In 1987 the Montreal Protocol banned ozone depleting substances (ODS) such as chlorofluorocarbons and other halogen-containing compounds. Jörg Mäder and colleagues, ETH Zurich, Switzerland, have shown that the steps taken to prevent further damage to the ozone layer are already having an effect.

Observed concentrations of chlorine and bromine in the stratosphere were measured from the global network of ground-based instruments. The data were compared to two models. The first contains an equivalent effective stratospheric chlorine (EESC) parameter as a measure of the effect of man-made ODS emissions on chemical ozone depletion. The second was based on projected concentration values as if they had continued to rise in the linear manner seen between 1970 and 1990.

For the majority of the stations, the EESC proxy fits the long term ozone evolution better than the linear trend.

• Evidence for the effectiveness of the Montreal Protocol to protect the ozone layer
  J. A. Mäder, J. Staehelin, T. Peter, D. Brunner, H. E. Rieder, W. A. Stahel,
  Atmos. Chem. Phys. Discuss. 2010, 10, 19005-19029.
  DOI: 10.5194/acpd-10-19005-2010