The results of a new study appear to support the view that the Montreal Protocol has been effective at protecting the ozone layer.
Researchers modelled the impacts of the Protocol's phasing out of ozone depleting substances and found that they matched actual changes of global ozone over the last 50 years.
Ozone in the stratosphere (the ozone layer) is depleted in particular by chlorine and bromine radicals, which are produced by substances such as man-made chlorofluorocarbons (CFCs) and halons.
In the mid-1980s it was recognised that these substances were creating a hole in the ozone layer above the Antarctic. In response, the Montreal Protocol regulated CFCs and other substances that deplete ozone in 1987.
Improvements to the ozone layer since the 1990s have been observed, but it has been difficult to directly attribute them to the Protocol.
For example, other influences, such as long-term climate variability, volcanic eruptions and peaks in solar activity, could affect the state of the ozone layer.
The new study was based on statistical modelling and examined the effectiveness of the Montreal Protocol in protecting the ozone layer by comparing two different models for the observed trend in stratospheric ozone.
One model assumed a continuing steady increase in ozone depleting substances (ODS), while the other assumed that the trend follows the effects of the Protocol, i.e., the level of ODS increases steadily until about 1997 and then starts to drop.
Considering these two trends in ODS, the study modelled the effects on ozone levels with and without the Protocol. It also considered other possible explanations for ozone variation, both locally and globally, such as the eruption of Mount Pinatubo in 1991 and changes in atmospheric composition and dynamics.
The study then compared the modelled ozone levels (with and without the Protocol) to real data on ozone levels that were taken from 116 stations worldwide between 1948 and 2007.
The results indicated that the scenario which considered the impacts of the Protocol provided, by far, the better description of changes in observed ozone over time - at least for the northern and southern latitudes.
At tropical latitudes, it remains unclear which scenario represents actual variations best. This can be explained by the high temporal variability of ozone compared to the overall rather small trend in increased levels of ozone. The study also indicated that, over time, the trend based on the impacts of the Protocol provided a better fit to real data, suggesting the Protocol is effective.
Since the study has not physically linked changes in the ozone layer to Protocol-led changes in ODS, but provided strong statistically evidence, it should not be interpreted as quantitative proof of the Protocol's effectiveness. But the results indicate that it is highly likely that the Montreal Protocol is having the desired impact on ozone through its reductions in ODS.
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