Good gawd. I see CO2 is being listed as an "alternative." Do you folks HAVE ANY FREEKIN IDEA how high operating head pressures would be with CO2???????????????????????????
I think it was the Germans who were looking into CO2 and yes the pressures would be ridiculous. As I recall the pressures were well over 1000 PSI. Given the high pressures required the idea didn't seem to go very far. I would imagine the costs involved to make it a safe system killed the idea.
It's the European Union who is really pushing to kill R-134A, although the phase out date in the US is supposed to be 2017. The replacement R-1234yf is slowly being phased in. It is supposed to be $60 to $70 a pound coming out of a joint venture Honeywell/ Dupont factory in China. As far as I know R-1234yf will not be available to us do-it-yourselfers.
Mercedes started using R-1234yf and then recalled 1500 cars and refitted them with R-134A citing safety issues with R-1234yf when it burns. The EU is in an uproar over that as R-134A is supposed to be illegal in the EU now.
We live in a very interesting world. I suspect when R-1234yf really hits the field R-134A will still be available but go to the R-1234yf price to keep people from converting back to R-134A when repairs are required.