Boston Globe: North of Canada`s capital, underneath an endless expanse of spruce, pine, and birch, ticks what some scientists are calling a carbon bomb: Peat. A thick layer of the black spongy soil, the remnants of ancient forests, wraps the globe`s northern tier. Deeper than 15 feet in places, the peat layer extends over more than 6 million square miles across Russia, Scandinavia, China, Canada, and the United States. Carbon that those forests absorbed from the air over thousands of years …