Coal is making a comeback as a cheap and reliable source of energy. With global energy demand rising at an unprecedented rate, the world's vast coal reserves are attracting growing interest from governments in Europe, the US and Asia. But while the efficiency and cleanliness of coal-fired power stations is improving, coal remains the dirtiest of all fossil fuels. Genuinely clean coal - i.e.: one that emits close to zero CO2 thanks to carbon capture and storage technology - is not expected to become economically viable before twenty years. And with the growing sense of emergency surrounding global warming, coal is simply not the answer, environmentalists argue.