As the mining reward rapidly dwindles relative to the cost of electricity, there is a huge incentive to reduce the electricity consumption while maintaining the security of the network. At what point does this reach an equilibrium depends largely on the market value of bitcoin. So it should be a somewhat fixed cost in proportion to the adoption of bitcoin, just like the fixed costs of any currency (e.g. minting coins, mining gold, etc.).
Mining rewards will diminish over time so it will no longer be feasible to mine as much as in the 'gold rush' phase. So the carbon footprint will not be growing all the time.
In fact, there's a race to make the current hardware to become more power efficient. If you live in a sunny place, you could probably pick up 240 Watt solar panels for around $300, a connect a Raspberry pi @ 3.5 watts, then a 10 GH/s Erupter blade @ 75 watts.
Current estimated power consumption of the Bitcoin network: 1.67 Mega watts. (6,028.57 megawatt hours http://blockchain.info/stats).
Typical power generated by a single Boeing 747: 140 Mega Watts. There's hundreds of these flying right now, yet no-one is appalled about the carbon footprint. http://en.wikipedia.org/wiki/Orders_of_magnitude_(power)