First off, the methane from cows is 1.8% of the greenhouse gas emissions in the US. Second off, methane doesn't come from cattle farts, it comes from cattle burps. I realize that might be a minor distinction, but the EPA, those high priests of junk science, jumped on the "regulate cattle farts" bandwagon under Obama. The UN claims cattle create 18% of global greenhouse gas emissions - more than comes from transportation - but they're lumping in all livestock, not just cattle, to include poultry, lamb and all sources of meat. They're also including the effects of animal feed production, feed harvesting, feeding the animals, the farm vehicles that tend to these animals and everything up to the emissions from the slaughterhouse. A third of that 18% is blamed on deforestation specifically in Brazil.
Both of those summaries are dishonest. First, it's not fair to blame methane production in chicken farming on cattle farming, and it's unfair to include everything that the goes into food production to just the tailpipe emissions of vehicles rather than the equivalent entire life cycle associated with transportation. Second, the part about deforestation is dishonest for two reasons; the easiest being that there's no equivalent deforestation in the US, or in other parts of the world. In the US the story is reforestation. We have more trees today than a hundred years ago. The other reason is that not all grassland could be forest and not all forest can convert to grasslands. There is some relation between the two, but it's not simple subtraction. Simply, much of the planet can't be dense forest and can only be grassland.
One of the most interesting videos from Ted talks about science is a 2013 talk about desertification by Allan Savory. Dr. Savory talks about discovering that large herds of grass eating animals restore grasslands and reverse the damages to the environment. Yes, herds of life stock "save the Earth". It's a story of how a hundred years of following the agricultural scientists' best recommendations and removing livestock converted lush grassland to wastelands, and how reintroducing livestock has restored millions of hectares of that wasteland to productive grassland. The before and after pictures toward the last few minutes of the video are jaw-dropping.
Dr. Savory recommends eating the livestock, which naturally makes the the vegans haaaate him with a white hot, burning hate.
An interesting guy I've heard talk on this subject several times is Dr. Peter Ballerstedt, who calls himself a leader of the Ruminati. He's an infrequent blogger, and regular speaker at various conferences. Here's a quick summary of the basis for a new video, We Need A Ruminant Revolution.
Human beings exist because of ruminants. Today’s societies rely upon them. Humanity’s future depends upon improvements in the productivity and efficiency of worldwide ruminant animal agriculture. Like the general public’s confusion of what constitutes a “healthy diet,” tremendous misunderstanding exists regarding the environmental role of ruminant animals. Human beings didn’t evolve to eat meat, they evolved because they ate meat - and because they learned to cook and process meat and other foodstuffs. Unsurprisingly then, diets rich in butter, meat and cheese have been shown to promote human health and development. Of significant worldwide impact, such diets can correct the symptoms of metabolic syndrome, offering hope in arresting the current worldwide epidemic of chronic diseases.The dire predictions from Anorexia Pistachio Kotex and her Democratic Socialists of America comrades that say we need to end cattle farts would be laughable without the need for a single fart joke. Except it's not funny, it would kill many people and make the planet worse off because the planet needs livestock. The planet needs ruminants. You can say they were designed for each other or you can say they co-evolved this way, but the result is the same. Like everything they say, this argument is demonstrably wrong.