Most carnivores of course hardly ever thought about their decision in detail because nobody seriously asks them. It's just the normal state of being. It's enough reason to just like meat. Vegetarians on the other hand need to mull over their decision and their reasons will be questioned. Yet maybe meat-eaters should be considering this topic just like they consider not drinking as much pop because they are worried about their figure.
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When you think about the food chain you find that to produce a certain amount of meat you need to feed the animal with cereals that could also be used for the starving millions in the world. This wheat or soy is produced on huge acres in places where there was rainforest until some years ago which was keeping our climate oxygenized. On the other hand, huge food plants with tens of thousands of cattle emit massive amounts of methane, a very strong greenhouse gas (18% of greenhouse gases attributed to livestock production!).
If you follow the steps further back that lead to this mountain of cereal you have water, and vast amounts of it. Water is the most important resource of the planet and arguably the wars of the 21st century will be about access to clean sources of it. As if that wasn't enough, livestock production takes place in remote Argentina which means that the average meat consumers in Europe and the North America have their steak shipped halfway around the globe. Here is a map of meat consumption from here.
Well, my point is made :) I hope you were open for all this today and will think more consciously about food. If you want data and entertaining talk about how to better eat, just watch this TED video. And if you still would like to know more, watch the founder of treehugger.com, why you should and how you could be a weekday vegetarian (starting minute 9).
Btw: I don't smoke either, but that's a whole different story ;)