The more correct facts we use and the fewer mistakes we make, the stronger our arguments and the fewer holes omnivores can poke in them.
That's hows come I'm always correcting yous guys. I correct because I care.
Not all others combined, just the ones we can actually reduce with our current infrastructure without starving to death and destroying the world economy; of the "optional" emissions, they are king.The meat- and dairy industries emit more greenhouse gasses than all other emissions combined, making them the single biggest contributors to global warming.
It is one of the biggest methane producers, and because methane is pretty much the most potent greenhouse gas (common one anyway), that's a big concern. It's hard to compare directly to CO2, but in the short term, around 70 times the potency by mass.
Suggested reading:
http://en.wikipedia.org/wiki/Greenhouse_gas
The totality of atmospheric methane contributes up to 9%. This is likely still an underestimate because we're still learning about its compounding effects, like on O3, but it's a safe one.
But that's deceptively small in statistics, because up to ~72% of the greenhouse effect is from water vapor and clouds. It's the little stuff that makes a big difference, persists in the atmosphere, and creates a kind of feedback loop (whereas water enters and exits the atmosphere very quickly depending on global temperatures and weather conditions).
[edit] 9% * 3.6 (=1/(1-.72)) = 32.4%
(assuming these two values)
All said, when you take into account some 40% of methane being natural, multiply by 3.6 to normalize for the effect of water, and count only Animal Agriculture's share of anthropogenic methane (It's usually called enteric fermentation and manure management, possibly to hide the fact that we're talking about cow farts? Maybe not), which is about 1/3rd (most of the rest is natural gas released from drilling and mining), you get that cow farts (and poops, and other farmed animals too) are responsible for about 6.5% of total persistent global warming gases [edit](aside from water, in the high estimate, but since they are well mixed this is probably similar elsewhere).
[This is a crude calculation at best]
However, if you want to count it for it's contribution to anthropogenic global warming, which would only be reasonable, you have to consider that CO2 and O3 is mostly natural (even now), and have only increased by about 40%, while CH4 (Methane) has increased 170% in the last couple hundred years, [edit] which means the CO2 contributions from humans is total relative to the total. This is easier said than done, and requires adding up everything else to normalize.
16% [EDIT: this is probably bad math, I've seen around 18% elsewhere from more credible sources than cowspiracy]
Normalizing for only CO2, Methane, and Ozone, it could be up to 21%... but still, there are many factors when gases are mixed like this and it's hard to account for more minor gases and catalytic effects.
That's just the farts (and it would be a little more than that, since cows did exist before 1750 too, but there weren't many of them).
Transportation and industrial infrastructure for the animals, their products, their food, and the processes needed to grow that food increases that footprint, but enough to make it a majority? No. 20% total almost certainly (given the CO2 emissions in agriculture and transportation), but I would balk at more than 30%. We spend a lot of energy in the home, at work, in unrelated industries (particularly construction), and on transportation of our persons around town.
Being conservative, 16% alone is very significant, and it pretty much comes down to us turning off our lights, computers, heating, and air conditioning (as it's comparable to the total of all emissions associated with residential energy usage), or not eating meat anymore to have a similar effect.
That's why I say it's the largest source of optional emissions -- the other things we can do right now aren't very realistic (Yes, less realistic than the world eating less meat, hard to believe).
