The dark side of ozone recovery

A recent statement by UNEP/WMO has claimed the size of the ozone hole is starting to decrease, but how certain can we be that this is the start of a recovery and what does this mean for the future of climate change? Around 300 scientists have contributed to a report by the UN which not only claims the ozone layer is starting to increase in thickness, but the size of the ozone hole is no longer expanding. This has been hailed as a great triumph for the environment, but with research suggesting that the ozone hole is actually helping to circumvent the effects of global warming in Antarctica, the picture may be more complicated than initially predicted. The ozone layer protects the Earth from the damaging effects of the sun’s ultraviolet rays. These UV rays create a whole host of problems including skin cancer, cataracts and damage to the immune system. During the 1980s, the increased use of chlorofluorocarbons (CFCs) in products such as refrigerators, air conditioners and hairspray caused an almighty problem for the environment: ozone depletion. The potential harmful effects of CFCs have been known since the 1970s, when scientists found that the UV rays break down chemical bonds in CFCs, releasing chlorine atoms which, in turn, destroy ozone molecules. However, it wasn’t until 1985 that a team at the British Antarctic Survey (BAS) shocked the world by announcing the presence of a huge hole in the ozone layer above Antarctica. The threat of health defects - caused by the ozone hole – resonated with the public on a personal level and sparked a global debate on the issue. With pressure mounting, the majority of UN member states agreed to sign the Montreal Protocol in 1987, which set out the plan to phase out the use of CFCs. This is seen as one of the greatest successes in international collaboration to tackle a global environmental problem. As a direct result of the Montreal Protocol, CFC levels in the atmosphere are slowly decreasing; the ozone hole has stopped growing and it now appears that it may even be shrinking. Data has been collected from numerous ground-based and satellite sources, including BAS and NASA. The BAS ground-based data, collected at the Halley research station in Antarctica, shows that since the 1990s there has been an upward trend in ozone levels. The UN report published in September 2014 by the United Nations Environment Programme (UNEP) and the World Meteorological Organisation (WMO) states that the “actions taken under the Montreal Protocol have led to decreases in the atmospheric abundance of controlled ozone-depleting substances and are enabling the return of the ozone layer toward 1980 levels.”  This review is the first comprehensive update on the subject in four years. Is the ozone problem fixed? Professor Martyn Chipperfield, whose research includes ozone depletion at the University of Leeds, warns that we must be cautious: “There is a lot of natural variability in atmospheric ozone so detecting unequivocally the first upturn in ozone due to decreasing CFCs is not straightforward.” The wrong combination of geological and meteorological circumstances could cause further expansion of the ozone hole and even if the current trend continues, it is likely that the hole will persist for at least another 50 years. It is expected that only by 2080 will we see a return to 1950s levels. However, as Chipperfield says: “The observed upturn in regions such as the Antarctic is fully consistent with what we would expect given the CFC decrease.” So it seems that UNEP/WMO are fully justified in stating that the ozone layer recovery has indeed begun: “This success is a clear case of our ability to undo the damage we have caused.” And now for the bad news – scientists have suggested that there may be some troubling side effects to the closing of the ozone hole. Firstly, the chemicals chosen to replace CFCs, while helping to decrease ozone depletion have significantly contributed to global warming. Researchers at the University of Leeds have also proposed that the closure of the ozone hole could lead to accelerated warming in the southern hemisphere.¹ They found that the high-speed winds beneath the ozone hole whip up large amounts of sea spray that can then form reflective clouds. These clouds reflect the sun’s rays, cooling the local environment and effectively cancelling out warming from carbon emissions. The findings caused quite a stir as they suggest that ozone depletion, an adverse, man-made effect on the environment has shielded most of Antarctica from another – global warming. As the ozone layer recovers, this feedback mechanism could decline in effectiveness leading to accelerated warming in the southern hemisphere. The consequences could be severe; warming in the southern ocean will cause changes in the Antarctic ecosystem. Rising temperatures may lead to ‘alien’ species migrating into the region, competing for resources with native species and decreasing the biodiversity in the Antarctic seabed. The melting of sea ice due to accelerated heating can also have disastrous effects on the local wildlife – seals, for example, rely on the ice to pull themselves out of the water to rest during feeding. So, could it be that the ozone hole is a good thing? The answer is still no. Despite the cooling effects caused by the westerly winds, those same winds have affected the Southern Ocean’s ability to absorb carbon dioxide (previously the largest sink for CO₂). The strong winds are also trapping the cold air over Antarctica, leading to warming in other continents, especially the Arctic and Australia which, it is predicted, will experience increased droughts over the next few decades. Jonathan Shanklin, a member of the BAS team who made the ozone hole discovery, warns: “Trying to stop global warming by enhancing the ozone hole would be a futile move, as the gases that cause the ozone hole are also greenhouse gases.” The reason for the ozone layer recovery is the removal of CFCs from the atmosphere; as these are also greenhouse gases, this can only be a good thing; indeed the consequences of unabated CFC growth would have been disastrous for the environment. There is a general consensus among the environmental experts that the removal of CFCs from the atmosphere has done more to prevent future climate change than the cooling effect due to the ozone hole, which is seen as a side effect and not a net gain. When it comes to controlling both ozone depletion and global warming, the answer seems simple. “The only way to deal with both ozone destroying and greenhouse gases is not to put them in the atmosphere in the first place” says Shanklin.

Ozone depletion, an adverse, man-made effect on the environment has shielded most of Antarctica from another – global warming.

Unsurprisingly, purging the atmosphere of greenhouse gases and ozone depleting substances is anything but simple. With the phasing out of CFCs came a replacement in the form of ‘safer’ HCFCs (hydrochlorofluorocarbons) and HFCs (hydrofluorocarbons). HCFCs are broken down in the lower part of the atmosphere and so they pose a much smaller risk to the ozone layer. Unfortunately, they are potent greenhouse gases and are consequently undergoing a similar phasing out process. HFCs, on the other hand, don’t contain chlorine, so don’t destroy ozone. They do contribute to global warming, albeit by an order of magnitude less than CFCs. The main concern with these chemicals is the effect of certain by-products, such as trifluoroacetate, on aquatic environments. However, to date, these are the best option for reducing ozone depletion and global warming. Nitrous oxide (N₂O) has been identified by many, including Chipperfield, as the current most important ozone depleting substance and this is likely to remain the case throughout the century. N₂O is produced naturally in the soil during the microbial process of nitrification and denitrification. Once released into the atmosphere, most N₂O molecules are broken down into harmless nitrogen and oxygen gas although some is converted to NO, a substance capable of destroying ozone. It is worth noting here that N₂O does not affect the ozone hole but rather decreases the thickness of the global ozone layer. Nitrous oxide is also a formidable greenhouse gas that has 298 times the ability of CO₂, per molecule of gas, to trap heat in the atmosphere. Even though N₂O is naturally occurring, it is thought that human contributions account for around a third of global levels. To put this into perspective, human emissions of N₂O amount to 10 million tonnes per year, compared with 1 million tonnes from CFCs at the peak of their emissions. Increased agricultural demands, the use of nitrogen-based fertilisers, fossil fuel combustion and sewage treatment plants have been branded as the main culprits. N₂O is currently not regulated by the Montreal Protocol, and the solution may not be as straight forward as cutting down emissions, which would, as Shanklin says, be “like putting a sticking plaster on a broken leg.” It has been found that nitrogen and chlorine counteract each other’s effects on ozone – as CFCs are being removed from the atmosphere, nitrous oxide will become 50% more potent. The situation becomes even more muddled when taking into account other greenhouse gases, such as CO₂ and methane (CH₄), which actually increase the thickness of the ozone layer but have detrimental effects on global warming. Clearly, the effect we are having on the ozone layer and its role in climate change is incredibly complicated. The recent UN summit in New York, along with the thousands of protestors who took to the streets to demand action, has once again brought the topic to the forefront of the public’s attention, with increased pressure on world leaders to take action. The only way to control emissions of greenhouse gases and ozone-depleting substances is to tackle them synergistically. The UN report makes this clear: “As controlled ozone-depleting substances decline, the evolution of the ozone layer in the second half of the 21^st century will largely depend on the atmospheric abundance of CO₂, N₂O and CH₄.” If one thing is to be taken from the depletion of the ozone layer it is the unequivocal success of the Montreal Protocol. The recovery of the ozone hole is a testament to the sound scientific research and the nerve of the politicians to act on that science. This should encourage current world leaders to take a similar approach to rein in climate change. References Korhonen, H., Carslaw, K. S., Forster, P. M., Mikkonen, S., Gordon, N. D., Kokkola, H. 2010. Aerosol climate feedback due to decadal increases in Southern Hemisphere wind speeds. Geophysical Research Letters. 37: 1-6. Author Rebecca Dey is an Editorial Intern at Laboratory News and is currently undertaking a PhD in Polymer Chemistry at the University of Manchester.