Hugh Pickens writes writes: "Thomas K. Grose reports that the Environmental Protection Agency (EPA) estimates that greenhouse gas emissions in the US have fallen 8 percent from their 2007 peak to 6,703 million metric tons of CO2 equivalent, due largely to the drop in coal-fired electricity which in 2012 generated 37.4 percent of US electricity, down from 50 percent in 2005. But don't celebrate just yet. A major side effect of that cleaner air in the US has been the further darkening of skies over Europe and Asia as US coal producers have been shipping the most carbon-intensive fuel to energy-hungry markets overseas. US coal exports to China were on track to double last year and demand for US metallurgical coal, the high-heat content coking coal that is used for steelmaking, is so great in Asia that shipments make a round-the-world journey from Appalachia as they are sent by train to the port of Baltimore, where they steam to sea through the Chesapeake Bay, then south across the Atlantic Ocean and around Africa's Cape of Good Hope to reach Asian ports. The Tyndall Center study estimates that the burning of all that exported coal could erase fully half the gains the United States has made in reducing carbon emissions and if the trend continues, the dramatic changes in energy use in the United States — in particular, the switch from coal to newly abundant natural gas for generating electricity — will have only a modest impact on global warming, observers warn. "Without a meaningful cap on global carbon emissions, the exploitation of shale gas reserves is likely to increase total emissions," write Dr John Broderick and Prof Kevin Anderson. "For this not to be the case, consumption of displaced fuels must be reduced globally and remain suppressed indefinitely; in effect displaced coal must stay in the ground (PDF).""