WASHINGTON (Reuters) - Ancient roots and bones locked in long-frozen soil in Siberia are starting to thaw, and have the potential to unleash billions of tons of carbon and accelerate global warming, scientists said on Thursday. This vast carbon reservoir, contained in permafrost soil in northeastern Siberia, contains about 75 times more carbon than the amount released into the atmosphere each year by the burning of fossil fuels, the researchers said in a statement. Siberia isn't the only place on Earth with massive lodes of permafrost -- parts of Alaska, Canada and northern Europe have them too. The Siberian area...