The Truth about the Ozone Layer

The first truth about the ozone hole is that we can make a difference. We made a difference in its original creation. We are making a difference now in turning the tide.

On September 16, 1987, 196 nation-states committed to ending ozone depletion by phasing out the use of chloroflurocarbons (CFCs) and hydrochloroflurocarbons (HCFCs). The CFC Phase-Out Management Plan is complete: CFCs can no longer be found anywhere in the commercial market. The HCFC Phase-OUt Management Plan is scheduled to begin in earnest in 2013, with phasing out to be completed by 2030. Operation Sky Hole Patching has been implemented to address the problem of illegal trade in these substances.

Thanks to the Montreal Protocol, global CFC reduction has resulted in a 10% drop in the (atmospheric) effective equivalent chlorine level between the 1994 peak and 2008. This is still a drop in the bucket: yet it is enough that the ozone layer is now finally starting to stabilise. Ozone depletion has not yet stopped, but it is noticeably slowing down.

The second truth about the ozone hole is that many of us were highly reluctant to believe in our own power to influence our environment. Some of us still are. It took a long time for us to accept that our actions were making a difference, and not only in the intended, positive ways.

It took decades of research and argument to demonstrate convincingly that human-created CFCs were indeed having any kind of significant environmental impact. Frank Sherwood Rowland and Mario Molina were the first to connect the known reaction between chlorine and ozone with CFCs. Their 1974 seminal paper eventually won them the 1995 Nobel Prize for Chemistry, shared with Paul Crutzen, who had developed much of the underlying theory. Yet so determined was DuPont’s campaign against these findings that at one point, Rowland nearly lost his job over his public statements.

The arguments popularised by DuPont in the 1970s and 1980s still resound in some circles today. They take three primary forms:

1. Natural sources of chlorine far outweigh any human effect.
2. Man-made CFCs are heavy, so they cannot reach the stratosphere in any statistically significant numbers.
3. Any perceived loss of ozone is due to a natural annual cycle. There is no real ozone “hole”.

It is true that natural sources of tropospheric chlorine are as much as five times greater than manmade sources. Yet for the ozone layer, only stratospheric chlorine levels matter: and there the reverse is true, since the levels of stratospheric chlorine depend on the kinds of long-lived chlorine compounds not seen in nature. There is only one stratospheric halocarbon which has a predominantly natural source: methyl chloride. Together with hydrogen chloride (HCl), it is responsible for 20% of all stratospheric chlorine. The rest comes entirely from man-made CFCs and HCFCs.

It is also true that man-made CFCs are heavy. The lightest of them is twice as heavy as the hydrogen chloride ejected into the atmosphere by volcanoes. Oxygen and nitrogen molecules are even lighter. Yet the molecular weight of a gas makes no difference in an open volume where the air movement (wind) suffices to mix them. Much more important in this case is the extremely long life of a CFC molecule: which enables it to survive long enough to be mixed all the way up into the stratosphere. Once they are up there, they will remain up there. Over time, these long-lived molecules will inevitably outnumber their much shorter-lived natural counterparts.

The third argument also holds a seed of truth. There is indeed a natural thinning of the ozone layer over magnetic polar regions every year, of roughly 30%. This natural polar ozone cycle was first observed in the Antarctic by G.M.B. Dobson in 1956.

This natural cycle should not be confused with the ozone thinning caused by CFC-sourced atmospheric chlorine. In the natural cycle, ozone levels fall gradually during the winter, when there is no sunlight, and begin rising again in spring when the polar vortex collapses. The CFC-linked depletion has a completely different pattern: the greatest depletion of ozone happens in the spring, shortly after sunrise – and it is not gradual, but quite abrupt. This second depletion happens because during the sunless winter, free chlorine atoms and chlorine monoxide are able to pool within polar stratospheric clouds, which can only exist in polar winter. In the spring, the clouds dissipate, releasing the free chlorine to interact with the sunlight and the ozone. As a result, already low winter ozone levels are halved.

It should be emphasised that neither the natural cycle nor the CFC-aggravated depletion results in a true “hole”, not as we usually understand the word. The natural cycle results in a regular thinning and thickening of the ozone layer which is strongest over the magnetic polar regions. During the seasonal CFC-linked depletion which follows the natural low, ozone in the lower stratosphere is utterly destroyed, causing a “hole”. Yet even at the magnetic poles during the time of greatest overall depletion, roughly a third of the usual amount of ozone still remains in the upper stratosphere.

The most effective kind of spin is built on minimal truths, carefully spread so as to cast an obscuring veil across the full picture.

The final hard truth about the ozone hole is that we will not see a statistically significant decrease in its size until at least 2024. We cannot. The interaction between ozone (O3) and the free chlorine atoms (Cl) resulting from CFC breakdown is a self-perpetuating cycle:

Cl + O3 -> ClO + O2
ClO + O3 -> Cl + 2 O2

A single free chlorine atom will keep moving in and out of a loosely bound chlorine monoxide molecule, constantly producing stable oxygen molecules but also destroying ozone each and every time. It can keep on destroying up to 100,000 ozone molecules for as much as two years.

Yet if we continue along the track established by the Montreal Protocol, we can hope for ozone levels to return to 1980 levels by around 2070. It will have been nearly eighty years of CFC-free recovery to balance out a single decade of careless CFC use: but we will have turned the tide. We will have made a real, positive difference for our children and grandchildren. We will have given them back a world where they will once more be able to step outside under the warm sun without worrying about UV levels and sunscreen.