EcoWatch: Although oil remains the worlds leading energy source, coal and natural gas continue to grow in significance, according to new research conducted by the Worldwatch Institute for its Vital Signs Online service. Global consumption of coal increased 5.4 percent in 2011, to 3.72 billion tons of oil equivalent, while natural gas use grew 2.2 percent, to 2.91 billion tons of oil equivalent. Both are primary fuels for the worlds electricity market, and because they are often used as substitutes for one...