The use of electrical power to secure the network, and an increase in doing so fueled by the greed of miners (I'm one of them). The calculations completed have no intrinsic value - they're just a means to the end of securing the network. As time goes on, if bitcoin continues to succeed, power consumption will continue increasing.
IMO, Bitcoin use far less, order of magnitude less power than banks paiement system / network / servers
I bet diiff can go to several billions and Bitcoin will still uses a lot less power than the actual fiat/credit system/network