Let's say that Bitcoin takes the place of the USD within 10 years and see what the energy consumption of the bitcoin network will be.

facts:
 - USD "M2" money supply in 2009: 8E12 USD (http://money.howstuffworks.com/how-much-money-is-in-the-world.htm)
 - in 2023 there will be 19E6 BTC. Mining will be honoured with 6.25 BTC/10min => 329'000 BTC/y (https://en.bitcoin.it/wiki/Controlled_supply)
 
assumptions:
 - BTC takes the place of USD => one BTC will be worth 421'053$ (of course valuation of BTC will devaluate USD...but let's leave that aside.)
 - let's say 1kWh costs 10cts USD
 - all miners use the same recent technology. Or at least with equal efficiency in MH/J
 - people are not paying significant transaction fees by 2023 (the more people will pay fees, the higher the resulting profitable global energy consumption)

(edit) In a free market the following formula approximates energy consumption of any PoW based cryptocurrency:
GlobalEnergyConsumptionForMining ~= (MiningReward + TransactionFees) * bitcoinValueInUSD / EnergyCostInUSDperkWh

So mining would be profitable somewhere below an energy consumption of the bitcoin network of:

break even price: 421'053$/BTC*329'000BTC/y = 139E9$/y
equivalent energy: 1.39E12 kWh/y

Today's global electricity consumption is around 20E12 kWh/y (http://en.wikipedia.org/wiki/Electric_energy_consumption)

This means that the bitcoin network by itself would raise global electricity consumption by up to 7%

Please show me where I'm wrong!

We could of course put this less dramatic:
  - in 2033 it would be 174E9 kWh/y
  - in 2100 it would be 2.65 GWh/y

today we would be at 5MWh if all miners were using newest ASIC's (https://en.bitcoin.it/wiki/Mining_hardware_comparison)

(edit)
CONCLUSIONS:
 

• according to economical basics, energy consumption of the bitcoin network does not depend on mining gear efficiency!
• the problem described applies to every proof-of-work based currency (including Litecoin) because proof-of-work equals proof-of-energy-consumption at a market equilibrium
• bitcoin value is NOT directly backed by energy consumption. But mining rewards and transaction fees are.
• the slower the bitcoin value rises and the lower the transaction fees, the lower the energy consumption of the bitcoin network on the long run