Every greenhouse gas counts, whether it's part of the preexisting baseline or something recent we started emitting. The goal isn't a return to how things were, the goal is to keep the temperature from climbing too high too quickly.
Some problems you can fix in one place and be done with it. Harder problems you gotta work them every place you can make a difference. If livestock accounts for 14% of greenhouse gas, that seems like a viable angle of attack.
