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

The 1987 Montreal Protocol regulating emissions of chlorofluorocarbons (CFCs) and other ozone depleting 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 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); it is also widely apprecited that ozone depletionhas been the dominant driver of atmospheric circulation changes in the Southern Hemisphere in the last half century.

We will discuss that the Montreal Protocol also significantly protects the Earth's hydroclimate. Using the Community Atmospheric Model, version 3 (CAM3), coupled to a simple mixed layer ocean, we show that in the 'World Avoided' (i.e. a scenario with unregulated CFC emissions), the subtropical dry zones would be substantially drier, and the middle and high latitude regions considerably wetter in the coming decade (2020-29) than in a world without ozone depletion. Surprisingly, we find that these changes are very similar, in both pattern and magnitude, to those caused by projected increases in GHG concentrations over the same period. We further show that, by dynamical and thermodynamical mechanisms, both stratospheric ozone depletion and increased CFCs contribute to these changes. Our results 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.