"Globally [the livestock sector] is one of the largest sources of greenhouse gases and one of the leading causal factors in the loss of biodiversity, while in developed and emerging countries it is perhaps the leading source of water pollution."
(Livestock's Long Shadow, Food and Agriculture Organization of the United Nations)
Bottom line: Eat more vegetables and less animals.
Watch Cowspiracy to learn more (it's also on Netflix).
Note: The FAO still considers animal products a nutritious food source (i.e. they aren't "biased" toward a plant-based diet). However, if you want to learn more about a plant-based diet, check out the links below.
Joyful Vegan (Advice for Earth Day)
Plantriotic (Podcast Interview)
NutritionFacts.org (Why Plant-Based?)