The first Earth Summit was held in Rio de Janeiro in 1992. It was agreed that human interference was changing the climate and that something had to be done. Since then, conferences have been held all over the world--most notably in Kyoto in 1997 and Copenhagen in 2009 — without producing an enforceable strategy to cope with changes that are already under way or to prevent more change in the future. Now the leaders of nations covering 95% of the planet are convening in Paris to try again. Can even that limited goal survive differences between rich and developing nations and overcome domestic politics in the US and other countries?