Canadian Politics Today

At least according to the economic models that you like because they tell you want you want to hear. Any time someone makes "what if" claims about what would happen in the future if society did "something" they must make a huge number of assumptions that have the potential to invalidate their claims. In the studies that you like, it appears that they assume that humans will be happy eating animal feed. I don't consider that to be a valid assumption so the most important question becomes: what would rich humans with money to spend choose to eat if meat was not an option? Avocados? Difficult to cultivate nuts? What would be the environmental impact of those particular crops? It is simply not possible to make definitive claims about the net environmental impact of reducing meat consumption.

First of all it's not economic models I speak of, it's scientific ones. And there are numerous ones that demonstrate that raising animals for food is damaging not only to the environment, but also to human health. For instance  between 14-18% of green house gas emissions come from those animals. Then there is the pollution (manure) from them, a lot of which ends up in the water. And you probably don't need a scientific study to figure out that there is a lot of land being used to grow crops to feed those animals, thereby significantly reducing the volume of protein you end up with. Animals are very inefficient at converting food into flesh. I'm not sure why you would try to say that becoming more vegetarian means we need to eat "animal food". Have you seen a lot of hay being offered in the vegetable section of your grocery store? And I'm sure if rich people want to spend their money to get meat they will get meat. Don't get me wrong, I have eaten my share of meat in my day, and I still include it in my diet, although it's very seldom red meat as I just don't feel good after I eat it. But I am willing to move even more toward vegetables the more evidence there is that it's good for me and the planet.   

Any claims these studies made are completely dependent on their assumptions. If their assumptions are wrong the conclusions are worthless. What I am doing is questioning the assumptions that went into these claims. If you want to argue that these studies should be taken at face value you are going to have to defend the assumptions that these studies depend on. So far you prefer to avoid the question and create strawmen.

The argument I make: farming practices depend on the plant being grown. The net impact of animals depend on the exact set of foods that people switch to. Simple calculations based on protein in vs protein out are not sufficient.
Nope. The majority of emissions comes from the production of animal feed. Not the animals. If there was no meat production it would be necessary to replace meat with the production of plants that can replace meat in the human diet. The open question is how large is the impact of that additional production. You can't determine the impact of animal production unless you take that into account.

There is no "assumption" as to how much land is used to produce animal feed, it's simply a measurement of acreage. That's land that could be largely replanted with people food, which would be much more efficient for both humans and the environment. But you are somewhat correct in that the % of emissions associated with raising animals for food is a combination of what comes out of them, and the energy used growing their food. And yes there certain types of crops such as almonds that do require an inordinate amount of water, but they are few and far between, and if you want to play, you pay. But I don't think most vegetarians sit down to dinner to a plate full of almonds.

Assumption 1) The land is suitable for human crops. A lot of land used for grazing is not suitable;
Assumption 2) All plants produce the same amount of food per acre; Less productive, but more tasty food could require more acres;

All science is based on assumptions. If you don't know the assumptions you cannot assess the plausibility of the claims.
This is my point. You don't know how getting rid of meat would affect the mix of crops grown and you certainly can't make assumptions based on the patterns of willing vegetarians today.  The mix of crops would reflect the tastes of many unwilling former meat eaters in need of protein. The environmental impact will be larger than the impact crops grown for animal feed. How much more depends on the assumptions.

A lot more of the land that is used for grazing could be used for food crops. Cows are the most inefficient because they can't graze elsewhere other than on mostly flat ground where there is good dirt that could be used more efficiently. Dirt is dirt and it's done all over the world. Even in places like Afghanistan where the slopes of the Hindu Kush are terraced and the rocks chucked out and vegetables are grown.
And as to your silly statement that all science is based on assumptions: light a match and throw it in gasoline. Do you have any doubts that what scientist's say will happen is simply an assumption? Wait 'till I get out of the way first.