It doesn’t, at least not in a negative way. In theory, breathing is part of a closed-loop cycle that sees our exhalations get recycled by plants. In practice, whenever you breathe in you keep a little bit of carbon for your own body, so we actually come out ahead (when it comes to global warming prevention).
However, we each combust and release into the atmosphere more than our fair share of carbon every day just by driving to work. Breathing is small potatoes.
