Crack down on coal.
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Coal, the dirtiest fossil fuel, is responsible for a third of U.S. carbon pollution. In 2011, Obama passed strict new rules on toxic mercury pollution, but cutting down on carbon will be harder. After failing to pass cap and trade legislation in the first term, Obama has vowed to use the regulatory powers of the EPA to impose carbon limits on new and existing coal plants. Done poorly, the rules will be tied up in court for years; done right, they could have a major impact on global warming pollution.