Scientifically it's proven that animal agriculture accounts for an average of 50% of global green house gases emitted into the environment causing global warming. Global warming has the following negative effects;
Many sources, when doing quick fact checking state differently. As well, excessively large plots of land are used for crops as well which also release their own GHG's. I am not saying that crops are worse (they are better) but implying that roughly 50% of GHG's are scientifically proven to come from agriculture... I mean using USA for a benchmark, roughly 9% of its GHG's come from agriculture1. Yes that number will fluctate country to country, however the US is a major producer of GHG's (along with places like China) and with minimal extrapolation you can deduce that Agriculture would very very likely be below the number provided. More is wrong in this post too, I just wanted to clear this up for future readers.
[1] EPA GHG Source