Emitting carbon dioxide is not something we can opt out of. By the very fact of our existence, we are polluters of the Earth's atmosphere, breathing in oxygen and breathing out this greenhouse gas. In addition, we are (mostly) born into societies which are driven by fossil fuels.
Freegan: a portmanteau word combining 'free' and 'vegan'; freegans do not buy animal products. This is an anti-consumerist, anti-food waste movement, so they will eat animal products if they would otherwise be discarded.