A team of researchers from University of Hawaii, USA, have concluded that the energy spent to mine Bitcoins will accelerate global warming. In a study published in Nature, a team of researchers determined that if Bitcoin adoption continued at the current rate, the CO2 produced in mining Bitcoins alone could push global temperates up by 2 °C in less than 3 decades.
The University of Hawaii researchers made an assumption that Bitcoin’s growth will be similar to other tradition technologies like credit card, etc. (fig below)

Another key assumption is that fuels and renewables used to generate electricity will have similar trend as of today over next two decades. However, massive adoption of renewable energy may only party slow down the trend.