Global ozone has been declining since the 1970s due to human-made chemicals. Since these were banned, parts of the ozone layer have been recovering, particularly at the poles. However, new research, recently published in the journal Atmospheric Chemistry and Physics, showed that the bottom part of the ozone layer at populated, lower latitudes is not recovering. The exact cause is currently unknown.

The atmosphere is only a thin film when you look at earth from outer space. The protecting ozone layer is recovering at the poles, but unexpected decreases in parts of the atmosphere may be preventing recovery at lower latitudes. (Credit: NASA)
The atmosphere is only a thin film when you look at earth from outer space. The protecting ozone layer is recovering at the poles, but unexpected decreases in parts of the atmosphere may be preventing recovery at lower latitudes. (Credit: NASA)

Ozone is a gas that naturally forms in the stratosphere, between 10 and 50 km altitude. It is mainly produced in tropical latitudes and distributed around the globe. The resulting ozone layer resides in the lower part of the stratosphere and absorbs much of the harmful UV radiation from the sun, which, if it reached the earth's surface, would cause damage to DNA in plants, animals and humans. Scientists had theorized since the 1970s about the chemistry that could lead to ozone depletion, but it was only in the mid-1980s that they recognised that chlorofluorocarbons, called CFCs which were used in refrigeration and aerosols, were indeed destroying the ozone layer. The effect was worst in Antarctica, where the ozone 'hole' formed.

The ozone layer protects the earth from the ultraviolet rays from the sun.  If the ozone layer is depleted harmful UV radiation can reach the earth causing skin cancer and eye cataracts in humans and damage plants and materials. (Credit: Twenty Questions and Answers About the Ozone Layer, Scientific Assessment of Ozone Depletion)
The ozone layer protects the earth from the ultraviolet rays from the sun. If the ozone layer is depleted harmful UV radiation can reach the earth causing skin cancer and eye cataracts in humans and damage plants and materials. (Credit: Twenty Questions and Answers About the Ozone Layer, Scientific Assessment of Ozone Depletion)

In 1987, the Montreal Protocol was signed, which led to the phase-out of CFCs and, recently, the first signs of recovery of the Antarctic ozone layer. Current measurements showed that the decline in chlorine, has resulted in about 20 percent less ozone depletion during the Antarctic winter than there was in 2005 — the first year that measurements of chlorine and ozone during the Antarctic winter were made by NASA’s Aura satellite. “We see very clearly that chlorine from CFCs is going down in the ozone hole, and that less ozone depletion in Antarctica is occurring because of it,” said Susan Strahan, an atmospheric scientist from NASA’s Goddard Space Flight Center in Greenbelt, Maryland. These research results were published in the journal Geophysical Research Letters in January 2018.

Measurements from satellites in 2017 showed the hole in Earth’s ozone layer that forms over Antarctica each September was the smallest observed since 1988. At its peak on 11 Sept 2017, the ozone hole still extended across an area nearly two and a half times the size of the continental United States. The purple and blue colours are areas with the least ozone. (Credit: NASA/NASA Ozone Watch/Katy Mersmann)
Measurements from satellites in 2017 showed the hole in Earth’s ozone layer that forms over Antarctica each September was the smallest observed since 1988. At its peak on 11 Sept 2017, the ozone hole still extended across an area nearly two and a half times the size of the continental United States. The purple and blue colours are areas with the least ozone. (Credit: NASA/NASA Ozone Watch/Katy Mersmann)

Despite this success story in Antarctica, another research study recently found that stratospheric ozone is not recovering in the same way at lower latitudes, between 60°N and 60°S, due to unexpected decreases in ozone in the lower part of the stratosphere. The co-author of this study, Professor Joanna Haigh, Co-Director of the Grantham Institute for Climate Change and the Environment at Imperial College London, said: "Ozone has been seriously declining globally since the 1980s, but while the banning of CFCs is leading to a recovery at the poles, the same does not appear to be true for the lower latitudes. The potential for harm in lower latitudes may actually be worse than at the poles. The decreases in ozone are less than we saw at the poles before the Montreal Protocol was enacted, but UV radiation is more intense in these regions and more people live there." The cause of this decline is not certain, although the authors suggest a couple of possibilities. One is that climate change is altering the pattern of atmospheric circulation, causing more ozone to be carried away from the tropics. The other possibility is that very short-lived substances (VSLSs), which contain chlorine and bromine, could be destroying ozone in the lower stratosphere. VSLSs include chemicals used as solvents, paint strippers, and as degreasing agents. One is even used in the production of an ozone-friendly replacement for CFCs.

In Antarctica the ozone hole should continue to recover gradually as CFCs leave the atmosphere, but complete recovery will take decades since CFCs have lifetimes from 50 to 100 years, so they stay in the atmosphere for a very long time. Scientist looking at 2060 or 2080 for a recovery, but even then, there might still be a small hole. (Credit: Katja Riedel)
In Antarctica the ozone hole should continue to recover gradually as CFCs leave the atmosphere, but complete recovery will take decades since CFCs have lifetimes from 50 to 100 years, so they stay in the atmosphere for a very long time. Scientist looking at 2060 or 2080 for a recovery, but even then, there might still be a small hole. (Credit: Katja Riedel)

Dr William Ball from the ETH Zurich and the Physikalisch-Meteorologisches Observatorium Davos World Radiation Centre in Switzerland, who led the analysis, said: "The finding of declining low-latitude ozone is surprising, since our current best atmospheric circulation models do not predict this effect. Very short-lived substances could be the missing factor in these models." It was thought that very short-lived substances would not persist long enough in the atmosphere to reach the height of the stratosphere and affect ozone, but more research may be needed. To conduct the analysis, the team developed new algorithms to combine the efforts of multiple international teams that have worked to connect data from different satellite missions since 1985 and create a robust, long time series. Dr Ball said: "The study is an example of the concerted international effort to monitor and understand what is happening with the ozone layer; many people and organisations prepared the underlying data, without which the analysis would not have been possible." Although individual datasets had previously hinted at a decline, the application of advanced merging techniques and time series analysis has revealed a long-term trend of ozone decrease in the stratosphere at lower altitudes and latitudes. The researchers say the focus now should be on getting more precise data on the ozone decline, and determining what the cause most likely is, for example by looking for the presence of VSLSs in the stratosphere. Dr Justin Alsing from the Flatiron Institute in New York, who took on a major role in developing and implementing the statistical technique used to combine the data, said: "This research was only possible because of a great deal of cross-disciplinary collaboration. My field is normally cosmology, but the technique we developed can be used in any science looking at complex datasets." The study was conducted by researchers from institutions in Switzerland, the UK, the USA, Sweden, Canada and Finland, and included data gathered by satellite missions including those by NASA.

Source: Hayley Dunning, Imperial College, London / Science Daily