The good news: The biggest environmental gathering of government leaders in many years showed the world is finally waking up to the urgency of climate change. The bad news: Time is running out.

Ban Ki-Moon knows a thing or two about climate change and, as head of the UN, the world’s top diplomat knows a thing or two about politics. When our own executive director, Dr. Gerd Leipold, got in the lift at the UN bound for the 38th floor, he...