Raising livestock generates 9% of all CO2 emissions, 37% of Methane emissions and 65% of nitrous oxide emissions on the planet. 18% of all Greenhouse
gasses (in CO2 equivalents) come from the raising of livestock. Methane is 23 times as potent of greenhouse gas as CO2 and nitrous oxide is 296 times as potent.

Raising 1kg of meat causes the emissions equivalent to 80 lbs of CO2 or burning 4 gallons of gasoline.

For more information, check out this article from New Scientist or this one from the Guardian