Raising animals for food increases carbon emissions because of the fuel it takes to cultivate, harvest, and ship animal feed; transport animals to slaughter; and then process, package, store, and transport that plastic-wrapped slab of tenderloin you eventually pick up for dinner. Not to mention that animals—particularly beef cattle—produce methane, a greenhouse gas that's about 15 times more damaging to the atmosphere than carbon dioxide. Methane emissions are included in the calculations for carbon-dioxide emissions that we're using in the Green Challenge, via a carbon equivalency factor.