So far as I know, commercial CO2 is taken from stack gasses at power plants, etc., it's CO2 that would otherwise be wasted to the atmosphere anyway. While used in CO2 cartridges, it's also used as a shielding gas in welding processes, and that's probably a bigger application for it. But it shouldn't contribute to global warming, etc.
But, the reason I jumped into this old thread- I'm a Pump kind of a guy. Except on last week's ride, I broke the shaft on my Road Morph. Fortunately, I had the tire up to about 75 psi, and we were able to ride it on in. That's the second Road Morph I've broken, though. If you assume that pumps are more reliable than something else- well, that may not be the case.
One other time, I rode around for 6 months and finally discovered the pump in my bag was missing the little tip and was essentially useless, but I discovered that prior to needing it.
I've rescued one or two other people with non-working pumps, also.
Asking around in the local club, everyone was telling me, "Oh, I carry a pump AND CO2", "Oh, I carry a pump AND a minipump for backup", etc.
As a side note, I've been carrying a spare tire on the tandem for 3 or 4 years, and finally used it last spring. But in that same time, I've seen and experienced multiple pump failures. So based on that experience, a backup pump/CO2 is probably more likely to be useful than a spare tire.
I'm planning to replace the Road Morph with another one, but will also carry CO2 now.