The Montreal Protocol may not only be, as UNESCO claims, “one of the world’s most effective examples of environmental cooperation.” It also appears to have worked.
The Montreal Protocol on Substances that Deplete the Ozone Layer, which took effect globally in 1987, was among the first to invoke the precautionary principle, calling for a ban on chlorofluorocarbons (CFCs) even in the absence of conclusive evidence that they were damaging the earth’s ozone layer. Now, NASA has confirmed through satellite measurements that a decline in chlorine (a direct consequence of the Montreal Protocol) has led to roughly 20% less ozone depletion during the Antarctic winter than in 2005, the first year that satellite measurements were taken.
"We see very clearly that chlorine from CFCs is going down in the ozone hole, and that less ozone depletion is occurring because of it," said Susan Strahan, a NASA atmospheric scientist and the lead author of a study published in the journal Geophysical Research Letters. Scientists were able to calculate that the CFC ban had caused an annual decline in total atmospheric chlorine levels of roughly 0.8 percent.
NASA says the ozone hole should continue to recover gradually, but that complete recovery is still a long way off.
"CFCs have lifetimes from 50 to 100 years, so they linger in the atmosphere for a very long time," said Anne Douglass, a fellow atmospheric scientist and a co-author of the study.