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In 1974, Mario J. Molina and F. Sherwood Rowland warned that chlorofluorocarbons (CFCs) could destroy the stratospheric ozone layer, which protects Earth against the harmful effects of ultraviolet radiation [Molina and Rowland Nature 1974, 249, 810]. In 1975, Ramanathan warned that CFCs are powerful greenhouse gases (GHGs) and would rival carbon dioxide (CO2) in causing climate change if left unabated [Ramanathan Science 1975, 190, 50]. The 1987 Montreal Protocol on Substances that Deplete the Ozone Layer (Protocol), arguably the most successful global environmental treaty in history, was enacted in response to these warnings. This Protocol has phased out almost 99% of the production and consumption of ozone depleting substances (ODSs). Other papers have explored the world avoided” by actions under the Protocol [Prather et al. Nature 1996, 381, 551; Newman et al. Atmos. Chem. Phys. 2009, 9, 2113; Morgenstern et al. Geophys. Res. Lett. 2008, 35, 1]. They concluded that the ozone layer would have been highly depleted across the globe by the mid-21st century without the Protocol and that the Protocol contributed significantly to reduce climate change. This paper explores what could have been achieved if the world had acted against the continued use of ODSs, which were both ozone-depleting and greenhouse gases, immediately after Molina and Rowland warned of stratospheric ozone depletion and Ramanathan warned of climate forcing using chemicals and technology that were already globally available in the mid-1970s. We show that such precautionary principle” actions would have reduced global ozone layer depletion, reduced the extent of the ozone hole, brought forward the dates for ozone layer recovery, and helped minimize climate change.

There is growing international interest in mitigating climate change during the early part of this century by reducing emissions of short-lived climate pollutants (SLCPs), in addition to reducing emissions of CO2. The SLCPs include methane (CH4), black carbon aerosols (BC), tropospheric ozone (O3) and hydrofluorocarbons (HFCs). Recent studies have estimated that by mitigating emissions of CH4, BC, and O using available technologies, about 0.5 to 0.6◦C warming can be avoided by mid-21st century. Here we show that avoiding production and use of high-GWP (global warming potential) HFCs by using technologically feasible low-GWP substitutes to meet the increasing global demand can avoid as much as another 0.5◦C warming by the end of the century. This combined mitigation of SLCPs would cut the cumulative warming since 2005 by 50% at 2050 and by 60% at 2100 from the CO2-only mitigation scenarios, significantly reducing the rate of warming and lowering the probability of exceeding the 2◦C warming threshold during this century.

Inclusion of HFCs under the Montreal Protocol offers a path, starting in the short term, to preserve the climate benefits already achieved by this treaty.