[mcgin]

There are energy efficient ways of securing a block chain. Proof of Stake does not rely on vast energy consumption and in fact is better at securing the network, something one of the newer currencies Peercoin takes advantage of.

[humbledrone]

Thanks for the Proof of Stake reference, I was not familiar with that system but am reading about it now and it's very interesting. This site [1] seems to indicate a rather unfavorable transaction cost for Bitcoins, around 7%. However, I'm not sure whether their estimation of 650 watts per gigahash* is accurate (that warrants some more research), so I don't know whether to trust that figure or not. If it is accurate, it is not favorable when compared to Visa.

[1] https://blockchain.info/stats

* I assume they meant watts per gigahash/second.

[EDIT] It seems like the recent ASIC miners on the market are vastly more efficient (>100x) than blockchain.info's estimates [2] (which probably include CPU/GPU miners since ASICs are relatively new). Without knowing the makeup of the Bitcoin network (i.e. what portion is CPU/GPU/ASIC etc), though, it seems like any efficiency estimate will be pretty inaccurate.

[2] https://www.kncminer.com/categories/miners

[DennisP]

Proof of Burn is another interesting idea.