OXFORD, England -- Marc Stuart and Pedro Moura Costa have become multimillionaires in a booming new market designed to fight global warming. Now, their empire is under attack. Their firm, United Kingdom-based EcoSecurities Ltd., helps companies in the industrialized world meet their obligations to pollute less by selling them "credits" that fund clean-air projects in poorer nations. Last year, some $9.4 billion in these credits were traded, up from almost none four years earlier. The market's anything-goes early days now appear to be ending. United Nations officials who regulate the trade have started questioning scores of proposed projects, from hydroelectric...