Burning coal is the number one cause of climate change.
The chemical make up of coal means that a lot of carbon dioxide (the main heat trapping gas involved in global warming) is emitted for every unit of energy produced. Natural gas is about half as polluting as coal.
Coal is cheap and there is a lot of it in Australia. However, it is only cheap because the market fails to factor in the environmental and social cost of burning coal. If the true environmental cost of burning coal was factored into its price, then it would be very expensive.