If we really would see a 100 fold price increase in 18 months then energy usage will probably be very high as well but there are a few factors which you might not have considered:
1. If difficulty increases steadily at the required ~29% per month then we will reach block 210.000 already in about 13 months.
2. New mining hardware will not be bought anymore some time before block 210.000 because there is not enough time left to recoup the costs.
3. Peak energy usage will probably only be held for a relatively short period of time.
4. Increasing energy prices (which are probably to be expected in the next years) will further cut down on energy usage.
5. Intermediate price decreases or even crashes due to speculation might scare off some potential big mining investors.
There could also be other factors which would limit the power requirements if it really gets out of hand - for example, miners could agree that new blocks are only accepted within a 5-minute time window (granted - this is highly speculative and such changes would have to be very carefully designed to not change the basic dynamics of the system).
Having said that, I very much doubt that we would see a 100 fold increase in price within the next year, but if it really happens, then it is obvious that society deems Bitcoin as a very valuable service and therefore it is probably justified to use up some energy (which is hopefully saved in part with other financial services becoming less important then).
Last, I really hope that the percentage of energy coming from renewable sources increases in the next years, which would also mitigate some of the ecological impact.
No worries, I only had the question half formulated when I first typed it in. It'd be interesting to factor in those points above, and graph out the corresponding max value of BTC over time assuming a max of 1GW mining power.
The main point I'm trying to make is that with the current setup, bitcoins going to $10,000 or taking any significant percentage of world GDP or the world black market over the next few years isn't going to happen. It's too severely limited by the power available in the world.