A growing body of research now confirms that the Montreal agreement averted at least one catastrophic form of climate change, even if others still loom. "The Montreal Protocol is a major success story," says William Randel.

Clearly, the end of the Copenhagen conference isn’t the end of work on climate change—though, with regards to Winston Churchill, it might be the end of the beginning.

Years before Congress began debating greenhouse-gas reduction, more than 500 U.S. cities had pledged to reduce their carbon footprints in line with the goals of the Kyoto Protocol. Now American cities are leading the way on adapting to climate change.