"... using millions of gallons of fossil fuels to make synthetic fertilizers and pesticides."
Unless you burn fossil fuels, they do not produce carbon dioxide that does contribute to global warming,
The notion that eating meat is unhealthy ignores the fact humans were hunters and gatherers before they invented agriculture and domesticated both plant and animal species. A culture that hunts wild game and eats meat has to be nomadic because you must follow the wild game. Domesticating animals and herding them is superior to hunting because it allowed humans to become sedentary and it is safer to kill an animal that is in your herd than it is to chase a wild animal all over creation to get your dinner. Domestic animals also don't run away from you while wild animals are very tough to catch and kill.
Although it is possible to obtain a balanced vegan diet, it is much more difficult to get the proper nutrients if you exclude meat because many plant proteins do not have all of the essential amino acids. Plants are also lacking in iron; it is the blood in animals that is a good source of iron (all hemoglobin molecules contain iron and meat contains a very iron-containing protein called myoglobin; that is why both meat and blood are red).
If eating meat were not healthy, then predators would not have evolved. If eating meat were unhealthy then zoos should not feed meat to their predators for this would be harmful to the zoo animals. This would result in starvation because a predator like a wolf or lion will not eat anything but meat.
A common mistake non-scientists make is to observe a statistical correlation (humans who eat meat have a higher incidence of heart attacks and colon cancer) and conclude that this means eating meat causes colon cancer. If you really want to demonstrate the effect meat has on animals, then compare the health of predators with herbivores. This could easily be done with zoo animals. Why people have never done this escapes me, probably because they don't want to find out that predators on a meat-only diet are just as healthy as herbivores on a non-meat diet.
The best evidence that humans evolved to eat meat is that we now know there are taste receptors for cooked meat in the human tongue. This is why steak tastes good.
The best evidence that animal agriculture does not contribute to global warming (other than the internal combustion engine is used in agriculture; but, if you burn only biofuel in your tractor, you won't cause global warming) is that humans domesticated animals about 10,000 years ago. The dog was the first domesticated animal species, but dogs were not domesticated as a food source (although there are cultures that do eat dogs), they were domesticated because they are pack animals that hunt during the day (cats are not pack animals and are nocturnal). Humans are also pack animals that hunt during the day, so we domesticated dogs to help us hunt (actually the evidence is that wolves evolved traits to get us to domesticate them because both species benefited from this symbiotic relationship).
If animal agriculture contributed to global warming, then global warming would have become a problem long before the Industrial Revolution when humans discovered that burning fossil fuels could be used as a source of energy. The problem with burning fossil fuels is that this produces carbon dioxide that should not be in the atmosphere (the carbon in fossil fuels is sequestered deep in the earth and is very different from the carbon in, say, biofuels (ethanol and biodiesel). If you have ethanol in your gas tank, carbon dioxide molecules produced in combustion of ethanol simply replaces carbon dioxide molecules the corn plant used to produce ethanol, removed from the atmosphere when it was alive. Burning biofuels is a zero-sum game.
Before the Industrial Revolution, the natural carbon cycle (photosynthesis removes carbon dioxide from the atmosphere and respiration adds carbon dioxide to the atmosphere) was a steady state (this means the level of carbon dioxide in the atmosphere did not change (similar to your checking account balance not changing if you withdraw an amount of money that is equal to the amount you deposit each month). Humans started burning fossil fuels at the start of the Industrial Revolution and the carbon dioxide released upset the natural carbon cycle.
So, agriculture is actually good for the environment (so long as farmers use biofuels to power their equipment) because the plants that are cultivated remove carbon dioxide from the atmosphere.
Burning fossil fuels will cause climate change, but this is not necessarily bad. A catastrophic meteor impact wiped out the dinosaurs. The meteor impact spewed so much dust and debris in the atmosphere that less solar radiation was absorbed by the earth and this caused the temperature of the earth to drop. We know this will happen because a volcanic eruption does the same thing. The year after Mt. Tambora erupted was called the year without a summer. A cooler earth wiped out the dinosaurs because they could not adapt to the changed environment, but mammals flourished (they evolved rapidly to fill the niches once occupied by reptiles) and since we are mammals, climate change was good for us, but bad for dinosaurs.