Bitcoin mining is really contributing in the global warming but as others are saying even banks are doing the same maybe when the time comes that bitcoin replaced the currency fiat system then it will be an acceptable use of energy and contribution to the global warming.
If bitcoin will be successful, this will lessen the production of paper bills, hence lower the polution from factories and improve our environment. However, for us to produce cryptocurrencies, we need to mine them. So I guess this will still be the same.