well for sure this is happening, and not only the bitcoin network is soaking power like hell, all the GPU Crunching that currently goes on is also taking a lot of power. Personally i think we need to invent new systems of securing a coin than simply throw a lot of hashpower at it. Also new distribution methods need to be thought of than simply POW or POS ( of those 2 POS for sure is the much energy friendlier method).
What do you think where this at the time of writing this reply 78.17 Th/s for Ethereum come from ? Remember the GTX 1060 (one of the most energy saving cards) uses roundabout 90 Watts for 21.3Mh/s .
Short break for some Calculations :
Mega is 10^6, Giga 10^9 and Tera 10^12
Following this a 78.17 TH/s are then 78.170 000 000 000 Hashes per Second.
Like said above the card produces 21.300 000 Hashes per Second.
Which ultimately means if all cards would be GTX 1060 (for sure not true) that they would soak up 330295774,6478873 Watts. To make it more readable
330.295774 Megawatt
Following this thought onwards this means that at a day this mining would soak up
330.92 * 24h = 818,08 MWh a day. For the year this would be 365* 818,08 MWh = 297139,2 MWh.
I checked on Wikipedia, this is as much Energy as the Seychelles are using or Greenland or the Maldives.
On the other hand side there are power plants out there that are exactly delivering 300 MW, Coal plants for instance.

And remember, this is only one of the Altcoins (the one where most mining is happening for sure, but only one coin out of many).
If you want to try this out for another big coin like Zcash i just used the stats given on Whattomine.com

I know guys, this isnt really the case, not all cards are 1060Gtx and some are better in H/W. But there are many that are worse as well :) so i just tried to make things a bit more simple by reducing complexity.
One final thought, i for one am really into the topic that @scalextrix already mentioned. Using this energy for something that is really worthwhile and not just for heating up the air :)
Give BOINC and Gridcoin a try (but forget about getting rich in no time) and calculate for instance in projects about fighting cancer, or identifying more HIV treatments .

• if there is a mistake in my calculations, please be gentle, it has gotten very late in the meantime ... ( i wanted to write only a short reply ... well this didnt work out that well did it ?)