Soon we will be forced in to using greener energy (not from fossil fuels) and then even if the cost of energy required for BTC rises it won't be anywhere near as bad as it is currently. Solar and Geothermal power are far superior to fossil fuels in terms of protecting the environment and these alternative energy sources should soon phase out the heavily polluting methods. In the long run I don't think Crypto will generate that great of an issue for the environment