In its currents incarnation it's not green, neither are most other proof of work (POW) coins.
As wonderful as proof of work is as a concept, for securing blockchain networks, it comes with an unintended consequence.
Greed has led to the development of dedicated hardware to mine POW coins (ASIC's) and these get faster and faster all the time. This has resulted in a mining hardware and capacity arms race that is spiraling out of control. The result is greater and greater amounts of electricity are being wasted in this ever escalating race to mine the next block and secure the block rewards.
This is unsustainable, no matter how high the price of bitcoin goes there will soon not be enough electricity generated on this planet to power all the bitcoin mining equipment that is being thrown into the mix at an ever increasing rate.
Already the hashrate can be used to estimate that more power is being consumed by the bitcoin network than is utilized in entire countries.
Solar and other renewable energy is coming online faster than older power generation technologies. This will however never be sufficient if bitcoin electricity consumption continues to increase at the current exponential rate.
Surely it's time for a better solution? It is after all a simple maths equation that is being solved, but the solution derives no real world value other than determining who gets the block rewards.