1980s ozone layer to return by... 2050

Submitted by Matthew on 18 October, 2017 - 12:35 Author: Les Hearn

Good news! The ozone hole is shrinking at last, a rare success for collective action in response to scientific evidence.1 Unfortunately, it will take until 2050 to return to its 1980 levels.

This is because the chemicals largely responsible for its depletion are very stable and those already released will persist in the atmosphere until then, even if no more emissions take place. It’s 30 years since the signing of the Montreal Protocol which aimed to tackle the problem of the accelerating destruction of the ozone layer by chlorofluorocarbons (CFCs).

Ozone in the stratosphere absorbs most of the Sun’s ultraviolet radiation (UVR) and without it life would be difficult or impossible except several metres below the surface of the oceans. Ozone (O3) is made from oxygen (O2) by the action of UVR in the stratosphere. But for there to be oxygen in the stratosphere there first had to be oxygen in the lower atmosphere and this only appeared when Earth was about half the age it is now, with the evolution of photosynthesis by bacteria in the oceans.

These produced oxygen as a waste product which gradually began to accumulate in the atmosphere. Ozone started to accumulate also and by half a billion years ago was absorbing enough UVR for the land to become habitable. Scientists only became aware of these facts with: A. the prediction and then discovery of different types of light (radiation) with different wavelengths; B. the development of spectroscopy, the study of how matter absorbs and emits light; and C. the understanding of how hot objects emit energy in the form of light.

These were mostly the result of curiosity-driven research. It was realised that the Sun should emit radiation of different wavelengths in the proportions predicted for the spectrum of a “black body” of the same temperature (about 5500 degrees Celsius). Spectroscopy showed that it did, with the puzzling exception of a region of wavelengths shorter than 310 nanometres, just beyond the violet region. This, the UV region, was about 1% of the predicted intensity. This meant that about 99% of UVR was being absorbed by something and an exhaustive search of likely chemical substances found that ozone was largely responsible.

The amount of ozone differs in different parts of the world and at different times of year, as does the intensity of UVR, so the amount of UVR reaching the ground is variable. In general, UVR is highest when the Sun is higher in the sky, i.e. in equatorial regions and during summer in northern and southern regions.

The UVR that gets through can be damaging to life, including humans in whom it causes sunburn, cataracts, and potentially fatal skin cancers. Many humans have melanin pigment in their skin which can absorb UVR before damage can occur but lighter-skinned people in high-UVR regions are at risk. Australia and New Zealand have the highest rates of melanoma in the world. It was therefore alarming to learn in 1985 that there was a great hole in the ozone layer above Antarctica. However, the story started earlier.

Refrigerators use the evaporation and condensation of liquids to transfer heat from the contents to the outside (you may have noticed warmth from the back of a fridge). Early fridges used easily liquefied gases such as methyl chloride, ammonia or sulfur dioxide, but these were toxic if released. Chemist Thomas Midgley2 developed the efficient synthesis of chlorofluorocarbons (CFCs) around 1930 and proposed their use as safe refrigerants. CFCs are very unreactive which is excellent for a refrigerant. Midgley demonstrated their safety by inhaling some and blowing out a candle. However, if released when a fridge is damaged or scrapped, their very stability means that CFCs persist in the atmosphere, eventually reaching the stratosphere. Here the problem starts: a CFC molecule such as Freon (Cl2F2C) is hit by a UV photon and a chlorine atom (Cl) is knocked out.

If this collides with an ozone molecule, it grabs an oxygen atom to make a ClO molecule, leaving an ordinary oxygen molecule that doesn’t absorb UVR. The ClO collides with another ozone molecule, making more O2 and regenerating the original Cl atom…which can now repeat the process with more ozone. The Cl is thus a catalyst for the breakdown of ozone. Each cycle removes two ozone molecules and there can be thousands of cycles before the Cl atom collides with something else and the process stops.

This was realised in the ‘70s but no-one knew if the effect was significant until the late Joe Farman and colleagues found a massive hole in the ozone layer above Antarctica. The levels had dropped by some 40% in about ten years. Farman had been measuring the levels for about five years, first fearing that his instruments were faulty. NASA had failed to detect the drop as its computer software was programmed to ignore “unusual” readings.

The clear threat was that, as thinning of the layer spread, organisms would be affected by the increased UVR, particularly UVB. This would affect plant growth, harm populations of plankton in the upper levels of the oceans, and cause increased skin cancers and cataracts. Australia would be the first to be affected, with potential epidemic levels of skin cancer. Due to different weather patterns, the Arctic had not yet developed an ozone hole but would eventually if nothing changed as the amount has also declined. Farman published his results in 1985 and, despite the opposition of the chemicals industry, the Montreal Protocol phasing out CFCs was signed in 1987. Readers may be surprised to learn that Margaret Thatcher played a positive role in this.3

It will take a long time for the ozone layer to return to its original thickness. In the meantime, we must make sure that governments and businesses adhere to the Montreal Protocol. But there is another problem: CFCs are actually more potent “greenhouse” gases than carbon dioxide and some of their ozone-friendly replacements, such as hydrofluorocarbons (HFCs), are even worse. Phasing out CFCs has already reduced the rate of global warming.

One option is to amend the Montreal Protocol to include HFCs (they are already in the Kyoto Protocol) but the alternatives also have their own problems. Propane/methylpropane mixtures are very effective refrigerants but are flammable (but then so is methane, piped to most houses in the UK).

Notes

1. Still fresh at 30: Ozone hole healing, Montreal Protocol takes on climate change

2. Thomas Midgley had “form.” In 1921, he showed that tetraethyl lead when added to petrol prevented the damaging phenomenon of engine “knock.” Despite knowing of its toxicity Midgley insisted that it was safe. It was marketed as “Ethyl” with no mention of lead. Midgley then inadvertently initiated the destruction of the ozone layer through CFCs. It has been said that he “had more impact on the atmosphere than any other single organism in Earth’s history.”

3. You won’t often hear a good word from me about Margaret Thatcher but with a Chemistry degree and PhD, this may have partly explained why she protected the funding of the British Antarctic Survey. She could also understand the scientific evidence about CFCs and supported the Montreal Protocol. She also supported UK’s membership of CERN and the establishment of the IPCC to research climate change.