As of right now, Bitcoin uses about 50KWh of electricity per transaction, which roughly translates to 4 gallons of gas or 37 pounds of coal worth of CO₂ emissions.
Per transaction! There are other complications too — most obviously, the coins must be mined first (more electricity) and most of that is done in China, where electricity is cheap due to coal powered electrical plants — even more CO₂.
Personally, I cannot ethically support such a wasteful enterprise, regardless of the
freedom from state that it theoretically boasts. It's not a matter of scaling issues or small tweaks, it's a fundamental design design that all blockchain based technology shares — powering transactions through
greenhouse gas emissions (electricity use) in a purposefully wasteful (electrically expensive) manner.