• stratosphere-troposphere coupling;
  • ozone;
  • Montreal Protocol

[1] The impact of increased stratospheric chlorine, averted by controls imposed by the Montreal Protocol, is studied using the UKCA chemistry-climate model. We contrast an atmosphere with 9 ppbv Cly, which could have occurred by ∼2030, with the present-day loading (3.5 ppbv), and consider the response of climate to resulting ozone perturbations, disregarding the radiative impact of the additional CFCs. Ozone columns decline everywhere, with climate impacts in both polar regions. The additional chlorine leads to a strengthening of the Southern Annular Mode, versus the reference, and Antarctic surface temperature differences resemble observed changes. Over Eurasia, winter surface temperature changes project on the Northern Annular Mode. These high-latitude temperature perturbations (>1 K) are larger than the global mean temperature rises projected over the next few decades, and perhaps comparable with projected regional changes. The Montreal Protocol has not only averted further damage to the ozone layer but has helped prevent significant regional climate change.