ESRL/PSD Seminar Series

Abstract

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The 1987 Montreal Protocol to phase out emissions of CFCs and ozone depleting substances has succeeded in stemming the depletion of the ozone layer which is perhaps now showing signs of recovery. The Protocol was motivated by concerns about human, animal and ecosystem health caused by increased UV exposure. Barely getting a mention was the greenhouse effect of CFCs but we now know that the Protocol is protecting the Earth from even more serious global warming than expected in the future. What was entirely unknown at the time of Montreal was the impact that ozone depletion itself would have on surface climate. Here we use controlled modeling experiments (the so-called World Avoided scenario) to examine what the impact of continued CFC increases and ozone depletion would have been on the Earth's hydroclimate in the next decade. It is shown that the Montreal Protocol is protecting us against a hydroclimate change that is approximately of the same magnitude and pattern as that expected in the next decade due to rising GHGs. That is, because of Montreal, the hydroclimate change in the next decade will be only half as severe as it otherwise would have been. The modeling experiments show that both the CFC increase and the ozone depletion contribute to the total change with the former working strongly via thermodynamic mechanisms and the latter working exclusively via changes in the tropospheric circulation. Because these impacts of CFCs and ozone on the hydrological cycle were unknown pre-Montreal, this hydroclimate protection has been achieved by pure chance, the implications and lessons of which will be discussed.

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