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A new study finds the 1987 Montreal Protocol, which was negotiated to stop ozone-depleting emissions, also prevented significant climate change.
Why it matters: Former UN Secretary-General Kofi Annan called the Montreal Protocol "perhaps the single most effective international agreement," and its success in both reversing ozone depletion and slowing warming shows why.
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What's happening: In a study published in Nature this week, researchers simulated what would have happened to the world if the Montreal Protocol had never gone into effect.
The treaty phased out ozone-depleting chemicals like chlorofluorocarbons (CFCs) and hydrofluorocarbons (HFCs).
Previous research has shown the Antarctic ozone hole would have been 40% bigger in the treaty's absence.
The new study, though, shows continued use of ozone-depleting chemicals in the absence of Montreal could have led to an additional 2.5 °C of warming by the end of the century.
How it works: Some of that climate change would have been triggered by direct warming caused by CFC and HFC emissions, which act as a greenhouse gas.
But because a damaged ozone layer would let in more harmful UV radiation, plants would have had a reduced ability to store carbon, which would have further contributed to warming.
The big picture: The success of the Montreal Protocol has often been cited as proof the global community can successfully negotiate a solution to a global environmental challenge.
But carbon — and the fossil fuels that contain them — is far more central to the global economy than CFCs and HFCs, which could be replaced with ozone-safer alternatives at a relatively low price.
The geopolitical realm is arguably much more complex now than it was in 1987, when large developing countries like India and China — which will be the source of the bulk of current and future carbon emissions — were much smaller and had less of a voice.
The bottom line: The world's governments were unusually far-sighted in negotiating the Montreal Protocol, but climate change won't be as easy.
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