Cattle are ruminants and their food undergoes fermentation during digestion. This produces lots of methane, a powerful GHG, which is belched out and helps to trap heat in the atmospehere. Swine are not ruminants and their digestion does not involve fermentation, so no methane is created. Other aspects of raising cattle and swine, such as manure management and feed production, produce some GHG, but the fermentation aspect of cattle's digestion puts them ahead of swine by a huge margin. Per pound of meat produced, estimates range from 3 to 8 times as much GHG is produced for beef compared to pork. Yes that's a big difference! And by the way, poultry and farmed seafood are both somewhat better than pork in terms of GHG release relative to protein produced.