.S. Action on Climate Change Two years ago it was unclear whether the United States would ever adopt mandatory controls on its greenhouse gas emissions. Today, it seems almost certain that caps on carbon dioxide and other gases will be passed and implemented within the next five years. What remains wholly unclear is how strong such a cap will be and what market mechanisms, if any, will facilitate the transition to a low carbon economy. While many outside North America think it is inevitable that the U.S. Congress will pass cap-andtrade