But extracting shale energy involves environmental risks and uncertainties, among them groundwater contamination, higher methane emissions, and potential seismic effects. And shale gas emits CO2 when burned, even though it has half the carbon content of coal and has played a significant role in cutting US carbon emissions to mid-1990’s levels. &#160;
但提炼页岩能源会产生环境风险和不确定性，包括地下水污染、甲烷排量和潜在的地震效应。页岩气燃烧时还会释放二氧化碳，尽管页岩气比煤炭的含碳量低一半，并且对美国将碳排放量减至20世纪90年代中期水平居功至伟。