Early in the year, the United States, once the biggest contributor to greenhouse gas emissions, saw its carbon-dioxide emissions from energy use drop to the lowest level since 1992, a decline the Department of Energy attributed to a mild winter, a shift from coal to natural gas and a slow economy. In 2011, the United States contributed 16 percent to the world's emissions from fossil fuel use, behind the 28 percent contribution from the top emitter, now China, according to a report by the Global Carbon Project.
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