It is insane to waste energy on mining bitcoin in the age of peak oil where greedy energy companies destroy nature for profit.
Right now all bitcoin miners consume the amount of the energy produced by an average US nuclear power plant (30 million KwH/day), or the total oil production of Phillipines (12,000 barrels of oil per day)

The mining of bitcoin uses PoW (Proof of Work) to secure the network, which leads to an exponential growth of energy spent on mining.
All miners compete against each other and must increase mining power (energy usage) to increase their chance of being rewarded in the mining process.

But there are other models to secure a crypto currency without the need to waste energy: The security model of PoS (Proof of Stake) relies instead of the ownership of coins.
The reason is simple: the more coins you own, the less is the risk that you wish to damage the coin for short term profit, since you would lose the value of your current holdings.

So I hope Bitcoin evolves and starts to use PoS, or similar methods to save energy and save the nature.


Sources:

Proof of Stake
https://en.bitcoin.it/wiki/Proof_of_Stake

Bitcoin Mining Uses $15 Million's Worth Of Electricity Every Day
http://www.forbes.com/sites/timworstall/2013/12/03/fascinating-number-bitcoin-mining-uses-15-millions-worth-of-electricity-every-day/

How much electricity does a typical nuclear power plant generate?
http://www.eia.gov/tools/faqs/faq.cfm?id=104&t=21

List of countries by oil production
http://en.wikipedia.org/wiki/List_of_countries_by_oil_production

Canada, before and after tar sand mining: