Here is why I eat some chicken, no beef, I why I won't be eating shrimp anymore.
This chart is taken from the fantastic report by the UN, "Kick the Habit, a UN Guide to Climate Neutrality.", page 103. You need to combine it with the World Ressource Institute chart, pasted below, on the proportion of the entire problem that is caused by different activities, including agriculture and livestocks (it's 12.3%.)
It's also the reason I drive a 70 mpg motorcycle, and why I lobby for 100% clean electricity for Massachusetts by 2020.