The “exponential” use of energy is obviously not sustainable.
The energy demand has developed because of the soaring cost of producing and collecting virtual currencies. Computers are used to make the complex calculations that verify a running ledger of all the transactions in virtual currencies around the world.
In return, the miners claim a fraction of a coin not yet in circulation. In the case of bitcoin, a total of 21 million can be mined, leaving about 4.2 million left to create. As more bitcoin enter circulation, more powerful computers are needed to keep up with the calculations — and that means more energy.