Would that mean that there could be no carbonation? We enjoy the bubbles, but it's all carbon dioxide, isn't it? That would be asking a lot to forego the carbonation. It might spell the end for Coca-Cola!
If you mean green in the sense of being environmentally friendly, I'd expect that tea or juice would have the least impact on the environment, compared to soft drinks that are processed.
The closest I've ever come is Airforce Nutrisoda (http://www.nutrisoda.com/). They can be hard to find, but they're delicious and boast a wealth of nutrients that traditional sodas can't claim- even Coke Plus.
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