The “exponential” use of energy is obviously not sustainable.

Associated Press:

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.

More:

Bitcoin energy use in Iceland set to overtake homes, says local firm – Chris Baraniuk, BBC

In Iceland, bitcoin mining will soon use more energy than its residents – Cyrus Farivar, Ars Technica