worth pointing out, though, that increased levels of tropospheric ozone are very much a restricted, localized effect. Its formation depends on the local sources, and types, of fuel-burning and the pollution produced. It also depends on the weather, and on how effective pollution control measures have been.
 
A tale of two tropospheric worlds
 
In fact, in much of the developed world, tropospheric ozone concentrations have been reduced, as a result of such measures. But in the developing world they appear to have risen significantly, even if scientist can't pinpoint by how much. Overall, though, there's no doubt that tropospheric ozone is at higher levels than those seen in pre-industrial times.
 
 
Figure 3. Radiative forcings of climate between 1750 and 2005. Source: IPCC AR4
 
The IPCC[v] places the warming effect of tropospheric ozone at +0.35 W/ sq.m (see Figure 3 above). That is much more significant than the slight cooling from the depletion of stratospheric ozone (-0.5 W/sq.m). And it is higher still than proposed the warming effect (of +0.2 W/sq.m) caused by the shifts to the planet's winds and clouds by stratospheric ozone.
 
So how will the story of ozone play out in the wider drama of global warming from CO2? If the evidence scientists have gathered so far is anything to go by, then 'confusedly' is probably the best guess. To start with, it is thought that, as the CFCs slowly disappear from the atmosphere, then the stratospheric 'ozone hole' will start to fill in.
 
That will halt the poleward move of winds and clouds in the southern hemisphere, and so cut back on that particular warming effect. But equally, if higher levels of tropospheric ozone – thanks to man-made pollution – aren't cut back, then low-level, local warming will continue. It could conceivably even increase.
 
And remember that global warming isn't just being pushed along by changes in ozone. Some effects of global warming – like increased numbers of wildfires in forests – push the other way. They could increase low-level ozone, setting up a positive feedback loop that would further boost warming. The chapters yet to be written on the ozone story look like being just as convoluted as those that have already been inked.
 

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[ii]   Evolution of the Southern Annular Mode during the past millennium. Nature Climate Change,  Abram, N. J. et al (2014)