RIO DE JANEIRO, Brazil — Brazil's largest city sold millions of dollars worth of carbon credits at an auction Wednesday in a deal that experts said paves the way for developing countries to make money fighting global warming. Brazil's Mercantile and Futures Exchange called Sao Paulo's sale of $18.5 million in carbon credits to Dutch-Belgian Fortis Bank the first such sale to be held on a regulated stock market and a significant step toward institutionalizing the carbon market. Under the Kyoto Treaty on greenhouse gases blamed for global warming, companies that generate large amounts of polluting carbon dioxide and methane...