Contrary to recent scientific assumptions, the ozone layer, which protects life on the Earth from harmful ultra-violet radiation, continues to deplete on a global scale even after decades of efforts to protect it, according to an worldwide study on Tuesday.
A large portion of the resulting ozone layer resides in the lower part of the stratosphere.
However, total ozone levels in the atmosphere remained the same and now scientists at the federal institute of technology ETH Zurich showed that this is because ozone in the lower stratosphere declined steadily over the past three decades.
This comes as a bit of a blow following the good news previous year that the hole in the ozone above Antarctica appears to be healing thanks to a ban on chlorofluorocarbons (CFCs). The impact of the Protocol is undisputed, as evidenced by the trend reversal in the upper stratosphere and at the poles. This is partly due to the fact that ozone is also formed in the troposphere - at altitudes below about 15 km - by human activities. "But we have to keep an eye on the ozone layer and its function as a UV filter in the heavily populated mid-latitudes and tropics", he says. They found that from 1998 to 2016, ozone in the lower stratosphere ebbed by 2.2 Dobson units-a measure of ozone thickness-even as concentrations in the upper stratosphere rose by about 0.8 Dobson units. 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 reasons for the continuing decline are still unclear. One suggestion by the authors of the paper include the use of chlorine and bromine-containing chemicals known as very short-lived substances (VSLSs), used in paint strippers and solvents.
Although the exact cause of this thinning of the ozone layer at the lower latitudes could not be decoded, the scientists are suspecting that climate change might be obstructing the ozone layer to heal. "These short-lived substances could be an insufficiently considered factor in the models", says Ball.
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To conduct the analysis, the team developed new algorithms to combine the efforts of multiple worldwide teams that have worked to connect data from different satellite missions since 1985 and create a robust, long time series.
Previous research has hinted at changes in the Earth's ozone but this appears to be the first to combine datasets since 1985 to show that atmospheric thinning is a long-term trend.
The researchers from Imperial College London believe the continued decline may be down to climate change causing more ozone to be carried away from the tropics. According to the study, the bottom part of the ozone layer at more populated latitudes is not healing and the scientists could not find out why.
Dr Justin Alsing from the Flatiron Institute in NY, 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".
The study was conducted by researchers from institutions in Switzerland, the United Kingdom, the USA, Sweden, Canada and Finland, and included data gathered by satellite missions including those by NASA.