Edward I, known for being ruthless, banned the burning of coal in London because his mother didn’t like the smell of it. Despite threats of hangings, the ban didn’t work. People defied the king because coal was cheaper than wood.
Centuries later, coal is still the cheapest source of energy that we have. But the only way to get energy out of the dirty stuff is to burn it and release carbon dioxide in the process. It is now widely believed that increasing levels of carbon dioxide will raise global temperatures — which is why you might have heard a lot of talk about “clean coal” in recent years.