Digging up and selling our coal has a range of horrible environmental effects both short and long term. Short term includes: massive use of water that we need for agriculture and environmental flows, destruction of marine environment from dredging so coal ships can get in, giant holes in landscape. Long term: more of the same ocean warming that's killed about 50% of the Great Barrier Reef already. Google Adani coal mine for more on what Australians are pissed about. If you'd like a more strictly economic argument, the sale price sure doesn't cover the damage.