A multibillion-dollar experiment designed to curb global warming is stumbling as regulators question whether the program is doing enough environmental good. The United Nations is the main global policeman in an effort by wealthy nations to reduce the impact of their own pollution by paying for cleanups in the developing world. The program, known as the Clean Development Mechanism, is one of the most important coordinated efforts to attack global warming. [Graphic] In recent months, however, U.N. regulators who administer the program have objected to dozens of these developing-world projects, ranging from hydroelectric plants to wind farms, questioning whether the...