Coal could rival oil as the world’s largest energy source within five years as consumption continues to climb in most regions of the world, a trend that could have profound effects on the climate, the International Energy Agency (IEA) says. While coal consumption is expected to decline in the U.S., where it increasingly has been displaced by ample supplies of natural gas, that reduction in U.S. coal burning has helped drive down coal costs globally. According to the IEA’s annual Medium-Term Coal Market Report, the world will burn about 1.2 billion more tons of coal annually by 2017 than it does today. The surge in coal consumption will be driven largely by China and India, with China projected to pass the rest of the world in coal demand within five years, and India predicted to pass the U.S. as the world’s second-biggest coal consumer. Without a high carbon tax, the report says, only competition from cleaner natural gas will reduce coal demand.