Marine cloud whitening, and other ideas. We have precious little to show for nearly 20 years of efforts to prevent global warming. Promises in Rio de Janeiro in 1992 to cut carbon emissions went unfulfilled. Stronger pledges in Kyoto five years later failed to keep emissions in check. The only possible lesson is that agreements to reduce carbon emissions are costly, politically arduous and ultimately ineffective. But this is a lesson many are hell-bent on ignoring, as politicians plan to gather again—this time in Copenhagen, Denmark, in December—to negotiate a new carbon-emissions treaty.