Which protein sources have the lower carbon footprint [closed]

The lowest - actually, negative carbon footprint will be plant proteins - like legumes: beans, soy, lentil. (organically grown plants, produced without artificial fertilizer or heavy farming machinery break down more of CO2 than processing them afterwards creates.) To reduce it further you might pick up gardening and grow your own, organically - even the organic farming market has a bit of carbon footprint (cars bringing the foods, maintenance of the market, all the industry behind bureaucracy required, etc) but if you use simple hand tools, and grow a small plot by your house, the carbon footprint will be firmly in the negatives.

You may want to look at the source of your soy though. The fields are often replacing large swaths of the amazon forest. That would be heavily counter-productive. Other legumes don't require that sort of climate, so they have a lower impact on the environment.

The "leftovers" protein sources as you put it are actually a good choice too. They have a pretty large carbon footprint, but the problem is that this carbon footprint will exist whether you buy (provide demand, so incentive for production) or not - they will "happen" as byproduct of the main production regardless of demand, and the CO2 will be produced regardless of whether they are going to be sold, or just discarded (or redirected to animal feeder.)

Legumes are likely the lowest footprint of well known protein sources, but as @Carmi points, unless you are raising them yourself, they are not negative or zero, and maybe not then. Unless you are going completely off grid, there is fertilizer requirements, regardless of if it is organic or chemical, energy for watering and working the soil, storage, etc. and if you are not the source, transportation, and accounting for other items which may have been displaced by growing the item. Plant material other than legumes tend to go up, corn for instance tends to be a much higher feeder than most beans and peas so requires more inputs into the system. If you go with processed, such as tofu, then add in the costs of industrialized processing. Comparatively low, yes, but not free.

On meats, the dreaded broiler chicken, the nightmare to most who worry about carbon footprints, is likely the most efficient meat source readily available. They require small space, grow fast, and are much higher in efficiency in converting plant to meat than other major meat sources like beef and pork. This still makes them much less efficient that using the grains directly and will not go into opinions on the conditions that some commercial growers employ. You will pay a price to do so, but you can likely find local growers that use these birds and treat them in a manner you would more approve of it you are interested in meat in your diet. From my experience when I raised them, a regular chicken would take 6-8 months to mature and produce 3 lbs of usable meat on open pasture while consuming in a ballpark of 50-60 lbs of grain. A broil, on pasture not in those confined cages in buildings used commercially, would produce 6 lbs of usable meat in 7-8 weeks while eating 20 lbs or less of grain. Now, that is considered highly efficient protein conversion for a meat source, but makes it obvious why meat has a higher footprint than grain: The chicken required space to live, used the ground it was pastured on, produced waste product all of which cost, and still consumed up to 20 lbs of grain all of which also had footprint associated. One could have just consumed the grain and saved a lot. It is a trade off.

• My numbers are anecdotal and very rough approximations from personal experience. Though I would prefer solid documented data in an answer, I agree with earlier comments that I do not really think Seasoned Advice is the place for that detailed an argument, but a light touching of a more general answer seems OK to me in this case. Veto my opinion and remove as appropriate.