But the new study, published in the journal Atmospheric Chemistry and Physics, showed that despite the ban on chlorofluorocarbons (CFCs), the concentration of ozone in the lower part of the stratosphere (15 to 24 km) - where the ozone layer is at its densest - has continued to decline.
It is clear from Antarctic data that the ozone layer is beginning to recover where it was worst affected, though it will take many more decades before it is back to its condition of the 1970s.
Researchers conducted an analysis on 11 different locations and gathered information which helped them create a subtle model of the ozone throughout the last 30 years.
The ozone layer, which protects life on earth from harmful ultra-violet radiation, continues to deplete on a global scale contrary to recent scientific assumptions, say an worldwide team of scientists led by researchers in Zurich. Substitutes for the particularly harmful CFCs are less thinning for the ozone layer but are not neutral either.
The cause of the decline is unknown but might be the result of global warming.
The ozone layer is recovering at the poles, but unexpected decreases in part of the atmosphere may be preventing recovery at lower latitudes.
During the study, the researcher's team detected that Ozone layer in the stratosphere is not recovering as expected, between 60 N and 60 S. "Although the Montreal protocol has done what we wanted it to do in the upper stratosphere, there are other things going on that we don't understand", Haigh said. "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", informed Haigh.
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Although they're not certain what's causing this decline, the authors suggest two possibilities. These substances contain chlorine and bromine which destroy the layer presented in the lower stratosphere.
An global research team says the ozone, which protects humans and other species from harmful ultraviolet radiation, is continuing to recover at the poles. On the one hand, climate change is modifying the pattern of atmospheric circulation, moving air from the tropics faster and further in the polar direction, so that less ozone is formed (see illustration).
The chemicals that are used as solvents, including paint strippers and degreasing agents could be the reason behind Earth's ozone shield depleting. "These short-lived substances could be an insufficiently considered factor in the models", says Ball.
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. "What matters most for UV at Earth's surface is the total column amount of ozone overhead", says co-author Sean Davis, a research scientist with NOAA's Earth System Research Laboratory in Boulder, Colorado.
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.
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.