The Importance of the Montreal Protocol in Protecting Earth’s Hydroclimate

The 1987 Montreal Protocol regulating emissions of chlorofluorocarbons (CFCs) and other ozonedepleting substances (ODSs) was motivated primarily by the harm to human health and ecosystems arising from increased exposure to ultraviolet-B (UV-B) radiation associated with depletion of the ozone layer. It is now known that the Montreal Protocol has helped reduce radiative forcing of the climate system since CFCs are greenhouse gases (GHGs), and that ozone depletion (which is now on the verge of reversing) has been the dominant driver of atmospheric circulation changes in the Southern Hemisphere in the last half century. This paper demonstrates that theMontreal Protocol also significantly protects Earth’s hydroclimate. Using the Community Atmospheric Model, version 3 (CAM3), coupled to a simple mixed layer ocean, it is shown that in the ‘‘world avoided’’ (i.e., with CFC emissions not regulated), the subtropical dry zones would be substantially drier, and themiddleand high-latitude regions considerably wetter in the coming decade (2020– 29) than in a world without ozone depletion. Surprisingly, these changes are very similar, in both pattern and magnitude, to those caused by projected increases in GHG concentrations over the same period. It is further shown that, by dynamical and thermodynamical mechanisms, both the stratospheric ozone depletion and increased CFCs contribute to these changes. The results herein imply that, as a consequence of the Montreal Protocol, changes in the hydrological cycle in the coming decade will be only half as strong as what they otherwise would be.